static stbi_uc *stbi__gif_load_next(stbi__context *s, stbi__gif *g, int *comp, int req_comp, stbi_uc *two_back)
{
   int dispose;
   int first_frame;
   int pi;
   int pcount;
   STBI_NOTUSED(req_comp);

   // on first frame, any non-written pixels get the background colour (non-transparent)
   first_frame = 0;
   if (g->out == 0) {
      if (!stbi__gif_header(s, g, comp,0)) return 0; // stbi__g_failure_reason set by stbi__gif_header
      if (!stbi__mad3sizes_valid(4, g->w, g->h, 0))
         return stbi__errpuc("too large", "GIF image is too large");
      pcount = g->w * g->h;
      g->out = (stbi_uc *) stbi__malloc(4 * pcount);
      g->background = (stbi_uc *) stbi__malloc(4 * pcount);
      g->history = (stbi_uc *) stbi__malloc(pcount);
      if (!g->out || !g->background || !g->history)
         return stbi__errpuc("outofmem", "Out of memory");

      // image is treated as "transparent" at the start - ie, nothing overwrites the current background;
      // background colour is only used for pixels that are not rendered first frame, after that "background"
      // color refers to the color that was there the previous frame.
      memset(g->out, 0x00, 4 * pcount);
      memset(g->background, 0x00, 4 * pcount); // state of the background (starts transparent)
      memset(g->history, 0x00, pcount);        // pixels that were affected previous frame
      first_frame = 1;
   } else {
      // second frame - how do we dispose of the previous one?
      dispose = (g->eflags & 0x1C) >> 2;
      pcount = g->w * g->h;

      if ((dispose == 3) && (two_back == 0)) {
         dispose = 2; // if I don't have an image to revert back to, default to the old background
      }

      if (dispose == 3) { // use previous graphic
         for (pi = 0; pi < pcount; ++pi) {
            if (g->history[pi]) {
               memcpy( &g->out[pi * 4], &two_back[pi * 4], 4 );
            }
         }
      } else if (dispose == 2) {
         // restore what was changed last frame to background before that frame;
         for (pi = 0; pi < pcount; ++pi) {
            if (g->history[pi]) {
               memcpy( &g->out[pi * 4], &g->background[pi * 4], 4 );
            }
         }
      } else {
         // This is a non-disposal case eithe way, so just
         // leave the pixels as is, and they will become the new background
         // 1: do not dispose
         // 0:  not specified.
      }

      // background is what out is after the undoing of the previou frame;
      memcpy( g->background, g->out, 4 * g->w * g->h );
   }

   // clear my history;
   memset( g->history, 0x00, g->w * g->h );        // pixels that were affected previous frame

   for (;;) {
      int tag = stbi__get8(s);
      switch (tag) {
         case 0x2C: /* Image Descriptor */
         {
            stbi__int32 x, y, w, h;
            stbi_uc *o;

            x = stbi__get16le(s);
            y = stbi__get16le(s);
            w = stbi__get16le(s);
            h = stbi__get16le(s);
            if (((x + w) > (g->w)) || ((y + h) > (g->h)))
               return stbi__errpuc("bad Image Descriptor", "Corrupt GIF");

            g->line_size = g->w * 4;
            g->start_x = x * 4;
            g->start_y = y * g->line_size;
            g->max_x   = g->start_x + w * 4;
            g->max_y   = g->start_y + h * g->line_size;
            g->cur_x   = g->start_x;
            g->cur_y   = g->start_y;

            // if the width of the specified rectangle is 0, that means
            // we may not see *any* pixels or the image is malformed;
            // to make sure this is caught, move the current y down to
            // max_y (which is what out_gif_code checks).
            if (w == 0)
               g->cur_y = g->max_y;

            g->lflags = stbi__get8(s);

            if (g->lflags & 0x40) {
               g->step = 8 * g->line_size; // first interlaced spacing
               g->parse = 3;
            } else {
               g->step = g->line_size;
               g->parse = 0;
            }

            if (g->lflags & 0x80) {
               stbi__gif_parse_colortable(s,g->lpal, 2 << (g->lflags & 7), g->eflags & 0x01 ? g->transparent : -1);
               g->color_table = (stbi_uc *) g->lpal;
            } else if (g->flags & 0x80) {
               g->color_table = (stbi_uc *) g->pal;
            } else
               return stbi__errpuc("missing color table", "Corrupt GIF");

            o = stbi__process_gif_raster(s, g);
            if (!o) return NULL;

            // if this was the first frame,
            pcount = g->w * g->h;
            if (first_frame && (g->bgindex > 0)) {
               // if first frame, any pixel not drawn to gets the background color
               for (pi = 0; pi < pcount; ++pi) {
                  if (g->history[pi] == 0) {
                     g->pal[g->bgindex][3] = 255; // just in case it was made transparent, undo that; It will be reset next frame if need be;
                     memcpy( &g->out[pi * 4], &g->pal[g->bgindex], 4 );
                  }
               }
            }

            return o;
         }

         case 0x21: // Comment Extension.
         {
            int len;
            int ext = stbi__get8(s);
            if (ext == 0xF9) { // Graphic Control Extension.
               len = stbi__get8(s);
               if (len == 4) {
                  g->eflags = stbi__get8(s);
                  g->delay = 10 * stbi__get16le(s); // delay - 1/100th of a second, saving as 1/1000ths.

                  // unset old transparent
                  if (g->transparent >= 0) {
                     g->pal[g->transparent][3] = 255;
                  }
                  if (g->eflags & 0x01) {
                     g->transparent = stbi__get8(s);
                     if (g->transparent >= 0) {
                        g->pal[g->transparent][3] = 0;
                     }
                  } else {
                     // don't need transparent
                     stbi__skip(s, 1);
                     g->transparent = -1;
                  }
               } else {
                  stbi__skip(s, len);
                  break;
               }
            }
            while ((len = stbi__get8(s)) != 0) {
               stbi__skip(s, len);
            }
            break;
         }

         case 0x3B: // gif stream termination code
            return (stbi_uc *) s; // using '1' causes warning on some compilers

         default:
            return stbi__errpuc("unknown code", "Corrupt GIF");
      }
   }
}
Unknown