static stbi_uc *load_jpeg_image(stbi__jpeg *z, int *out_x, int *out_y, int *comp, int req_comp)
{
   int n, decode_n, is_rgb;
   z->s->img_n = 0; // make stbi__cleanup_jpeg safe

   // validate req_comp
   if (req_comp < 0 || req_comp > 4) return stbi__errpuc("bad req_comp", "Internal error");

   // load a jpeg image from whichever source, but leave in YCbCr format
   if (!stbi__decode_jpeg_image(z)) { stbi__cleanup_jpeg(z); return NULL; }

   // determine actual number of components to generate
   n = req_comp ? req_comp : z->s->img_n >= 3 ? 3 : 1;

   is_rgb = z->s->img_n == 3 && (z->rgb == 3 || (z->app14_color_transform == 0 && !z->jfif));

   if (z->s->img_n == 3 && n < 3 && !is_rgb)
      decode_n = 1;
   else
      decode_n = z->s->img_n;

   // nothing to do if no components requested; check this now to avoid
   // accessing uninitialized coutput[0] later
   if (decode_n <= 0) { stbi__cleanup_jpeg(z); return NULL; }

   // resample and color-convert
   {
      int k;
      unsigned int i,j;
      stbi_uc *output;
      stbi_uc *coutput[4] = { NULL, NULL, NULL, NULL };

      stbi__resample res_comp[4];

      for (k=0; k < decode_n; ++k) {
         stbi__resample *r = &res_comp[k];

         // allocate line buffer big enough for upsampling off the edges
         // with upsample factor of 4
         z->img_comp[k].linebuf = (stbi_uc *) stbi__malloc(z->s->img_x + 3);
         if (!z->img_comp[k].linebuf) { stbi__cleanup_jpeg(z); return stbi__errpuc("outofmem", "Out of memory"); }

         r->hs      = z->img_h_max / z->img_comp[k].h;
         r->vs      = z->img_v_max / z->img_comp[k].v;
         r->ystep   = r->vs >> 1;
         r->w_lores = (z->s->img_x + r->hs-1) / r->hs;
         r->ypos    = 0;
         r->line0   = r->line1 = z->img_comp[k].data;

         if      (r->hs == 1 && r->vs == 1) r->resample = resample_row_1;
         else if (r->hs == 1 && r->vs == 2) r->resample = stbi__resample_row_v_2;
         else if (r->hs == 2 && r->vs == 1) r->resample = stbi__resample_row_h_2;
         else if (r->hs == 2 && r->vs == 2) r->resample = z->resample_row_hv_2_kernel;
         else                               r->resample = stbi__resample_row_generic;
      }

      // can't error after this so, this is safe
      output = (stbi_uc *) stbi__malloc_mad3(n, z->s->img_x, z->s->img_y, 1);
      if (!output) { stbi__cleanup_jpeg(z); return stbi__errpuc("outofmem", "Out of memory"); }

      // now go ahead and resample
      for (j=0; j < z->s->img_y; ++j) {
         stbi_uc *out = output + n * z->s->img_x * j;
         for (k=0; k < decode_n; ++k) {
            stbi__resample *r = &res_comp[k];
            int y_bot = r->ystep >= (r->vs >> 1);
            coutput[k] = r->resample(z->img_comp[k].linebuf,
                                     y_bot ? r->line1 : r->line0,
                                     y_bot ? r->line0 : r->line1,
                                     r->w_lores, r->hs);
            if (++r->ystep >= r->vs) {
               r->ystep = 0;
               r->line0 = r->line1;
               if (++r->ypos < z->img_comp[k].y)
                  r->line1 += z->img_comp[k].w2;
            }
         }
         if (n >= 3) {
            stbi_uc *y = coutput[0];
            if (z->s->img_n == 3) {
               if (is_rgb) {
                  for (i=0; i < z->s->img_x; ++i) {
                     out[0] = y[i];
                     out[1] = coutput[1][i];
                     out[2] = coutput[2][i];
                     out[3] = 255;
                     out += n;
                  }
               } else {
                  z->YCbCr_to_RGB_kernel(out, y, coutput[1], coutput[2], z->s->img_x, n);
               }
            } else if (z->s->img_n == 4) {
               if (z->app14_color_transform == 0) { // CMYK
                  for (i=0; i < z->s->img_x; ++i) {
                     stbi_uc m = coutput[3][i];
                     out[0] = stbi__blinn_8x8(coutput[0][i], m);
                     out[1] = stbi__blinn_8x8(coutput[1][i], m);
                     out[2] = stbi__blinn_8x8(coutput[2][i], m);
                     out[3] = 255;
                     out += n;
                  }
               } else if (z->app14_color_transform == 2) { // YCCK
                  z->YCbCr_to_RGB_kernel(out, y, coutput[1], coutput[2], z->s->img_x, n);
                  for (i=0; i < z->s->img_x; ++i) {
                     stbi_uc m = coutput[3][i];
                     out[0] = stbi__blinn_8x8(255 - out[0], m);
                     out[1] = stbi__blinn_8x8(255 - out[1], m);
                     out[2] = stbi__blinn_8x8(255 - out[2], m);
                     out += n;
                  }
               } else { // YCbCr + alpha?  Ignore the fourth channel for now
                  z->YCbCr_to_RGB_kernel(out, y, coutput[1], coutput[2], z->s->img_x, n);
               }
            } else
               for (i=0; i < z->s->img_x; ++i) {
                  out[0] = out[1] = out[2] = y[i];
                  out[3] = 255; // not used if n==3
                  out += n;
               }
         } else {
            if (is_rgb) {
               if (n == 1)
                  for (i=0; i < z->s->img_x; ++i)
                     *out++ = stbi__compute_y(coutput[0][i], coutput[1][i], coutput[2][i]);
               else {
                  for (i=0; i < z->s->img_x; ++i, out += 2) {
                     out[0] = stbi__compute_y(coutput[0][i], coutput[1][i], coutput[2][i]);
                     out[1] = 255;
                  }
               }
            } else if (z->s->img_n == 4 && z->app14_color_transform == 0) {
               for (i=0; i < z->s->img_x; ++i) {
                  stbi_uc m = coutput[3][i];
                  stbi_uc r = stbi__blinn_8x8(coutput[0][i], m);
                  stbi_uc g = stbi__blinn_8x8(coutput[1][i], m);
                  stbi_uc b = stbi__blinn_8x8(coutput[2][i], m);
                  out[0] = stbi__compute_y(r, g, b);
                  out[1] = 255;
                  out += n;
               }
            } else if (z->s->img_n == 4 && z->app14_color_transform == 2) {
               for (i=0; i < z->s->img_x; ++i) {
                  out[0] = stbi__blinn_8x8(255 - coutput[0][i], coutput[3][i]);
                  out[1] = 255;
                  out += n;
               }
            } else {
               stbi_uc *y = coutput[0];
               if (n == 1)
                  for (i=0; i < z->s->img_x; ++i) out[i] = y[i];
               else
                  for (i=0; i < z->s->img_x; ++i) { *out++ = y[i]; *out++ = 255; }
            }
         }
      }
      stbi__cleanup_jpeg(z);
      *out_x = z->s->img_x;
      *out_y = z->s->img_y;
      if (comp) *comp = z->s->img_n >= 3 ? 3 : 1; // report original components, not output
      return output;
   }
}