diff --git a/MANIFEST.in b/MANIFEST.in
index ecc747a..b5f518e 100644
--- a/MANIFEST.in
+++ b/MANIFEST.in
@@ -6,4 +6,5 @@ include sample.settings.py
 recursive-include contrib/ *.py
 recursive-include overviewer_core/ *.py
 recursive-include overviewer_core/src/ *.c *.h
+recursive-include overviewer_core/src/primitives/ *.c *.h
 recursive-include overviewer_core/data/ *
diff --git a/docs/config.rst b/docs/config.rst
index d6d26bd..5b073dd 100644
--- a/docs/config.rst
+++ b/docs/config.rst
@@ -430,6 +430,26 @@ SmoothLighting
 
     (same as Lighting)
 
+ClearBase
+    Forces the background to be transparent. Use this in place of Base
+    for rendering pure overlays.
+
+SpawnOverlay
+    Color the map red in areas where monsters can spawn. Either use
+    this on top of other modes, or on top of ClearBase to create a
+    pure overlay.
+
+MineralOverlay
+    Color the map according to what minerals can be found
+    underneath. Either use this on top of other modes, or on top of
+    ClearBase to create a pure overlay.
+    
+    **Options**
+    
+    minerals
+        A list of (blockid, (r, g, b)) tuples to use as colors. If not
+        provided, a default list of common minerals is used.
+
 Defining Custom Rendermodes
 ---------------------------
 Each rendermode primitive listed above is a Python *class* that is automatically
diff --git a/overviewer.py b/overviewer.py
index 9ab4019..83ee257 100755
--- a/overviewer.py
+++ b/overviewer.py
@@ -365,8 +365,11 @@ dir but you forgot to put quotes around the directory, since it contains spaces.
         if rset == None: # indicates no such dimension was found:
             logging.error("Sorry, you requested dimension '%s' for %s, but I couldn't find it", render['dimension'], render_name)
             return 1
+        
+        # If this is to be a rotated regionset, wrap it in a RotatedRegionSet
+        # object
         if (render['northdirection'] > 0):
-            rset = rset.rotate(render['northdirection'])
+            rset = world.RotatedRegionSet(rset, render['northdirection'])
         logging.debug("Using RegionSet %r", rset) 
 
         # create our TileSet from this RegionSet
@@ -380,6 +383,12 @@ dir but you forgot to put quotes around the directory, since it contains spaces.
         tset = tileset.TileSet(rset, assetMrg, tex, tileSetOpts, tileset_dir)
         tilesets.append(tset)
 
+    # Do tileset preprocessing here, before we start dispatching jobs
+    for ts in tilesets:
+        ts.do_preprocessing()
+
+    # Output initial static data and configuration
+    assetMrg.initialize(tilesets)
    
     # multiprocessing dispatcher
     if config['processes'] == 1:
diff --git a/overviewer_core/assetmanager.py b/overviewer_core/assetmanager.py
index 5d60cae..26734b4 100644
--- a/overviewer_core/assetmanager.py
+++ b/overviewer_core/assetmanager.py
@@ -72,7 +72,23 @@ directory.
         # TODO based on the type, so something
         POI[regionset.name].append
 
-    def finalize(self, tilesets):
+    def initialize(self, tilesets):
+        """Similar to finalize() but calls the tilesets' get_initial_data()
+        instead of get_persistent_data() to compile the generated javascript
+        config.
+
+        """
+        return self.finalize(tilesets, True)
+
+    def finalize(self, tilesets, initial=False):
+        """Called to output the generated javascript and all static files to
+        the output directory
+
+        """
+        if not initial:
+            get_data = lambda tileset: tileset.get_persistent_data()
+        else:
+            get_data = lambda tileset: tileset.get_initial_data()
 
         # dictionary to hold the overviewerConfig.js settings that we will dumps
         dump = dict()
@@ -94,7 +110,7 @@ directory.
         # based on the tilesets we have, group them by worlds
         worlds = []
         for tileset in tilesets:
-            full_name = tileset.get_persistent_data()['world']
+            full_name = get_data(tileset)['world']
             if full_name not in worlds:
                 worlds.append(full_name)
 
@@ -120,7 +136,7 @@ directory.
 
 
         for tileset in tilesets:
-            dump['tilesets'].append(tileset.get_persistent_data())
+            dump['tilesets'].append(get_data(tileset))
 
             # write a blank image
             blank = Image.new("RGBA", (1,1), tileset.options.get('bgcolor'))
@@ -128,11 +144,10 @@ directory.
 
 
         jsondump = json.dumps(dump, indent=4)
-        with codecs.open(os.path.join(self.outputdir, 'overviewerConfig.js'), 'w', encoding='UTF-8') as f:
-            f.write("var overviewerConfig = " + jsondump + ";\n")
+        with util.FileReplacer(os.path.join(self.outputdir, "overviewerConfig.js")) as tmpfile:
+            with codecs.open(tmpfile, 'w', encoding='UTF-8') as f:
+                f.write("var overviewerConfig = " + jsondump + ";\n")
 
-          
-        
         # copy web assets into destdir:
         global_assets = os.path.join(util.get_program_path(), "overviewer_core", "data", "web_assets")
         if not os.path.isdir(global_assets):
@@ -143,23 +158,16 @@ directory.
         js_src = os.path.join(util.get_program_path(), "overviewer_core", "data", "js_src")
         if not os.path.isdir(js_src):
             js_src = os.path.join(util.get_program_path(), "js_src")
-        with open(os.path.join(self.outputdir, "overviewer.js"), "w") as fout:
-            # first copy in js_src/overviewer.js
-            with open(os.path.join(js_src, "overviewer.js")) as f:
-                fout.write(f.read())
-            # now copy in the rest
-            for js in os.listdir(js_src):
-                if not js.endswith("overviewer.js"):
-                    with open(os.path.join(js_src,js)) as f:
-                        fout.write(f.read())
-        
-        # do the same with the local copy, if we have it
-        # TODO 
-        # if self.web_assets_path:
-        #    util.mirror_dir(self.web_assets_path, self.outputdir)
-
-
-
+        with util.FileReplacer(os.path.join(self.outputdir, "overviewer.js")) as tmpfile:
+            with open(tmpfile, "w") as fout:
+                # first copy in js_src/overviewer.js
+                with open(os.path.join(js_src, "overviewer.js"), 'r') as f:
+                    fout.write(f.read())
+                # now copy in the rest
+                for js in os.listdir(js_src):
+                    if not js.endswith("overviewer.js") and js.endswith(".js"):
+                        with open(os.path.join(js_src,js)) as f:
+                            fout.write(f.read())
         # helper function to get a label for the given rendermode
         def get_render_mode_label(rendermode):
             info = get_render_mode_info(rendermode)
@@ -177,7 +185,6 @@ directory.
         versionstr = "%s (%s)" % (overviewer_version.VERSION, overviewer_version.HASH[:7])
         index = index.replace("{version}", versionstr)
 
