GIF: Support transparency in the native decoder.

Allow the transparency index to be passed to the native decoder.  If not
-1, pixels with this index will be left at their previous value.

This only adds the decoder support and isn't active yet.

src/PIL/GifImagePlugin.py

@@ -247,7 +247,7 @@ class GifImageFile(ImageFile.ImageFile):
                 self.tile = [("gif",
                              (x0, y0, x1, y1),
                              self.__offset,
-                             (bits, interlace))]
+                             (bits, interlace, -1))]
                 break
 
             else:

src/decode.c

@@ -435,7 +435,8 @@ PyImaging_GifDecoderNew(PyObject* self, PyObject* args)
     char* mode;
     int bits = 8;
     int interlace = 0;
-    if (!PyArg_ParseTuple(args, "s|ii", &mode, &bits, &interlace))
+    int transparency = -1;
+    if (!PyArg_ParseTuple(args, "s|iii", &mode, &bits, &interlace, &transparency))
         return NULL;
 
     if (strcmp(mode, "L") != 0 && strcmp(mode, "P") != 0) {
@@ -451,6 +452,7 @@ PyImaging_GifDecoderNew(PyObject* self, PyObject* args)
 
     ((GIFDECODERSTATE*)decoder->state.context)->bits = bits;
     ((GIFDECODERSTATE*)decoder->state.context)->interlace = interlace;
+    ((GIFDECODERSTATE*)decoder->state.context)->transparency = transparency;
 
     return (PyObject*) decoder;
 }

src/libImaging/Gif.h

@@ -33,6 +33,9 @@ typedef struct {
      */
     int interlace;
 
+    /* The transparent palette index, or -1 for no transparency. */
+    int transparency;
+
     /* PRIVATE CONTEXT (set by decoder) */
 
     /* Interlace parameters */

src/libImaging/GifDecode.c

@@ -264,29 +264,33 @@ ImagingGifDecode(Imaging im, ImagingCodecState state, UINT8* buffer, int bytes)
 	/* To squeeze some extra pixels out of this loop, we test for
 	   some common cases and handle them separately. */
 
-	/* FIXME: should we handle the transparency index in here??? */
-
-	if (i == 1) {
-	    if (state->x < state->xsize-1) {
-		/* Single pixel, not at the end of the line. */
-		*out++ = p[0];
-		state->x++;
+	/* If we have transparency, we need to use the regular loop. */
+	if(context->transparency == -1)
+	{
+	    if (i == 1) {
+		if (state->x < state->xsize-1) {
+		    /* Single pixel, not at the end of the line. */
+		    *out++ = p[0];
+		    state->x++;
+		    continue;
+		}
+	    } else if (state->x + i <= state->xsize) {
+		/* This string fits into current line. */
+		memcpy(out, p, i);
+		out += i;
+		state->x += i;
+		if (state->x == state->xsize) {
+		    NEWLINE(state, context);
+		}
 		continue;
 	    }
-	} else if (state->x + i <= state->xsize) {
-	    /* This string fits into current line. */
-	    memcpy(out, p, i);
-            out += i;
-	    state->x += i;
-	    if (state->x == state->xsize) {
-		NEWLINE(state, context);
-	    }
-	    continue;
 	}
 
 	/* No shortcut, copy pixel by pixel */
 	for (c = 0; c < i; c++) {
-	    *out++ = p[c];
+	    if(p[c] != context->transparency)
+		*out = p[c];
+	    out++;
 	    if (++state->x >= state->xsize) {
 		NEWLINE(state, context);
 	    }