SCI: Implementation of kCelInfo subop 4

kCelInfo subop 4 returns the pixel color at the
passed in x,y coordinates for the passed in view,
loop, cel.  Shivers uses this function for the
red door puzzle, room 23601 to determine what
blocks on the puzzle board are already occupied
by pieces.

engines/sci/engine/kgraphics.cpp

@@ -1471,10 +1471,8 @@ reg_t kSetShowStyle(EngineState *s, int argc, reg_t *argv) {
 }
 
 reg_t kCelInfo(EngineState *s, int argc, reg_t *argv) {
-	// TODO: This is all a stub/skeleton, thus we're invoking kStub() for now
-	kStub(s, argc, argv);
-
-	// Used by Shivers 1, room 23601
+	// Used by Shivers 1, room 23601 to determine what blocks on the red door puzzle board
+	// are occupied by pieces already
 
 	// 6 arguments, all integers:
 	// argv[0] - subop (0 - 4). It's constantly called with 4 in Shivers 1
@@ -1490,7 +1488,21 @@ reg_t kCelInfo(EngineState *s, int argc, reg_t *argv) {
 	// 2, 3 - nop
 	// 4 - return value of pixel at x, y
 
-	return s->r_acc;
+	switch (argv[0].toUint16()) {
+		case 4: {
+			GuiResourceId viewId = argv[1].toSint16();
+			int16 loopNo = argv[2].toSint16();
+			int16 celNo = argv[3].toSint16();
+			int16 x = argv[4].toUint16();
+			int16 y = argv[5].toUint16();
+			byte color = g_sci->_gfxCache->kernelViewGetColorAtCoordinate(viewId, loopNo, celNo, x, y);
+			return make_reg(0, color);
+		}
+		default: {
+			kStub(s, argc, argv);
+			return s->r_acc;
+		}
+	}
 }
 
 reg_t kScrollWindow(EngineState *s, int argc, reg_t *argv) {

engines/sci/graphics/cache.cpp

@@ -102,4 +102,8 @@ int16 GfxCache::kernelViewGetCelCount(GuiResourceId viewId, int16 loopNo) {
 	return getView(viewId)->getCelCount(loopNo);
 }
 
+byte GfxCache::kernelViewGetColorAtCoordinate(GuiResourceId viewId, int16 loopNo, int16 celNo, int16 x, int16 y) {
+	return getView(viewId)->getColorAtCoordinate(loopNo, celNo, x, y);
+}
+
 } // End of namespace Sci

engines/sci/graphics/cache.h

@@ -49,6 +49,8 @@ class GfxCache {
 	int16 kernelViewGetLoopCount(GuiResourceId viewId);
 	int16 kernelViewGetCelCount(GuiResourceId viewId, int16 loopNo);
 
+	byte kernelViewGetColorAtCoordinate(GuiResourceId viewId, int16 loopNo, int16 celNo, int16 x, int16 y);
+
 private:
 	void purgeFontCache();
 	void purgeViewCache();

engines/sci/graphics/view.cpp

@@ -837,4 +837,13 @@ void GfxView::adjustBackUpscaledCoordinates(int16 &y, int16 &x) {
 	_screen->adjustBackUpscaledCoordinates(y, x, _sci2ScaleRes);
 }
 
+byte GfxView::getColorAtCoordinate(int16 loopNo, int16 celNo, int16 x, int16 y) {
+	const CelInfo *celInfo = getCelInfo(loopNo, celNo);
+	const byte *bitmap = getBitmap(loopNo, celNo);
+	const int16 celWidth = celInfo->width;
+
+	bitmap += (celWidth * y);
+	return bitmap[x];
+}
+
 } // End of namespace Sci

engines/sci/graphics/view.h

@@ -85,6 +85,8 @@ class GfxView {
 	void adjustToUpscaledCoordinates(int16 &y, int16 &x);
 	void adjustBackUpscaledCoordinates(int16 &y, int16 &x);
 
+	byte getColorAtCoordinate(int16 loopNo, int16 celNo, int16 x, int16 y);
+
 private:
 	void initData(GuiResourceId resourceId);
 	void unpackCel(int16 loopNo, int16 celNo, byte *outPtr, uint32 pixelCount);