-        with codecs.open(os.path.join(self.outputdir, "index.html"), 'w', encoding='UTF-8') as output:
-            output.write(index)
-
-
+        with util.FileReplacer(indexpath) as indexpath:
+            with codecs.open(indexpath, 'w', encoding='UTF-8') as output:
+                output.write(index)
diff --git a/overviewer_core/data/js_src/views.js b/overviewer_core/data/js_src/views.js
index bde253e..70867e0 100644
--- a/overviewer_core/data/js_src/views.js
+++ b/overviewer_core/data/js_src/views.js
@@ -36,21 +36,23 @@ overviewer.views.WorldView = Backbone.View.extend({
 
 overviewer.views.WorldSelectorView = Backbone.View.extend({
     initialize: function() {
-        // a div will have already been created for us, we just
-        // need to register it with the google maps control
-        var selectBox = document.createElement('select');
-        $.each(overviewer.collections.worldViews, function(index, elem) {
-            var o = document.createElement("option");
-            o.value = elem.model.get("name");
-            o.innerHTML = elem.model.get("name");
-            $(o).data("viewObj", elem);
-            selectBox.appendChild(o);
+        if(overviewer.collections.worldViews.length > 1) {
+            // a div will have already been created for us, we just
+            // need to register it with the google maps control
+            var selectBox = document.createElement('select');
+            $.each(overviewer.collections.worldViews, function(index, elem) {
+                var o = document.createElement("option");
+                o.value = elem.model.get("name");
+                o.innerHTML = elem.model.get("name");
+                $(o).data("viewObj", elem);
+                selectBox.appendChild(o);
 
-        });
+            });
 
-        this.el.appendChild(selectBox);
-        overviewer.map.controls[google.maps.ControlPosition.TOP_LEFT].push(this.el);
-        },
+            this.el.appendChild(selectBox);
+            overviewer.map.controls[google.maps.ControlPosition.TOP_LEFT].push(this.el);
+        }
+    },
     events: {
         "change select":  "changeWorld"
     },
diff --git a/overviewer_core/dispatcher.py b/overviewer_core/dispatcher.py
index 2e8e353..9823eef 100644
--- a/overviewer_core/dispatcher.py
+++ b/overviewer_core/dispatcher.py
@@ -49,14 +49,9 @@ class Dispatcher(object):
         """
         # TODO use status callback
         
-        # preprocessing
-        for tileset in tilesetlist:
-            tileset.do_preprocessing()
-        
         # setup tilesetlist
         self.setup_tilesets(tilesetlist)
         
-        
         # iterate through all possible phases
         num_phases = [tileset.get_num_phases() for tileset in tilesetlist]
         for phase in xrange(max(num_phases)):
diff --git a/overviewer_core/rendermodes.py b/overviewer_core/rendermodes.py
index 12ab93b..6548cb9 100644
--- a/overviewer_core/rendermodes.py
+++ b/overviewer_core/rendermodes.py
@@ -146,6 +146,45 @@ class Lighting(RenderPrimitive):
 class SmoothLighting(Lighting):
     name = "smooth-lighting"
 
+class ClearBase(RenderPrimitive):
+    name = "clear-base"
+
+class Overlay(RenderPrimitive):
+    name = "overlay"
+
+    @property
+    def whitecolor(self):
+        whitecolor = getattr(self, "_whitecolor", None)
+        if whitecolor:
+            return whitecolor
+        white = Image.new("RGBA", (24,24), (255, 255, 255, 255))
+        self._whitecolor = white
+        return white
+    
+    @property
+    def facemask_top(self):
+        facemask_top = getattr(self, "_facemask_top", None)
+        if facemask_top:
+            return facemask_top
+        
+        white = Image.new("L", (24,24), 255)
+        top = Image.new("L", (24,24), 0)
+        toppart = textures.Textures.transform_image_top(white)
+        top.paste(toppart, (0,0))
+        for x,y in [(3,4), (7,2), (11,0)]:
+            top.putpixel((x,y), 255)
+        self._facemask_top = top
+        return top
+
+class SpawnOverlay(Overlay):
+    name = "overlay-spawn"
+
+class MineralOverlay(Overlay):
+    name = "overlay-mineral"
+    options = {
+        'minerals' : ('a list of (blockid, (r, g, b)) tuples for coloring minerals', None),
+    }
+
 # Built-in rendermodes for your convenience!
 normal = [Base(), EdgeLines()]
 lighting = [Base(), EdgeLines(), Lighting()]
diff --git a/overviewer_core/settingsDefinition.py b/overviewer_core/settingsDefinition.py
index 91d8560..4c3dd02 100644
--- a/overviewer_core/settingsDefinition.py
+++ b/overviewer_core/settingsDefinition.py
@@ -71,7 +71,7 @@ renders = Setting(required=True, default={},
             "optimizeimg": Setting(required=True, validator=validateOptImg, default=0),
             "nomarkers": Setting(required=False, validator=validateBool, default=None),
             "texturepath": Setting(required=False, validator=validateTexturePath, default=None),
-            "renderchecks": Setting(required=True, validator=validateInt, default=0),
+            "renderchecks": Setting(required=False, validator=validateInt, default=None),
             "rerenderprob": Setting(required=True, validator=validateFloat, default=0),
             
             # Remove this eventually (once people update their configs)
diff --git a/overviewer_core/signals.py b/overviewer_core/signals.py
index 3e8dc5e..ca2eab8 100755
--- a/overviewer_core/signals.py
+++ b/overviewer_core/signals.py
@@ -89,3 +89,12 @@ class Signal(object):
     # convenience
     def __call__(self, *args, **kwargs):
         self.emit(*args, **kwargs)
+    
+    # force pickled signals to redirect to existing signals
+    def __getstate__(self):
+        return self.fullname
+    def __setstate__(self, fullname):
+        for attr in dir(self.signals[fullname]):
+            if attr.startswith('_'):
+                continue
+            setattr(self, attr, getattr(self.signals[fullname], attr))
diff --git a/overviewer_core/src/overviewer.h b/overviewer_core/src/overviewer.h
index d402795..89b3c6e 100644
--- a/overviewer_core/src/overviewer.h
+++ b/overviewer_core/src/overviewer.h
@@ -26,7 +26,7 @@
 
 // increment this value if you've made a change to the c extesion
 // and want to force users to rebuild
-#define OVERVIEWER_EXTENSION_VERSION 25
+#define OVERVIEWER_EXTENSION_VERSION 26
 
