diff --git a/src/composite.c b/src/composite.c
index b7454c8..3400453 100644
--- a/src/composite.c
+++ b/src/composite.c
@@ -59,33 +59,42 @@ imaging_python_to_c(PyObject *obj)
 }
 
 /**
- * img should be an Image object, type 'L'
+ * img should be an Image object, type 'L' or 'RGBA'
+ * for RGBA images, operates on the alpha only
  * factor should be between 0 and 1, inclusive
  */
-PyObject *brightness(PyObject *img, double factor) {
+PyObject *brightness(PyObject *img, float factor) {
     Imaging imDest;
 
     imDest = imaging_python_to_c(img);
-    assert(imDest);
+    if (!imDest)
+        return NULL;
 
+    if (strcmp(imDest->mode, "RGBA") != 0 && strcmp(imDest->mode, "L") != 0) {
+        PyErr_SetString(PyExc_ValueError,
+                        "given image does not have mode \"RGBA\" or \"L\"");
+        return NULL;
+    }
+    
+    /* how far into image the first alpha byte resides */
+    int offset = (imDest->pixelsize == 4 ? 3 : 0);
+    /* how many bytes to skip to get to the next alpha byte */
+    int stride = imDest->pixelsize;
+    
     int x, y;
 
     int xsize = imDest->xsize;
     int ysize = imDest->ysize;
 
     for (y = 0; y < ysize; y++) {
-        UINT8 *out = (UINT8 *)imDest->image[y];
-        //UINT8 *outmask = (UINT8 *)imDest->image[y] + 3;
+        UINT8 *out = (UINT8 *)imDest->image[y] + offset;
 
         for (x = 0; x < xsize; x++) {
-            //printf("old out: %d\n", *out);
             *out *= factor;
-            //printf("new out: %d\n", *out);
-            out++;
+            out += stride;
         }
     }
     return NULL;
-
 }
 
 /* the alpha_over function, in a form that can be called from C */
diff --git a/src/overviewer.h b/src/overviewer.h
index 5ae9079..d704a83 100644
--- a/src/overviewer.h
+++ b/src/overviewer.h
@@ -41,7 +41,7 @@ Imaging imaging_python_to_c(PyObject *obj);
 PyObject *alpha_over(PyObject *dest, PyObject *src, PyObject *mask, int dx,
                      int dy, int xsize, int ysize);
 PyObject *alpha_over_wrap(PyObject *self, PyObject *args);
-PyObject *brightness(PyObject *img, double factor);
+PyObject *brightness(PyObject *img, float factor);
 
 /* in iterate.c */
 typedef struct {
diff --git a/src/rendermode-lighting.c b/src/rendermode-lighting.c
index e9b5aae..10d5bcd 100644
--- a/src/rendermode-lighting.c
+++ b/src/rendermode-lighting.c
@@ -133,8 +133,8 @@ get_lighting_coefficient(RenderModeLighting *self, RenderState *state,
 /* shades the drawn block with the given facemask/black_color, based on the
    lighting results from (x, y, z) */
 static inline void
-do_shading_for_face(RenderModeLighting *self, RenderState *state,
-                    int x, int y, int z, PyObject *facemask) {
+do_shading_with_mask(RenderModeLighting *self, RenderState *state,
+                     int x, int y, int z, PyObject *facemask) {
     /* first, check for occlusion if the block is in the local chunk */
     if (x >= 0 && x < 16 && y >= 0 && y < 16 && z >= 0 && z < 128) {
         unsigned char block = getArrayByte3D(state->blocks, x, y, z);
@@ -220,10 +220,15 @@ rendermode_lighting_draw(void *data, RenderState *state, PyObject *src, PyObject
     RenderModeLighting* self = (RenderModeLighting *)data;
     int x = state->x, y = state->y, z = state->z;
     
-    // TODO whole-block shading for transparent blocks
-    do_shading_for_face(self, state, x, y, z+1, self->facemasks[0]);
-    do_shading_for_face(self, state, x-1, y, z, self->facemasks[1]);
-    do_shading_for_face(self, state, x, y+1, z, self->facemasks[2]);
+    if (is_transparent(state->block)) {
+        /* transparent: do shading on whole block */
+        do_shading_with_mask(self, state, x, y, z, mask);
+    } else {
+        /* opaque: do per-face shading */
+        do_shading_with_mask(self, state, x, y, z+1, self->facemasks[0]);
+        do_shading_with_mask(self, state, x-1, y, z, self->facemasks[1]);
+        do_shading_with_mask(self, state, x, y+1, z, self->facemasks[2]);
+    }
 }
 
 RenderModeInterface rendermode_lighting = {