Allow transparency in GIF images

Thanks to preperation work done by Harald

backend/gif.c

@@ -43,11 +43,6 @@
 
 #define SSET    "0123456789ABCDEF"
 
-/* Index of transparent color, -1 for no transparent color
- * This might be set into a variable if transparency is activated as an option
- */
-#define TRANSPARENT_INDEX (-1)
-
 typedef struct s_statestruct {
     unsigned char * pOut;
     unsigned char *pIn;
@@ -290,6 +285,8 @@ INTERNAL int gif_pixel_plot(struct zint_symbol *symbol, char *pixelbuf) {
     int paletteBitSize;
     int paletteSize;
     statestruct State;
+    int transparent_index;
+    int bgindex, fgindex;
 
     unsigned char backgroundColourIndex;
     unsigned char RGBCur[3];
@@ -439,6 +436,9 @@ INTERNAL int gif_pixel_plot(struct zint_symbol *symbol, char *pixelbuf) {
             paletteRGB[paletteIndex][2] = RGBCur[2];
 
             paletteCount++;
+
+            if (pixelColour == '0') bgindex = paletteIndex;
+            if (pixelColour == '1') fgindex = paletteIndex;
         }
         /* Add palette index to current colour code */
         (State.colourCode)[colourCount] = pixelColour;
@@ -447,6 +447,23 @@ INTERNAL int gif_pixel_plot(struct zint_symbol *symbol, char *pixelbuf) {
     }
     State.colourCount = colourCount;
 
+    /* Set transparency */
+    /* Note: does not allow both transparent foreground and background -
+     * background takes prioroty */
+    transparent_index = -1;
+    if (strlen(symbol->fgcolour) > 6) {
+        if ((symbol->fgcolour[6] == '0') && (symbol->fgcolour[7] == '0')) {
+            // Transparent foreground
+            transparent_index = fgindex;
+        }
+    }
+    if (strlen(symbol->bgcolour) > 6) {
+        if ((symbol->bgcolour[6] == '0') && (symbol->bgcolour[7] == '0')) {
+            // Transparent background
+            transparent_index = bgindex;
+        }
+    }
+
     /* find palette bit size from palette size*/
 
     /* 1,2 -> 1, 3,4 ->2, 5,6,7,8->3 */
@@ -465,7 +482,7 @@ INTERNAL int gif_pixel_plot(struct zint_symbol *symbol, char *pixelbuf) {
 
     /* GIF signature (6) */
     memcpy(outbuf, "GIF87a", 6);
-    if (TRANSPARENT_INDEX != -1)
+    if (transparent_index != -1)
         outbuf[4] = '9';
 
     fwrite(outbuf, 6, 1, gif_file);
@@ -516,7 +533,7 @@ INTERNAL int gif_pixel_plot(struct zint_symbol *symbol, char *pixelbuf) {
     /* A graphic control extension block is used for overlay gifs.
      * This is necessary to define a transparent color.
      */
-    if (TRANSPARENT_INDEX != -1) {
+    if (transparent_index != -1) {
         /* Extension Introducer = '!' */
         outbuf[0] = '\x21';
         /* Graphic Control Label */
@@ -534,7 +551,7 @@ INTERNAL int gif_pixel_plot(struct zint_symbol *symbol, char *pixelbuf) {
         outbuf[4] = 0;
         outbuf[5] = 0;
         /* Transparent Color Index */
-        outbuf[6] = (unsigned char) TRANSPARENT_INDEX;
+        outbuf[6] = (unsigned char) transparent_index;
         /* Block Terminator */
         outbuf[7] = 0;
         fwrite(outbuf, 8, 1, gif_file);

docs/manual.txt

@@ -420,7 +420,7 @@ will give different results for PNG and SVG. Experimentation is advised!
 Also note that these options don't work properly with Maxicode yet.
 
 In addition the --nobackground option will simply remove the background from
-PNG, SVG, EMF and EPS files.
+PNG, GIF, SVG, EMF and EPS files.
 
 4.8 Rotating the Symbol
 -----------------------
@@ -931,8 +931,9 @@ int main(int argc, char **argv)
     return 0;
 }
 
-Background removal for PNG, SVG, EMF and EPS files can be achieved by setting
+Background removal for PNG, GIF, SVG, EMF and EPS files can be achieved by
-the background alpha to "00" where the values for R, G and B will be ignored:
+setting the background alpha to "00" where the values for R, G and B will be
+ignored:
 
 strcpy(my_symbol->bgcolour, "55555500");