gimp r27421 - in trunk: . app/gegl

[martinn svn gnome org]

Author: martinn
Date: Sun Oct 26 19:28:47 2008
New Revision: 27421
URL: http://svn.gnome.org/viewvc/gimp?rev=27421&view=rev

Log:
* app/gegl/gimpoperationpointlayermode.c: Clamp Addition and
Subtract so that we acheieve consistent blending results. Also,
our Addition is fine, it is the formula for 'plus' in the SVG 1.2
draft that is wrong as far as I can see.


Modified:
   trunk/ChangeLog
   trunk/app/gegl/gimpoperationpointlayermode.c

Modified: trunk/app/gegl/gimpoperationpointlayermode.c
==============================================================================
--- trunk/app/gegl/gimpoperationpointlayermode.c	(original)
+++ trunk/app/gegl/gimpoperationpointlayermode.c	Sun Oct 26 19:28:47 2008
@@ -40,13 +40,13 @@
 #define outC out[c]
 #define outA out[A]
 
-#define BLEND(mode, expr)       \
+#define BLEND(mode, expr)               \
         case (mode):                    \
           for (c = RED; c < ALPHA; c++) \
             {                           \
               expr;                     \
             }                           \
-         break;
+          break;
 
 enum
 {
@@ -277,22 +277,39 @@
               outC = (inC * layA + (sqrt (inC / inA) * inA - inC) * (2 * layC - layA)) + layC * (1 - inA) + inC * (1 - layA);
             });
 
-          /* To be more mathematically correct we would have to either
-           * adjust the formula for the resulting opacity or adapt the
-           * other channels to the change in opacity. Compare to the
-           * 'plus' compositing operation in SVG 1.2.
+          /* Custom SVG 1.2:
            *
-           * Since this doesn't matter for completely opaque layers, and
-           * since consistency in how the alpha channel of layers is
-           * interpreted is more important than mathematically correct
-           * results, we don't bother.
+           * if Dc + Sc >= 1
+           *   f(Sc, Dc) = 1
+           * otherwise
+           *   f(Sc, Dc) = Dc + Sc
            */
           BLEND (GIMP_ADDITION_MODE,
-          outC = inC + layC);
+          if (layC * inA + inC * layA >= layA * inA)
+            {
+              outC = layA * inA + layC * (1 - inA) + inC * (1 - layA);
+            }
+          else
+            {
+              outC = inC + layC;
+            });
 
-          /* Derieved from SVG 1.2 formulas, f(Sc, Dc) = Dc - Sc */
+          /* Custom SVG 1.2:
+           *
+           * if Dc - Sc <= 0
+           *   f(Sc, Dc) = 0
+           * otherwise
+           *   f(Sc, Dc) = Dc - Sc
+           */
           BLEND (GIMP_SUBTRACT_MODE,
-          outC = inC + layC - 2 * layC * inA);
+          if (inC * layA - layC * inA <= 0)
+            {
+              outC = layC * (1 - inA) + inC * (1 - layA);
+            }
+          else
+            {
+              outC = inC + layC - 2 * layC * inA;
+            });
 
           /* Derieved from SVG 1.2 formulas, f(Sc, Dc) = Dc - Sc + 0.5 */
           BLEND (GIMP_GRAIN_EXTRACT_MODE,