Small fix in the bilinear formula

After consulting Gimp again, the formula was missing a final divide. This
takes care of the 1:1 discrepancy.


git-svn-id: file:///fltk/svn/fltk/branches/branch-1.3@10270 ea41ed52-d2ee-0310-a9c1-e6b18d33e121

src/Fl_Image.cxx

@@ -316,7 +316,6 @@ Fl_Image *Fl_RGB_Image::copy(int W, int H) {
         memcpy(downleft, array + dlefty * line_d + dleftx * d(), d());
         memcpy(downright, array + drighty * line_d + drightx * d(), d());
 
-        // TODO: how to check if it's premultiplied alpha?
         int i;
         if (d() == 4) {
           for (i = 0; i < 3; i++) {
@@ -338,6 +337,12 @@ Fl_Image *Fl_RGB_Image::copy(int W, int H) {
                    (downleft[i] * leftf +
                    downright[i] * rightf) * downf;
         }
+
+        if (d() == 4) {
+          for (i = 0; i < 3; i++) {
+            new_ptr[i] /= new_ptr[3] / 255.0f;
+          }
+        }
       }
     }
   }