Function remapPaletteForPNG

mapimageio.c:194–250

* sort a given list of rgba entries so that all the opaque pixels are at the end */

 * sort a given list of rgba entries so that all the opaque pixels are at the end
 */
int remapPaletteForPNG(rasterBufferObj *rb, rgbPixel *rgb, unsigned char *a, int *num_a) {
   int bot_idx, top_idx, x;
   int remap[256];
   unsigned int maxval = rb->data.palette.scaling_maxval; 
  
   assert(rb->type == MS_BUFFER_BYTE_PALETTE);
   
   /*
   ** remap the palette colors so that all entries with
   ** the maximal alpha value (i.e., fully opaque) are at the end and can
   ** therefore be omitted from the tRNS chunk.  Note that the ordering of
   ** opaque entries is reversed from how they were previously arranged
   ** --not that this should matter to anyone.
   */
   
   for (top_idx = rb->data.palette.num_entries-1, bot_idx = x = 0;  x < rb->data.palette.num_entries;  ++x) {
     if (rb->data.palette.palette[x].a == maxval)
         remap[x] = top_idx--;
     else
         remap[x] = bot_idx++;
   }
   /* sanity check:  top and bottom indices should have just crossed paths */
   if (bot_idx != top_idx + 1) {
     msSetError(MS_MISCERR,"quantization sanity check failed","createPNGPalette()");
     return MS_FAILURE;
   }
   
   *num_a = bot_idx;
   
   for(x=0;x<rb->width*rb->height;x++)
      rb->data.palette.pixels[x] = remap[rb->data.palette.pixels[x]];
   

   for (x = 0; x < rb->data.palette.num_entries; ++x) {
      if(maxval == 255) {
         a[remap[x]] = rb->data.palette.palette[x].a;
         rgb[remap[x]].r = rb->data.palette.palette[x].r;
         rgb[remap[x]].g = rb->data.palette.palette[x].g;
         rgb[remap[x]].b = rb->data.palette.palette[x].b;
      } else {
         /* rescale palette */
         rgb[remap[x]].r = (rb->data.palette.palette[x].r * 255 + (maxval >> 1)) / maxval;
         rgb[remap[x]].g = (rb->data.palette.palette[x].g * 255 + (maxval >> 1)) / maxval;
         rgb[remap[x]].b = (rb->data.palette.palette[x].b * 255 + (maxval >> 1)) / maxval;
         a[remap[x]] = (rb->data.palette.palette[x].a * 255 + (maxval >> 1)) / maxval;
      }
      if(a[remap[x]] != 255) {
         /* un-premultiply pixels */
         double da = 255.0/a[remap[x]];
         rgb[remap[x]].r *=  da;
         rgb[remap[x]].g *=  da;
         rgb[remap[x]].b *=  da;
      }
   }
   
   return MS_SUCCESS;
}