 /* Python PIL, and numpy headers */
 #include <Python.h>
diff --git a/overviewer_core/src/primitives/clear-base.c b/overviewer_core/src/primitives/clear-base.c
new file mode 100644
index 0000000..3ee36dc
--- /dev/null
+++ b/overviewer_core/src/primitives/clear-base.c
@@ -0,0 +1,49 @@
+/* 
+ * This file is part of the Minecraft Overviewer.
+ *
+ * Minecraft Overviewer is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Minecraft Overviewer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "../overviewer.h"
+
+static int
+clear_base_occluded(void *data, RenderState *state, int x, int y, int z) {
+    if ( (x != 0) && (y != 15) && (z != 127) &&
+         !render_mode_hidden(state->rendermode, x-1, y, z) &&
+         !render_mode_hidden(state->rendermode, x, y, z+1) &&
+         !render_mode_hidden(state->rendermode, x, y+1, z) &&
+         !is_transparent(getArrayByte3D(state->blocks, x-1, y, z)) &&
+         !is_transparent(getArrayByte3D(state->blocks, x, y, z+1)) &&
+         !is_transparent(getArrayByte3D(state->blocks, x, y+1, z))) {
+        return 1;
+    }
+    
+    return 0;
+}
+
+static void
+clear_base_draw(void *data, RenderState *state, PyObject *src, PyObject *mask, PyObject *mask_light) {
+    /* clear the draw space -- set alpha to 0 within mask */
+    tint_with_mask(state->img, 255, 255, 255, 0, mask, state->imgx, state->imgy, 0, 0);
+}
+
+RenderPrimitiveInterface primitive_clear_base = {
+    "clear-base",
+    0,
+    NULL,
+    NULL,
+    clear_base_occluded,
+    NULL,
+    clear_base_draw,
+};
diff --git a/overviewer_core/src/rendermode-mineral.c b/overviewer_core/src/primitives/overlay-mineral.c
similarity index 64%
rename from overviewer_core/src/rendermode-mineral.c
rename to overviewer_core/src/primitives/overlay-mineral.c
index 858333b..5d6f22e 100644
--- a/overviewer_core/src/rendermode-mineral.c
+++ b/overviewer_core/src/primitives/overlay-mineral.c
@@ -15,7 +15,14 @@
  * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
  */
 
-#include "overviewer.h"
+#include "overlay.h"
+
+typedef struct {
+    /* inherits from overlay */
+    RenderPrimitiveOverlay parent;
+    
+    void *minerals;
+} RenderPrimitiveMineral;
 
 struct MineralColor {
     unsigned char blockid;
@@ -43,15 +50,16 @@ static struct MineralColor default_minerals[] = {
 static void get_color(void *data, RenderState *state,
                       unsigned char *r, unsigned char *g, unsigned char *b, unsigned char *a) {
     
-    int x = state->x, y = state->y, z_max = state->z + 1, z;
+    int x = state->x, z = state->z, y_max, y;
     int max_i = -1;
-    RenderModeMineral* self = (RenderModeMineral *)data;
+    RenderPrimitiveMineral* self = (RenderPrimitiveMineral *)data;
     struct MineralColor *minerals = (struct MineralColor *)(self->minerals);
     *a = 0;
     
-    for (z = 0; z <= z_max; z++) {
+    y_max = state->y + 1;
+    for (y = state->chunky * -16; y <= y_max; y++) {
         int i, tmp;
-        unsigned char blockid = getArrayByte3D(state->blocks, x, y, z);
+        unsigned short blockid = get_data(state, BLOCKS, x, y, z);
         
         for (i = 0; (max_i == -1 || i < max_i) && minerals[i].blockid != 0; i++) {
             if (minerals[i].blockid == blockid) {
@@ -59,7 +67,7 @@ static void get_color(void *data, RenderState *state,
                 *g = minerals[i].g;
                 *b = minerals[i].b;
                 
-                tmp = (128 - z_max + z) * 2 - 40;
+                tmp = (128 - y_max + y) * 2 - 40;
                 *a = MIN(MAX(0, tmp), 255);
                 
                 max_i = i;
@@ -70,20 +78,21 @@ static void get_color(void *data, RenderState *state,
 }
 
 static int
-rendermode_mineral_start(void *data, RenderState *state, PyObject *options) {
+overlay_mineral_start(void *data, RenderState *state, PyObject *support) {
     PyObject *opt;
-    RenderModeMineral* self;
+    RenderPrimitiveMineral* self;
 
     /* first, chain up */
-    int ret = rendermode_overlay.start(data, state, options);
+    int ret = primitive_overlay.start(data, state, support);
     if (ret != 0)
         return ret;
     
     /* now do custom initializations */
-    self = (RenderModeMineral *)data;
+    self = (RenderPrimitiveMineral *)data;
     
-    opt = PyDict_GetItemString(options, "minerals");
-    if (opt) {
+    if (!render_mode_parse_option(support, "minerals", "O", &(opt)))
+        return 1;
+    if (opt && opt != Py_None) {
         struct MineralColor *minerals = NULL;
         Py_ssize_t minerals_size = 0, i;
         /* create custom minerals */
@@ -110,6 +119,7 @@ rendermode_mineral_start(void *data, RenderState *state, PyObject *options) {
     } else {
         self->minerals = default_minerals;
     }
+    Py_XDECREF(opt);
     
     /* setup custom color */
     self->parent.get_color = get_color;
@@ -118,50 +128,24 @@ rendermode_mineral_start(void *data, RenderState *state, PyObject *options) {
 }
 
 static void
-rendermode_mineral_finish(void *data, RenderState *state) {
+overlay_mineral_finish(void *data, RenderState *state) {
     /* first free all *our* stuff */
-    RenderModeMineral* self = (RenderModeMineral *)data;
+    RenderPrimitiveMineral* self = (RenderPrimitiveMineral *)data;
     
     if (self->minerals && self->minerals != default_minerals) {
         free(self->minerals);
     }
     
     /* now, chain up */
-    rendermode_overlay.finish(data, state);
+    primitive_overlay.finish(data, state);
 }
 
