diff -r 4049803a71c9 src/gfx.cpp
--- a/src/gfx.cpp	Fri Mar 09 18:45:07 2012 +0000
+++ b/src/gfx.cpp	Tue Mar 13 12:43:13 2012 -0600
@@ -1712,6 +1712,7 @@
  *
  * @see SetDirtyBlocks
  */
+extern uint dirtyblock_colour;
 void DrawDirtyBlocks()
 {
 	byte *b = _dirty_blocks;
@@ -1801,6 +1802,7 @@
 		} while (b++, (x += DIRTY_BLOCK_WIDTH) != w);
 	} while (b += -(int)(w / DIRTY_BLOCK_WIDTH) + _dirty_bytes_per_line, (y += DIRTY_BLOCK_HEIGHT) != h);
 
+	++dirtyblock_colour;
 	_invalid_rect.left = w;
 	_invalid_rect.top = h;
 	_invalid_rect.right = 0;
diff -r 4049803a71c9 src/viewport.cpp
--- a/src/viewport.cpp	Fri Mar 09 18:45:07 2012 +0000
+++ b/src/viewport.cpp	Tue Mar 13 12:43:13 2012 -0600
@@ -1344,6 +1344,10 @@
  * Draws the bounding boxes of all ParentSprites
  * @param psd Array of ParentSprites
  */
+#include "table/palettes.h"
+static void ViewportDrawDirtyBoxes();
+uint dirtyblock_colour = 0;
+
 static void ViewportDrawBoundingBoxes(const ParentSpriteToSortVector *psd)
 {
 	const ParentSpriteToDraw * const *psd_end = psd->End();
@@ -1361,6 +1365,26 @@
 	}
 }
 
+static void ViewportDrawDirtyBoxes()
+{
+	Blitter *blitter = BlitterFactoryBase::GetCurrentBlitter();
+	const DrawPixelInfo *dpi = _cur_dpi;
+	void *dst;
+	int right = dpi->width >> dpi->zoom;
+	int bottom = dpi->height >> dpi->zoom;
+
+	dirtyblock_colour %= 16;
+	int colour = _string_colourmap[dirtyblock_colour];
+
+	dst = dpi->dst_ptr;
+
+	byte bo = ((dpi->left + dpi->top) >> dpi->zoom) & 1;
+	do {
+		for (int i = (bo ^= 1); i < right; i += 2) blitter->SetPixel(dst, i, 0, (uint8)colour);
+		dst = blitter->MoveTo(dst, 0, 1);
+	} while (--bottom > 0);
+}
+
 static void ViewportDrawStrings(DrawPixelInfo *dpi, const StringSpriteToDrawVector *sstdv)
 {
 	DrawPixelInfo dp;
@@ -1457,6 +1481,7 @@
 	ViewportDrawParentSprites(&_vd.parent_sprites_to_sort, &_vd.child_screen_sprites_to_draw);
 
 	if (_draw_bounding_boxes) ViewportDrawBoundingBoxes(&_vd.parent_sprites_to_sort);
+	if (_draw_bounding_boxes) ViewportDrawDirtyBoxes();
 
 	if (_vd.string_sprites_to_draw.Length() != 0) ViewportDrawStrings(&_vd.dpi, &_vd.string_sprites_to_draw);