issue_181.patch

diff --git a/lib/blending.hpp b/lib/blending.hpp
index d61c741..0c800ff 100644
--- a/lib/blending.hpp
+++ b/lib/blending.hpp
@@ -107,6 +107,32 @@ class BufferCombineFunc <DSTALPHA, BUFSIZE, BlendNormal, CompositeDestinationOut
 // Da'  = Da × (1 - Sa)
 
 
+template <bool DSTALPHA, unsigned int BUFSIZE>
+class BufferCombineFunc <DSTALPHA, BUFSIZE, BlendNormal, CompositeDestinationAtop>
+{
+    // Partial specialization for normal lainting layers,
+    // working in premultiplied alpha for speed.
+  public:
+    inline void operator() (const fix15_short_t * const src,
+                            fix15_short_t * const dst,
+                            const fix15_short_t opac) const
+    {
+        for (unsigned int i=0; i<BUFSIZE; i+=4) {
+            const fix15_t as = fix15_mul(src[i+3], opac);
+            const fix15_t one_minus_ab = fix15_one - dst[i+3];
+            const fix15_t ab_mul_as = fix15_mul(dst[i+3], as);
+            const fix15_t as_mul_one_minus_ab = fix15_mul(as, one_minus_ab);
+            dst[i+0] = fix15_sumprods(src[i+0], as_mul_one_minus_ab, dst[i+0], ab_mul_as);
+            dst[i+1] = fix15_sumprods(src[i+1], as_mul_one_minus_ab, dst[i+1], ab_mul_as);
+            dst[i+2] = fix15_sumprods(src[i+2], as_mul_one_minus_ab, dst[i+2], ab_mul_as);
+            if (DSTALPHA) {
+                dst[i+3] = fix15_short_clamp(fix15_mul(as, one_minus_ab) + ab_mul_as);
+            }
+        }
+    }
+};
+
+
 // Multiply: http://www.w3.org/TR/compositing/#blendingmultiply
 
 class BlendMultiply : public BlendFunc
diff --git a/lib/compositing.hpp b/lib/compositing.hpp
index 5d983f7..dc956f4 100644
--- a/lib/compositing.hpp
+++ b/lib/compositing.hpp
@@ -269,6 +269,33 @@ class CompositeDestinationOut : public CompositeFunc
 };
 
 
+// Destination-Atop: Destination which overlaps the source replaces the source.
+// Source is placed elsewhere.
+// http://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_dstatop
+
+class CompositeDestinationAtop : public CompositeFunc
+{
+  public:
+    inline void operator() (const fix15_t Rs, const fix15_t Gs,
+                            const fix15_t Bs, const fix15_t as,
+                            fix15_short_t &rb, fix15_short_t &gb,
+                            fix15_short_t &bb, fix15_short_t &ab) const
+    {
+        const fix15_t one_minus_ab = fix15_one - ab;
+        const fix15_t as_mul_one_minus_ab = fix15_mul(as, one_minus_ab);
+        const fix15_t ab_mul_as = fix15_mul(as, ab);
+
+        rb = fix15_short_clamp(fix15_sumprods(as_mul_one_minus_ab, Rs, ab_mul_as, rb));
+        gb = fix15_short_clamp(fix15_sumprods(as_mul_one_minus_ab, Gs, ab_mul_as, gb));
+        bb = fix15_short_clamp(fix15_sumprods(as_mul_one_minus_ab, Bs, ab_mul_as, bb));
+        ab = fix15_short_clamp(as_mul_one_minus_ab + ab_mul_as);
+    }
+
+    static const bool zero_alpha_has_effect = true;
+    static const bool can_decrease_alpha = true;
+};
+
+
 // W3C "Lighter", a.k.a. Porter-Duff "plus", a.k.a. "svg:plus". This just adds
 // together corresponding channels of the destination and source.
 // Ref: http://www.w3.org/TR/compositing-1/#porterduffcompositingoperators_plus
diff --git a/lib/layer/consts.py b/lib/layer/consts.py
index f50b95f..701db19 100644
--- a/lib/layer/consts.py
+++ b/lib/layer/consts.py
@@ -113,6 +113,10 @@ MODE_STRINGS = {
         _("Destination Out"),
         _("Uses the backdrop only where this layer doesn't cover it. "
           "Everything else is ignored.")),
+    lib.mypaintlib.CombineDestinationAtop: (
+        _("Destination Atop"),
+        _("Destination which overlaps the source replaces the source. "
+          "Source is placed elsewhere.")),
 }
 for mode in STANDARD_MODES + STACK_MODES:
     assert mode in MODE_STRINGS
diff --git a/lib/pixops.cpp b/lib/pixops.cpp
index 794ce4b..e0a9e54 100644
--- a/lib/pixops.cpp
+++ b/lib/pixops.cpp
@@ -675,7 +675,8 @@ static const TileDataCombineOp * combine_mode_info[NumCombineModes] =
     // Normal blend mode + various compositing operators
     new TileDataCombine<BlendNormal, CompositeLighter>("svg:plus"),
     new TileDataCombine<BlendNormal, CompositeDestinationIn>("svg:dst-in"),
-    new TileDataCombine<BlendNormal, CompositeDestinationOut>("svg:dst-out")
+    new TileDataCombine<BlendNormal, CompositeDestinationOut>("svg:dst-out"),
+    new TileDataCombine<BlendNormal, CompositeDestinationAtop>("svg:dst-atop")
 };
 
 
diff --git a/lib/pixops.hpp b/lib/pixops.hpp
index 1e12c9f..972cf35 100644
--- a/lib/pixops.hpp
+++ b/lib/pixops.hpp
@@ -110,6 +110,7 @@ enum CombineMode {
     CombineLighter,   // normal blend mode, and W3C lightER (Porter-Duff PLUS)
     CombineDestinationIn,
     CombineDestinationOut,
+    CombineDestinationAtop,
     NumCombineModes
 };
 
diff --git a/lib/tiledsurface.py b/lib/tiledsurface.py
index 931873f..3be1678 100644
--- a/lib/tiledsurface.py
+++ b/lib/tiledsurface.py
@@ -319,7 +319,7 @@ class MyPaintSurface (object):
         """
 
         if opacity == 0:
-            if mode == mypaintlib.CombineDestinationIn:
+            if mode == mypaintlib.CombineDestinationIn or mode == mypaintlib.CombineDestinationAtop:
                 if dst_has_alpha:
                     mypaintlib.tile_clear_rgba16(dst)
                     return
@@ -333,7 +333,7 @@ class MyPaintSurface (object):
 
         with self.tile_request(tx, ty, readonly=True) as src:
             if src is transparent_tile.rgba:
-                if mode == mypaintlib.CombineDestinationIn:
+                if mode == mypaintlib.CombineDestinationIn or mode == mypaintlib.CombineDestinationAtop:
                     if dst_has_alpha:
                         mypaintlib.tile_clear_rgba16(dst)
                         return