-static int
-rendermode_mineral_occluded(void *data, RenderState *state, int x, int y, int z) {
-    /* no special occlusion here */
-    return rendermode_overlay.occluded(data, state, x, y, z);
-}
-
-static int
-rendermode_mineral_hidden(void *data, RenderState *state, int x, int y, int z) {
-    /* no special hiding here */
-    return rendermode_overlay.hidden(data, state, x, y, z);
-}
-
-static void
-rendermode_mineral_draw(void *data, RenderState *state, PyObject *src, PyObject *mask, PyObject *mask_light) {
-    /* draw normally */
-    rendermode_overlay.draw(data, state, src, mask, mask_light);
-}
-
-const RenderModeOption rendermode_mineral_options[] = {
-    {"minerals", "a list of (blockid, (r, g, b)) tuples for coloring minerals"},
-    {NULL, NULL}
-};
-
-RenderModeInterface rendermode_mineral = {
-    "mineral", "Mineral",
-    "draws a colored overlay showing where ores are located",
-    rendermode_mineral_options,
-    &rendermode_overlay,
-    sizeof(RenderModeMineral),
-    rendermode_mineral_start,
-    rendermode_mineral_finish,
-    rendermode_mineral_occluded,
-    rendermode_mineral_hidden,
-    rendermode_mineral_draw,
+RenderPrimitiveInterface primitive_overlay_mineral = {
+    "overlay-mineral",
+    sizeof(RenderPrimitiveMineral),
+    overlay_mineral_start,
+    overlay_mineral_finish,
+    NULL,
+    NULL,
+    overlay_draw,
 };
diff --git a/overviewer_core/src/primitives/overlay-spawn.c b/overviewer_core/src/primitives/overlay-spawn.c
new file mode 100644
index 0000000..872dc0f
--- /dev/null
+++ b/overviewer_core/src/primitives/overlay-spawn.c
@@ -0,0 +1,92 @@
+/*
+ * This file is part of the Minecraft Overviewer.
+ *
+ * Minecraft Overviewer is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Minecraft Overviewer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "overlay.h"
+#include <math.h>
+
+typedef struct {
+    /* inherits from overlay */
+    RenderPrimitiveOverlay parent;
+} RenderPrimitiveSpawn;
+
+static void get_color(void *data, RenderState *state,
+                      unsigned char *r, unsigned char *g, unsigned char *b, unsigned char *a) {
+    
+    RenderPrimitiveSpawn* self = (RenderPrimitiveSpawn *)data;
+    int x = state->x, y = state->y, z = state->z;
+    int y_light = y + 1;
+    unsigned char blocklight, skylight;
+    PyObject *block_py;
+    
+    /* set a nice, pretty red color */
+    *r = 229;
+    *g = 36;
+    *b = 38;
+    
+    /* default to no overlay, until told otherwise */
+    *a = 0;
+    
+    if (block_has_property(state->block, NOSPAWN)) {
+        /* nothing can spawn on this */
+        return;
+    }
+    
+    blocklight = get_data(state, BLOCKLIGHT, x, y_light, z);
+    skylight = get_data(state, SKYLIGHT, x, y_light, z);
+    
+    if (MAX(blocklight, skylight) <= 7) {
+        /* hostile mobs spawn in daylight */
+        *a = 240;
+    } else if (MAX(blocklight, skylight - 11) <= 7) {
+        /* hostile mobs spawn at night */
+        *a = 150;
+    }
+}
+
+static int
+overlay_spawn_start(void *data, RenderState *state, PyObject *support) {
+    RenderPrimitiveSpawn* self;
+
+    /* first, chain up */
+    int ret = primitive_overlay.start(data, state, support);
+    if (ret != 0)
+        return ret;
+    
+    /* now do custom initializations */
+    self = (RenderPrimitiveSpawn *)data;
+    self->parent.get_color = get_color;
+    
+    return 0;
+}
+
+static void
+overlay_spawn_finish(void *data, RenderState *state) {
+    RenderPrimitiveSpawn* self = (RenderPrimitiveSpawn *)data;
+    
+    /* chain up */
+    primitive_overlay.finish(data, state);
+}
+
+RenderPrimitiveInterface primitive_overlay_spawn = {
+    "overlay-spawn",
+    sizeof(RenderPrimitiveSpawn),
+    overlay_spawn_start,
+    overlay_spawn_finish,
+    NULL,
+    NULL,
+    overlay_draw,
+};
diff --git a/overviewer_core/src/primitives/overlay.c b/overviewer_core/src/primitives/overlay.c
new file mode 100644
index 0000000..6c1215b
--- /dev/null
+++ b/overviewer_core/src/primitives/overlay.c
@@ -0,0 +1,99 @@
+/* 
+ * This file is part of the Minecraft Overviewer.
+ *
+ * Minecraft Overviewer is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Minecraft Overviewer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "overlay.h"
+
+static void get_color(void *data, RenderState *state,
+                      unsigned char *r, unsigned char *g, unsigned char *b, unsigned char *a) {
+    *r = 200;
+    *g = 200;
+    *b = 255;
+    *a = 155;
+}
+
+static int
+overlay_start(void *data, RenderState *state, PyObject *support) {
+    PyObject *facemasks_py;
+    RenderPrimitiveOverlay *self = (RenderPrimitiveOverlay *)data;
+    
+    self->facemask_top = PyObject_GetAttrString(support, "facemask_top");
+    self->white_color = PyObject_GetAttrString(support, "whitecolor");
+    self->get_color = get_color;
+    
+    return 0;
+}
+
+static void
+overlay_finish(void *data, RenderState *state) {
+    RenderPrimitiveOverlay *self = (RenderPrimitiveOverlay *)data;
+    
+    Py_DECREF(self->facemask_top);
+    Py_DECREF(self->white_color);
+}
+
+void
+overlay_draw(void *data, RenderState *state, PyObject *src, PyObject *mask, PyObject *mask_light) {
+    RenderPrimitiveOverlay *self = (RenderPrimitiveOverlay *)data;
+    unsigned char r, g, b, a;
+    unsigned short top_block;
+
+    // exactly analogous to edge-line code for these special blocks
+    int increment=0;
+    if (state->block == 44)  // half-step
+        increment=6;
+    else if (state->block == 78) // snow
+        increment=9;
+    
+    /* skip rendering the overlay if we can't see it */
+    top_block = get_data(state, BLOCKS, state->x, state->y+1, state->z);
+    if (!is_transparent(top_block)) {
+        return;
+    }
+    
+    /* check to be sure this block is solid/fluid */
+    if (block_has_property(top_block, SOLID) || block_has_property(top_block, FLUID)) {
+        
+        /* top block is fluid or solid, skip drawing */
+        return;
+    }
+    
+    /* check to be sure this block is solid/fluid */
+    if (!block_has_property(state->block, SOLID) && !block_has_property(state->block, FLUID)) {
+        
+        /* not fluid or solid, skip drawing the overlay */
+        return;
+    }
+
+    /* get our color info */
+    self->get_color(data, state, &r, &g, &b, &a);
+    
+    /* do the overlay */
+    if (a > 0) {
+        alpha_over(state->img, self->white_color, self->facemask_top, state->imgx, state->imgy + increment, 0, 0);
+        tint_with_mask(state->img, r, g, b, a, self->facemask_top, state->imgx, state->imgy + increment, 0, 0);
+    }
+}
+
+RenderPrimitiveInterface primitive_overlay = {
+    "overlay",
+    sizeof(RenderPrimitiveOverlay),
+    overlay_start,
+    overlay_finish,
+    NULL,
+    NULL,
+    overlay_draw,
+};
diff --git a/overviewer_core/src/primitives/overlay.h b/overviewer_core/src/primitives/overlay.h
new file mode 100644
index 0000000..cc028bb
--- /dev/null
+++ b/overviewer_core/src/primitives/overlay.h
@@ -0,0 +1,31 @@
+/* 
+ * This file is part of the Minecraft Overviewer.
+ *
+ * Minecraft Overviewer is free software: you can redistribute it and/or
+ * modify it under the terms of the GNU General Public License as published
+ * by the Free Software Foundation, either version 3 of the License, or (at
+ * your option) any later version.
+ *
+ * Minecraft Overviewer is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
+ * Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License along
+ * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
+ */
+
+#include "../overviewer.h"
+
+typedef struct {
+    /* top facemask and white color image, for drawing overlays */
+    PyObject *facemask_top, *white_color;
+    /* can be overridden in derived classes to control
+       overlay alpha and color
+       last four vars are r, g, b, a out */
+    void (*get_color)(void *, RenderState *,
+                      unsigned char *, unsigned char *, unsigned char *, unsigned char *);
+} RenderPrimitiveOverlay;
+extern RenderPrimitiveInterface primitive_overlay;
+
+void overlay_draw(void *data, RenderState *state, PyObject *src, PyObject *mask, PyObject *mask_light);
diff --git a/overviewer_core/src/rendermode-overlay.c b/overviewer_core/src/rendermode-overlay.c
deleted file mode 100644
index 9ae4e25..0000000
--- a/overviewer_core/src/rendermode-overlay.c
+++ /dev/null
@@ -1,132 +0,0 @@
-/* 
- * This file is part of the Minecraft Overviewer.
- *
- * Minecraft Overviewer is free software: you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as published
- * by the Free Software Foundation, either version 3 of the License, or (at
- * your option) any later version.
- *
- * Minecraft Overviewer is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
- * Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along
- * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-#include "overviewer.h"
-
-static void get_color(void *data, RenderState *state,
-                      unsigned char *r, unsigned char *g, unsigned char *b, unsigned char *a) {
-    *r = 200;
-    *g = 200;
-    *b = 255;
-    *a = 155;
-}
-
-static int
-rendermode_overlay_start(void *data, RenderState *state, PyObject *options) {
-    PyObject *facemasks_py;
-    RenderModeOverlay *self = (RenderModeOverlay *)data;
-    
-    facemasks_py = PyObject_GetAttrString(state->support, "facemasks");
-    /* borrowed reference, needs to be incref'd if we keep it */
-    self->facemask_top = PyTuple_GetItem(facemasks_py, 0);
-    Py_INCREF(self->facemask_top);
-    Py_DECREF(facemasks_py);
-    
-    self->white_color = PyObject_GetAttrString(state->support, "white_color");
-    self->get_color = get_color;
-    
-    return 0;
-}
-
-static void
-rendermode_overlay_finish(void *data, RenderState *state) {
-    RenderModeOverlay *self = (RenderModeOverlay *)data;
-    
-    Py_DECREF(self->facemask_top);
-    Py_DECREF(self->white_color);
-}
-
-static int
-rendermode_overlay_occluded(void *data, RenderState *state, int x, int y, int z) {
-    if ( (x != 0) && (y != 15) && (z != 127) &&
-         !render_mode_hidden(state->rendermode, x-1, y, z) &&
-         !render_mode_hidden(state->rendermode, x, y, z+1) &&
-         !render_mode_hidden(state->rendermode, x, y+1, z) &&
-         !is_transparent(getArrayByte3D(state->blocks, x-1, y, z)) &&
-         !is_transparent(getArrayByte3D(state->blocks, x, y, z+1)) &&
-         !is_transparent(getArrayByte3D(state->blocks, x, y+1, z))) {
-        return 1;
-    }
-    
-    return 0;
-}
-
-static int
-rendermode_overlay_hidden(void *data, RenderState *state, int x, int y, int z) {
-    /* overlays hide nothing by default */
-    return 0;
-}
-
-static void
-rendermode_overlay_draw(void *data, RenderState *state, PyObject *src, PyObject *mask, PyObject *mask_light) {
-    RenderModeOverlay *self = (RenderModeOverlay *)data;
-    unsigned char r, g, b, a;
-
-    // exactly analogous to edge-line code for these special blocks
-    int increment=0;
-    if (state->block == 44)  // half-step
-        increment=6;
-    else if (state->block == 78) // snow
-        increment=9;
-    
-    /* clear the draw space -- set alpha to 0 within mask */
-    tint_with_mask(state->img, 255, 255, 255, 0, mask, state->imgx, state->imgy, 0, 0);
-
-    /* skip rendering the overlay if we can't see it */
-    if (state->z != 127) {
-        unsigned char top_block = getArrayByte3D(state->blocks, state->x, state->y, state->z+1);
-        if (!is_transparent(top_block)) {
-            return;
-        }
-        
-        /* check to be sure this block is solid/fluid */
-        if (block_has_property(top_block, SOLID) || block_has_property(top_block, FLUID)) {
-            
-            /* top block is fluid or solid, skip drawing */
-            return;
-        }
-    }
-    
-    /* check to be sure this block is solid/fluid */
-    if (!block_has_property(state->block, SOLID) && !block_has_property(state->block, FLUID)) {
-        
-        /* not fluid or solid, skip drawing the overlay */
-        return;
-    }
-
-    /* get our color info */
-    self->get_color(data, state, &r, &g, &b, &a);
-    
-    /* do the overlay */
-    if (a > 0) {
-        alpha_over(state->img, self->white_color, self->facemask_top, state->imgx, state->imgy + increment, 0, 0);
-        tint_with_mask(state->img, r, g, b, a, self->facemask_top, state->imgx, state->imgy + increment, 0, 0);
-    }
-}
-
-RenderModeInterface rendermode_overlay = {
-    "overlay", "Overlay",
-    "base rendermode for informational overlays",
-    NULL,
-    NULL,
-    sizeof(RenderModeOverlay),
-    rendermode_overlay_start,
-    rendermode_overlay_finish,
-    rendermode_overlay_occluded,
-    rendermode_overlay_hidden,
-    rendermode_overlay_draw,
-};
diff --git a/overviewer_core/src/rendermode-spawn.c b/overviewer_core/src/rendermode-spawn.c
deleted file mode 100644
index 0c8027b..0000000
--- a/overviewer_core/src/rendermode-spawn.c
+++ /dev/null
@@ -1,122 +0,0 @@
-/*
- * This file is part of the Minecraft Overviewer.
- *
- * Minecraft Overviewer is free software: you can redistribute it and/or
- * modify it under the terms of the GNU General Public License as published
- * by the Free Software Foundation, either version 3 of the License, or (at
- * your option) any later version.
- *
- * Minecraft Overviewer is distributed in the hope that it will be useful,
- * but WITHOUT ANY WARRANTY; without even the implied warranty of
- * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General
- * Public License for more details.
- *
- * You should have received a copy of the GNU General Public License along
- * with the Overviewer.  If not, see <http://www.gnu.org/licenses/>.
- */
-
-#include "overviewer.h"
-#include <math.h>
-
-static void get_color(void *data, RenderState *state,
-                      unsigned char *r, unsigned char *g, unsigned char *b, unsigned char *a) {
-    
-    RenderModeSpawn* self = (RenderModeSpawn *)data;
-    int x = state->x, y = state->y, z = state->z;
-    int z_light = z + 1;
-    unsigned char blocklight, skylight;
-    PyObject *block_py;
-    
-    /* set a nice, pretty red color */
-    *r = 229;
-    *g = 36;
-    *b = 38;
-    
-    /* default to no overlay, until told otherwise */
-    *a = 0;
-    
-    if (block_has_property(state->block, NOSPAWN)) {
-        /* nothing can spawn on this */
-        return;
-    }
-    
-    blocklight = getArrayByte3D(self->blocklight, x, y, MIN(127, z_light));
-    
-    /* if we're at the top, force 15 (brightest!) skylight */
-    if (z_light == 128) {
-        skylight = 15;
-    } else {
-        skylight = getArrayByte3D(self->skylight, x, y, z_light);
-    }
-    
-    if (MAX(blocklight, skylight) <= 7) {
-        /* hostile mobs spawn in daylight */
-        *a = 240;
-    } else if (MAX(blocklight, skylight - 11) <= 7) {
-        /* hostile mobs spawn at night */
-        *a = 150;
-    }
-}
-
-static int
-rendermode_spawn_start(void *data, RenderState *state, PyObject *options) {
-    RenderModeSpawn* self;
-
-    /* first, chain up */
-    int ret = rendermode_overlay.start(data, state, options);
-    if (ret != 0)
-        return ret;
-    
-    /* now do custom initializations */
-    self = (RenderModeSpawn *)data;
-    self->blocklight = get_chunk_data(state, CURRENT, BLOCKLIGHT, 1);
-    self->skylight = get_chunk_data(state, CURRENT, SKYLIGHT, 1);
-    
-    /* setup custom color */
-    self->parent.get_color = get_color;
-    
-    return 0;
-}
-
-static void
-rendermode_spawn_finish(void *data, RenderState *state) {
-    /* first free all *our* stuff */
-    RenderModeSpawn* self = (RenderModeSpawn *)data;
-    
-    Py_DECREF(self->blocklight);
-    Py_DECREF(self->skylight);
-    
-    /* now, chain up */
-    rendermode_overlay.finish(data, state);
-}
-
-static int
-rendermode_spawn_occluded(void *data, RenderState *state, int x, int y, int z) {
-    /* no special occlusion here */
-    return rendermode_overlay.occluded(data, state, x, y, z);
-}
-
-static int
-rendermode_spawn_hidden(void *data, RenderState *state, int x, int y, int z) {
-    /* no special hiding here */
-    return rendermode_overlay.hidden(data, state, x, y, z);
-}
-
-static void
-rendermode_spawn_draw(void *data, RenderState *state, PyObject *src, PyObject *mask, PyObject *mask_light) {
-    /* draw normally */
-    rendermode_overlay.draw(data, state, src, mask, mask_light);
-}
-
-RenderModeInterface rendermode_spawn = {
-    "spawn", "Spawn",
-    "draws a red overlay where monsters can spawn at night",
-    NULL,
-    &rendermode_overlay,
-    sizeof(RenderModeSpawn),
-    rendermode_spawn_start,
-    rendermode_spawn_finish,
-    rendermode_spawn_occluded,
-    rendermode_spawn_hidden,
-    rendermode_spawn_draw,
-};
diff --git a/overviewer_core/src/rendermodes.h b/overviewer_core/src/rendermodes.h
index 23df5e6..6408760 100644
--- a/overviewer_core/src/rendermodes.h
+++ b/overviewer_core/src/rendermodes.h
@@ -102,38 +102,4 @@ void render_mode_draw(RenderMode *self, PyObject *img, PyObject *mask, PyObject
    works like PyArg_ParseTuple on a support object */
 int render_mode_parse_option(PyObject *support, const char *name, const char *format, ...);
 
