src/lib/gifwrite.c 1.8

Index: src/lib/gifwrite.c
===================================================================
RCS file: /projects/compbio/cvsroot/kent/src/lib/gifwrite.c,v
retrieving revision 1.7
retrieving revision 1.8
diff -b -B -U 4 -r1.7 -r1.8
--- src/lib/gifwrite.c	22 Apr 2009 01:57:00 -0000	1.7
+++ src/lib/gifwrite.c	19 Aug 2009 22:28:36 -0000	1.8
@@ -7,11 +7,27 @@
 static char const rcsid[] = "$Id$";
 
 static char gifsig[] = "GIF87a";
 
+// GIF Graphic Control Extension, for making the background color transparent:
+static struct gif_gce
+    {
+    unsigned char extensionIntroducer, graphicControlLabel, blockSize;
+    unsigned char flags;
+    unsigned char delayTimeLo,delayTimeHi;
+    unsigned char transparentColorIndex;
+    unsigned char blockTerminator;
+    } gce = {0x21, 0xF9, 0x04, // fixed bytes from spec
+	     0x01,             // transparency on (no user input or disposal options)
+	     0x00, 0x00,       // no animation delay time
+	     0x00,             // color index 0 (white) is transparent
+	     0x00};
 
-boolean mgSaveToGif(FILE *gif_file, struct memGfx *screen)
-/* Save GIF to an already open file. */
+boolean mgSaveToGif(FILE *gif_file, struct memGfx *screen, boolean useTransparency)
+/* Save GIF to an already open file.
+ * If useTransparency, then the first color in memgfx's colormap/palette is
+ * assumed to be the image background color, and pixels of that color
+ * are made transparent. */
 {
 int i;
 struct gif_header gif;
 struct gif_image gim;
@@ -30,8 +46,13 @@
     goto TRUNCOUT;
 /* write global color map */
 if (fwrite(screen->colorMap, 3, 256, gif_file) < 256)
     goto TRUNCOUT;
+
+if (useTransparency)
+    if (fwrite(&gce, sizeof(gce), 1, gif_file ) < 1)
+	goto TRUNCOUT;
+
 if (fputc(',', gif_file) < 0) /* comma to start image */
     goto TRUNCOUT;
 if (fwrite(&gim, sizeof(gim), 1, gif_file) < 1)
     goto TRUNCOUT;
@@ -58,12 +79,16 @@
 BADOUT:
 return(FALSE);
 }
 
-void mgSaveGif(struct memGfx *screen, char *name)
+void mgSaveGif(struct memGfx *screen, char *name, boolean useTransparency)
+/* Save memory bitmap as a gif.
+ * If useTransparency, then the first color in memgfx's colormap/palette is
+ * assumed to be the image background color, and pixels of that color
+ * are made transparent. */
 {
 FILE *gifFile = mustOpen(name, "wb");
-if (!mgSaveToGif(gifFile, screen))
+if (!mgSaveToGif(gifFile, screen, useTransparency))
     {
     remove(name);
     errAbort("Couldn't save %s", name);
     }