fix gdImageCopyResampled() for non-constant alpha layer

diff --git a/ext/gd/libgd/gd.c b/ext/gd/libgd/gd.c
index 5a439e7a03fa58ecd0c4b0035d63122f109b5d54..ff6d4425cfbbed39e8ba6762e91f7467a454faf0 100644 (file)
--- a/ext/gd/libgd/gd.c
+++ b/ext/gd/libgd/gd.c
@@ -2047,6 +2047,7 @@ gdImageCopyResampled (gdImagePtr dst,
          float sx, sy;
          float spixels = 0.0f;
          float red = 0.0f, green = 0.0f, blue = 0.0f, alpha = 0.0f;
+         float alpha_factor, alpha_sum = 0.0f, contrib_sum = 0.0f;
          sy1 = ((float)(y - dstY)) * (float)srcH /
            (float)dstH;
          sy2 = ((float)(y + 1 - dstY)) * (float) srcH /
@@ -2104,10 +2105,13 @@ gdImageCopyResampled (gdImagePtr dst,
                                                 src,
                                                 (int) sx + srcX,
                                                 (int) sy + srcY);
-                 red += gdTrueColorGetRed (p) * pcontribution;
-                 green += gdTrueColorGetGreen (p) * pcontribution;
-                 blue += gdTrueColorGetBlue (p) * pcontribution;
+                 alpha_factor = ((gdAlphaMax - gdTrueColorGetAlpha(p))) * pcontribution;
+                 red += gdTrueColorGetRed (p) * alpha_factor;
+                 green += gdTrueColorGetGreen (p) * alpha_factor;
+                 blue += gdTrueColorGetBlue (p) * alpha_factor;
                  alpha += gdTrueColorGetAlpha (p) * pcontribution;
+                 alpha_sum += alpha_factor;
+                 contrib_sum += pcontribution;
                  spixels += xportion * yportion;
                  sx += 1.0f;
                }
@@ -2122,6 +2126,13 @@ gdImageCopyResampled (gdImagePtr dst,
              blue /= spixels;
              alpha /= spixels;
            }
+          if ( alpha_sum != 0.0f)
+            {
+              if( contrib_sum != 0.0f ) alpha_sum /= contrib_sum;
+              red /= alpha_sum;
+              green /= alpha_sum;
+              blue /= alpha_sum;
+            }
          /* Clamping to allow for rounding errors above */
          if (red > 255.0f)
            {