Fix blending of semi-opaque pixels in average and bilinear resamplers (#4875)

Author: tbonfort

mapresample.c

@@ -671,7 +671,7 @@ msAverageRasterResampler( imageObj *psSrcImage, rasterBufferObj *src_rb,
               alpha = (unsigned char)
                       MAX(0,MIN(255,255*dfAlpha01+0.5));
 
-              RB_SET_PIXEL(dst_rb,nDstX,nDstY,
+              RB_MIX_PIXEL(dst_rb,nDstX,nDstY,
                            red, green, blue, alpha );
             }
 #ifdef USE_GD

maputil.c

@@ -2096,19 +2096,19 @@ void msAlphaBlendPM( unsigned char red_src, unsigned char green_src,
   /*      Cases with actual blending.                                     */
   /* -------------------------------------------------------------------- */
   if(!alpha_dst || *alpha_dst == 255) {
-    int weight_dst = 256 - alpha_src;
+    int weight_dst = 255 - alpha_src;
 
-    *red_dst   = (256 * red_src   + *red_dst   * weight_dst) >> 8;
-    *green_dst = (256 * green_src + *green_dst * weight_dst) >> 8;
-    *blue_dst  = (256 * blue_src  + *blue_dst  * weight_dst) >> 8;
+    *red_dst   = (alpha_src * red_src   + *red_dst   * weight_dst) >> 8;
+    *green_dst = (alpha_src * green_src + *green_dst * weight_dst) >> 8;
+    *blue_dst  = (alpha_src * blue_src  + *blue_dst  * weight_dst) >> 8;
   } else {
-    int   weight_dst = (256 - alpha_src);
+    int   weight_dst = (255 - alpha_src);
 
-    *red_dst   = (256 * red_src   + *red_dst   * weight_dst) >> 8;
-    *green_dst = (256 * green_src + *green_dst * weight_dst) >> 8;
-    *blue_dst  = (256 * blue_src  + *blue_dst  * weight_dst) >> 8;
+    *red_dst   = (alpha_src * red_src   + *red_dst   * weight_dst) >> 8;
+    *green_dst = (alpha_src * green_src + *green_dst * weight_dst) >> 8;
+    *blue_dst  = (alpha_src * blue_src  + *blue_dst  * weight_dst) >> 8;
 
-    *alpha_dst = (256 * alpha_src + *alpha_dst * weight_dst) >> 8;
+    *alpha_dst = (255 * alpha_src + *alpha_dst * weight_dst) >> 8;
   }
 }