Fix the pipi_merge() semantics so that it matches Gimp’s, and don’t ignore

the image’s alpha layers.

git-svn-id: file:///srv/caca.zoy.org/var/lib/svn/libpipi/trunk@3545 92316355-f0b4-4df1-b90c-862c8a59935f

pipi/combine/merge.c

@@ -28,7 +28,6 @@ pipi_image_t *pipi_merge(pipi_image_t *img1, pipi_image_t *img2, double t)
     pipi_image_t *dst;
     pipi_pixels_t *img1p, *img2p, *dstp;
     float *img1data, *img2data, *dstdata;
-    double t2;
     int x, y, w, h;
 
     if(img1->w != img2->w || img1->h != img2->h)
@@ -38,7 +37,6 @@ pipi_image_t *pipi_merge(pipi_image_t *img1, pipi_image_t *img2, double t)
         t = 0.0;
     else if(t > 1.0)
         t = 1.0;
-    t2 = 1.0 - t;
 
     w = img1->w;
     h = img1->h;
@@ -57,22 +55,24 @@ pipi_image_t *pipi_merge(pipi_image_t *img1, pipi_image_t *img2, double t)
         for(x = 0; x < w; x++)
         {
             float p, q;
+            double t1 = t * img2data[4 * (y * w + x) + 3];
+            double t2 = 1.0 - t1;
 
             p = img1data[4 * (y * w + x)];
             q = img2data[4 * (y * w + x)];
-            dstdata[4 * (y * w + x)] = t * p + t2 * q;
+            dstdata[4 * (y * w + x)] = t2 * p + t1 * q;
 
             p = img1data[4 * (y * w + x) + 1];
             q = img2data[4 * (y * w + x) + 1];
-            dstdata[4 * (y * w + x) + 1] = t * p + t2 * q;
+            dstdata[4 * (y * w + x) + 1] = t2 * p + t1 * q;
 
             p = img1data[4 * (y * w + x) + 2];
             q = img2data[4 * (y * w + x) + 2];
-            dstdata[4 * (y * w + x) + 2] = t * p + t2 * q;
+            dstdata[4 * (y * w + x) + 2] = t2 * p + t1 * q;
 
             p = img1data[4 * (y * w + x) + 3];
             q = img2data[4 * (y * w + x) + 3];
-            dstdata[4 * (y * w + x) + 3] = t * p + t2 * q;
+            dstdata[4 * (y * w + x) + 3] = t2 * p + t1 * q;
         }
     }