-/* XXX individual rendermode interface declarations follow */
-#ifdef OLD_MODES
-
-/* OVERLAY */
-typedef struct {
-    /* top facemask and white color image, for drawing overlays */
-    PyObject *facemask_top, *white_color;
-    /* can be overridden in derived classes to control
-       overlay alpha and color
-       last four vars are r, g, b, a out */
-    void (*get_color)(void *, RenderState *,
-                      unsigned char *, unsigned char *, unsigned char *, unsigned char *);
-} RenderModeOverlay;
-extern RenderModeInterface rendermode_overlay;
-
-/* SPAWN */
-typedef struct {
-    /* inherits from overlay */
-    RenderModeOverlay parent;
-    
-    PyObject *skylight, *blocklight;
-} RenderModeSpawn;
-extern RenderModeInterface rendermode_spawn;
-
-/* MINERAL */
-typedef struct {
-    /* inherits from overlay */
-    RenderModeOverlay parent;
-    
-    void *minerals;
-} RenderModeMineral;
-extern RenderModeInterface rendermode_mineral;
-#endif /* OLD_MODES */
-
 #endif /* __RENDERMODES_H_INCLUDED__ */
diff --git a/overviewer_core/tileset.py b/overviewer_core/tileset.py
index 4a45a08..3bfd35d 100644
--- a/overviewer_core/tileset.py
+++ b/overviewer_core/tileset.py
@@ -29,6 +29,7 @@ from itertools import product, izip
 from PIL import Image
 
 from .util import iterate_base4, convert_coords, unconvert_coords, roundrobin
+from .util import FileReplacer
 from .optimizeimages import optimize_image
 import c_overviewer
 
@@ -189,7 +190,9 @@ class TileSet(object):
 
         renderchecks
             An integer indicating how to determine which tiles need updating
-            and which don't. This is one of three levels:
+            and which don't. This key is optional; if not specified, an
+            appropriate mode is determined from the persistent config obtained
+            from the asset manager. This is one of three levels:
 
             0
                 Only render tiles that have chunks with a greater mtime than
@@ -258,8 +261,38 @@ class TileSet(object):
         self.am = assetmanagerobj
         self.textures = texturesobj
 
-        self.last_rendertime = self.am.get_tileset_config(self.options.get("name")).get('last_rendertime', 0)
-        self.this_rendertime = time.time()
+        config = self.am.get_tileset_config(self.options.get("name"))
+        self.config = config
+
+        self.last_rendertime = config.get('last_rendertime', 0)
+
+        if "renderchecks" not in self.options:
+            if not config:
+                # No persistent config? This is a full render then.
+                self.options['renderchecks'] = 2
+                logging.debug("This is the first time rendering %s. Doing" +
+                        " a full-render",
+                        self.options['name'])
+            elif config.get("render_in_progress", False):
+                # The last render must have been interrupted. The default should be
+                # 1 then, not 0
+                logging.warning(
+                        "The last render for %s didn't finish. I'll be " +
+                        "scanning all the tiles to make sure everything's up "+
+                        "to date.",
+                        self.options['name'],
+                        )
+                logging.warning("You won't get percentage progress for "+
+                        "this run only, because I don't know how many tiles "+
+                        "need rendering. I'll be checking them as I go")
+                self.options['renderchecks'] = 1
+            else:
+                logging.debug("No rendercheck mode specified for %s. "+
+                        "Rendering tile whose chunks have changed since %s",
+                        self.options['name'],
+                        time.strftime("%x %X", time.localtime(self.last_rendertime)),
+                        )
+                self.options['renderchecks'] = 0
 
         # Throughout the class, self.outputdir is an absolute path to the
         # directory where we output tiles. It is assumed to exist.
@@ -298,8 +331,11 @@ class TileSet(object):
             logging.warning("Just letting you know, your map requries %s zoom levels. This is REALLY big!",
                     self.treedepth)
 
-        # Do any tile re-arranging if necessary
-        self._rearrange_tiles()
+        # Do any tile re-arranging if necessary. Skip if there was no config
+        # from the asset-manager, which typically indicates this is a new
+        # render
+        if self.config:
+            self._rearrange_tiles()
 
         # Do the chunk scan here
         self.dirtytree = self._chunk_scan()
@@ -383,6 +419,20 @@ class TileSet(object):
                 name = str(tilepath[-1])
             self._render_compositetile(dest, name)
 
+    def get_initial_data(self):
+        """This is called similarly to get_persistent_data, but is called after
+        do_preprocessing but before any work is acutally done.
+
+        """
+        d = self.get_persistent_data()
+        # This is basically the same as get_persistent_data() with the
+        # following exceptions:
+        # * last_rendertime is not changed
+        # * A key "render_in_progress" is set to True
+        d['last_rendertime'] = self.last_rendertime
+        d['render_in_progress'] = True
+        return d
+
     def get_persistent_data(self):
         """Returns a dictionary representing the persistent data of this
         TileSet. Typically this is called by AssetManager
@@ -400,7 +450,7 @@ class TileSet(object):
                 bgcolor = bgcolorformat(self.options.get('bgcolor')),
                 world = self.options.get('worldname_orig') + 
                     (" - " + self.options.get('dimension') if self.options.get('dimension') != 'default' else ''),
-                last_rendertime = self.this_rendertime,
+                last_rendertime = self.max_chunk_mtime,
                 imgextension = self.imgextension,
                 )
         try:
@@ -463,16 +513,17 @@ class TileSet(object):
 
         """
         try:
-            curdepth = get_dirdepth(self.outputdir)
-        except Exception:
-            logging.critical("Could not determine existing tile tree depth. Does it exist?")
-            raise
+            curdepth = self.config['zoomLevels']
+        except KeyError:
+            return
         
         if curdepth == 1:
             # Skip a depth 1 tree. A depth 1 tree pretty much can't happen, so
             # when we detect this it usually means the tree is actually empty
             return
-        logging.debug("Current tree depth was detected to be %s. Target tree depth is %s", curdepth, self.treedepth)
+        logging.debug("Current tree depth for %s is reportedly %s. Target tree depth is %s",
+                self.options['name'],
+                curdepth, self.treedepth)
         if self.treedepth != curdepth:
             if self.treedepth > curdepth:
                 logging.warning("Your map seems to have expanded beyond its previous bounds.")
@@ -648,8 +699,8 @@ class TileSet(object):
                     dirty.add(tile.path)
 
         t = int(time.time()-stime)
-        logging.debug("%s finished chunk scan. %s chunks scanned in %s second%s", 
-                self, chunkcount, t,
+        logging.debug("Finished chunk scan for %s. %s chunks scanned in %s second%s", 
+                self.options['name'], chunkcount, t,
                 "s" if t != 1 else "")
 
         self.max_chunk_mtime = max_chunk_mtime
@@ -728,22 +779,23 @@ class TileSet(object):
                 img.paste(quad, path[0])
             except Exception, e:
                 logging.warning("Couldn't open %s. It may be corrupt. Error was '%s'", path[1], e)
-                logging.warning("I'm going to try and delete it. You will need to run the render again")
+                logging.warning("I'm going to try and delete it. You will need to run the render again and with --check-tiles")
                 try:
                     os.unlink(path[1])
                 except Exception, e:
                     logging.error("While attempting to delete corrupt image %s, an error was encountered. You will need to delete it yourself. Error was '%s'", path[1], e)
 
         # Save it
-        if imgformat == 'jpg':
-            img.save(imgpath, quality=self.options['imgquality'], subsampling=0)
-        else: # png
-            img.save(imgpath)
-            
-        if self.options['optimizeimg']:
-            optimize_image(imgpath, imgformat, self.options['optimizeimg'])
+        with FileReplacer(imgpath) as tmppath:
+            if imgformat == 'jpg':
+                img.save(tmppath, "jpeg", quality=self.options['imgquality'], subsampling=0)
+            else: # png
+                img.save(tmppath, "png")
+                
+            if self.options['optimizeimg']:
+                optimize_image(tmppath, imgformat, self.options['optimizeimg'])
 
-        os.utime(imgpath, (max_mtime, max_mtime))
+            os.utime(tmppath, (max_mtime, max_mtime))
 
     def _render_rendertile(self, tile):
         """Renders the given render-tile.
@@ -829,15 +881,16 @@ class TileSet(object):
             #draw.text((96,96), "c,r: %s,%s" % (col, row), fill='red')
         
         # Save them
-        if self.imgextension == 'jpg':
-            tileimg.save(imgpath, quality=self.options['imgquality'], subsampling=0)
-        else: # png
-            tileimg.save(imgpath)
+        with FileReplacer(imgpath) as tmppath:
+            if self.imgextension == 'jpg':
+                tileimg.save(tmppath, "jpeg", quality=self.options['imgquality'], subsampling=0)
+            else: # png
+                tileimg.save(tmppath, "png")
 
-        if self.options['optimizeimg']:
-            optimize_image(imgpath, self.imgextension, self.options['optimizeimg'])
+            if self.options['optimizeimg']:
+                optimize_image(tmppath, self.imgextension, self.options['optimizeimg'])
 
-        os.utime(imgpath, (max_chunk_mtime, max_chunk_mtime))
+            os.utime(tmppath, (max_chunk_mtime, max_chunk_mtime))
 
     def _iterate_and_check_tiles(self, path):
         """A generator function over all tiles that need rendering in the
@@ -946,34 +999,6 @@ class TileSet(object):
                     # Nope.
                     yield path, max_child_mtime, False
 
-
-def get_dirdepth(outputdir):
-    """Returns the current depth of the tree on disk
-
-    """
-    # Traverses down the first directory until it reaches one with no
-    # subdirectories. While all paths of the tree may not exist, all paths
-    # of the tree should and are assumed to be the same depth
-
-    # This function returns a list of all subdirectories of the given
-    # directory. It's slightly more complicated than you'd think it should be
-    # because one must turn directory names returned by os.listdir into
-    # relative/absolute paths before they can be passed to os.path.isdir()
-    getsubdirs = lambda directory: [
-            abssubdir
-            for abssubdir in
-                (os.path.join(directory,subdir) for subdir in os.listdir(directory))
-            if os.path.isdir(abssubdir)
-            ]
-
-    depth = 1
-    subdirs = getsubdirs(outputdir)
-    while subdirs:
-        subdirs = getsubdirs(subdirs[0])
-        depth += 1
-
-    return depth
-
 ######################
 # The following two functions define the mapping from chunks to tiles and back.
 # The mapping from chunks to tiles (get_tiles_by_chunk()) is used during the
diff --git a/overviewer_core/util.py b/overviewer_core/util.py
index 3661b2e..ded3f11 100644
--- a/overviewer_core/util.py
+++ b/overviewer_core/util.py
@@ -152,6 +152,74 @@ def unconvert_coords(col, row):
     # col - row = chunkx + chunkx => (col - row)/2 = chunkx
     return ((col - row) / 2, (col + row) / 2)
 
+# Define a context manager to handle atomic renaming or "just forget it write
+# straight to the file" depending on whether os.rename provides atomic
+# overwrites.
+# Detect whether os.rename will overwrite files
+import tempfile
+with tempfile.NamedTemporaryFile() as f1:
+    with tempfile.NamedTemporaryFile() as f2:
+        try:
+            os.rename(f1.name,f2.name)
+        except OSError:
+            renameworks = False
+        else:
+            renameworks = True
+            # re-make this file so it can be deleted without error
+            open(f1.name, 'w').close()
+del tempfile,f1,f2
+doc = """This class acts as a context manager for files that are to be written
+out overwriting an existing file.
+
+The parameter is the destination filename. The value returned into the context
+is the filename that should be used. On systems that support an atomic
+os.rename(), the filename will actually be a temporary file, and it will be
+atomically replaced over the destination file on exit.
+
+On systems that don't support an atomic rename, the filename returned is the
+filename given.
+
+If an error is encountered, the file is attempted to be removed, and the error
+is propagated.
+
+Example:
+
+with FileReplacer("config") as configname:
+    with open(configout, 'w') as configout:
+        configout.write(newconfig)
+"""
+if renameworks:
+    class FileReplacer(object):
+        __doc__ = doc
+        def __init__(self, destname):
+            self.destname = destname
+            self.tmpname = destname + ".tmp"
+        def __enter__(self):
+            # rename works here. Return a temporary filename
+            return self.tmpname
+        def __exit__(self, exc_type, exc_val, exc_tb):
+            if exc_type:
+                # error
+                try:
+                    os.remove(self.tmpname)
+                except Exception, e:
+                    logging.warning("An error was raised, so I was doing "
+                            "some cleanup first, but I couldn't remove "
+                            "'%s'!", self.tmpname)
+            else:
+                # atomic rename into place
+                os.rename(self.tmpname, self.destname)
+else:
+    class FileReplacer(object):
+        __doc__ = doc
+        def __init__(self, destname):
+            self.destname = destname
+        def __enter__(self):
+            return self.destname
+        def __exit__(self, exc_type, exc_val, exc_tb):
+            return
+del renameworks
+
 # Logging related classes are below
 
 # Some cool code for colored logging:
diff --git a/overviewer_core/world.py b/overviewer_core/world.py
index 1d50983..2ab2480 100644
--- a/overviewer_core/world.py
+++ b/overviewer_core/world.py
@@ -34,10 +34,10 @@ class ChunkDoesntExist(Exception):
 
 def log_other_exceptions(func):
     """A decorator that prints out any errors that are not
-    ChunkDoesntExist errors. This decorates get_chunk because the C
-    code is likely to swallow exceptions, so this will at least make
-    them visible.
-
+    ChunkDoesntExist errors. This should decorate any functions or
+    methods called by the C code, such as get_chunk(), because the C
+    code is likely to swallow exceptions. This will at least make them
+    visible.
     """
     functools.wraps(func)
     def newfunc(*args):
@@ -359,9 +359,6 @@ class RegionSet(object):
         return chunk_data      
     
 
-    def rotate(self, north_direction):
-        return RotatedRegionSet(self.regiondir, north_direction)
-    
     def iterate_chunks(self):
         """Returns an iterator over all chunk metadata in this world. Iterates
         over tuples of integers (x,z,mtime) for each chunk.  Other chunk data
@@ -412,6 +409,33 @@ class RegionSet(object):
             x = int(p[1])
             y = int(p[2])
             yield (x, y, path)
+
+class RegionSetWrapper(object):
+    """This is the base class for all "wrappers" of RegionSet objects. A
+    wrapper is an object that acts similarly to a subclass: some methods are
+    overridden and functionality is changed, others may not be. The difference
+    here is that these wrappers may wrap each other, forming chains.
+
+    In fact, subclasses of this object may act exactly as if they've subclassed
+    the original RegionSet object, except the first parameter of the
+    constructor is a regionset object, not a regiondir.
+
+    This class must implement the full public interface of RegionSet objects
+
+    """
+    def __init__(self, rsetobj):
+        self._r = rsetobj
+
+    def get_type(self):
+        return self._r.get_type()
+    def get_biome_data(self, x, z):
+        return self._r.get_biome_data(x,z)
+    def get_chunk(self, x, z):
+        return self._r.get_chunk(x,z)
+    def iterate_chunks(self):
+        return self._r.iterate_chunks()
+    def get_chunk_mtime(self, x, z):
+        return self._r.get_chunk_mtime(x,z)
     
 # see RegionSet.rotate.  These values are chosen so that they can be
 # passed directly to rot90; this means that they're the number of
@@ -421,7 +445,7 @@ UPPER_RIGHT = 1 ## - Return the world such that north is down the +X axis (rotat
 LOWER_RIGHT = 2 ## - Return the world such that north is down the +Z axis (rotate 180 degrees)
 LOWER_LEFT  = 3 ## - Return the world such that north is down the -X axis (rotate 90 degrees clockwise)
 
-class RotatedRegionSet(RegionSet):
+class RotatedRegionSet(RegionSetWrapper):
     """A regionset, only rotated such that north points in the given direction
 
     """
@@ -433,39 +457,41 @@ class RotatedRegionSet(RegionSet):
     _ROTATE_180 =                lambda x,z: (-x,-z)
     
     # These take rotated coords and translate into un-rotated coords
-    _unrotation_funcs = {
-        0: _NO_ROTATION,
-        1: _ROTATE_COUNTERCLOCKWISE,
-        2: _ROTATE_180,
-        3: _ROTATE_CLOCKWISE,
-    }
+    _unrotation_funcs = [
+        _NO_ROTATION,
+        _ROTATE_COUNTERCLOCKWISE,
+        _ROTATE_180,
+        _ROTATE_CLOCKWISE,
+    ]
     
     # These translate un-rotated coordinates into rotated coordinates
-    _rotation_funcs = {
-        0: _NO_ROTATION,
-        1: _ROTATE_CLOCKWISE,
-        2: _ROTATE_180,
-        3: _ROTATE_COUNTERCLOCKWISE,
-    }
+    _rotation_funcs = [
+        _NO_ROTATION,
+        _ROTATE_CLOCKWISE,
+        _ROTATE_180,
+        _ROTATE_COUNTERCLOCKWISE,
+    ]
     
-    def __init__(self, regiondir, north_dir):
+    def __init__(self, rsetobj, north_dir):
         self.north_dir = north_dir
         self.unrotate = self._unrotation_funcs[north_dir]
         self.rotate = self._rotation_funcs[north_dir]
 
-        super(RotatedRegionSet, self).__init__(regiondir)
+        super(RotatedRegionSet, self).__init__(rsetobj)
 
     
-    # Re-initialize upon unpickling
+    # Re-initialize upon unpickling. This is needed because we store a couple
+    # lambda functions as instance variables
     def __getstate__(self):
-        return (self.regiondir, self.north_dir)
+        return (self._r, self.north_dir)
     def __setstate__(self, args):
         self.__init__(args[0], args[1])
     
     def get_chunk(self, x, z):
         x,z = self.unrotate(x,z)
-        chunk_data = super(RotatedRegionSet, self).get_chunk(x,z)
+        chunk_data = dict(super(RotatedRegionSet, self).get_chunk(x,z))
         for section in chunk_data['Sections']:
+            section = dict(section)
             for arrayname in ['Blocks', 'Data', 'SkyLight', 'BlockLight']:
                 array = section[arrayname]
                 # Since the anvil change, arrays are arranged with axes Y,Z,X