Use PushGroup()/PopGroup() to implement transparency blending using cairo

gfx/src/thebes/nsThebesRenderingContext.cpp

@@ -849,23 +849,30 @@ NS_IMETHODIMP
 nsThebesRenderingContext::PushFilter(const nsRect& twRect, PRBool aAreaIsOpaque, float aOpacity)
 {
     gfxPoint p0(FROM_TWIPS(twRect.x), FROM_TWIPS(twRect.y));
-    gfxPoint p1(FROM_TWIPS(twRect.XMost()), FROM_TWIPS(twRect.YMost()));
+    gfxSize ps(FROM_TWIPS(twRect.XMost() - twRect.x),
+               FROM_TWIPS(twRect.YMost() - twRect.y));
 
-    p0 = mThebes->UserToDevice(p0);
-    p1 = mThebes->UserToDevice(p1);
+    mOpacityArray.AppendElement(aOpacity);
 
-    nsIntRect deviceRect(TO_TWIPS(floor(PR_MIN(p0.x, p1.x))),
-                         TO_TWIPS(floor(PR_MIN(p0.y, p1.y))),
-                         TO_TWIPS(ceil(PR_MAX(p0.x, p1.x))),
-                         TO_TWIPS(ceil(PR_MAX(p0.y, p1.y))));
-
-    return mDrawingSurface->PushFilter(deviceRect, aAreaIsOpaque, aOpacity);
+    mThebes->Save();
+    mThebes->Clip(gfxRect(p0, ps));
+    mThebes->PushGroup();
+    return NS_OK;
 }
 
 NS_IMETHODIMP
 nsThebesRenderingContext::PopFilter()
 {
-    mDrawingSurface->PopFilter();
+    if (mOpacityArray.Length() > 0) {
+        float f = mOpacityArray[mOpacityArray.Length()-1];
+        mOpacityArray.RemoveElementAt(mOpacityArray.Length()-1);
+
+        mThebes->PopGroupToSource();
+        mThebes->Paint(f);
+    }
+
+    mThebes->Restore();
+
     return NS_OK;
 }
 

gfx/src/thebes/nsThebesRenderingContext.h

@@ -40,6 +40,7 @@
 #define NSTHEBESRENDERINGCONTEXT__H__
 
 #include "nsCOMPtr.h"
+#include "nsTArray.h"
 #include "nsIRenderingContext.h"
 #include "nsRenderingContextImpl.h"
 #include "nsIDeviceContext.h"
@@ -273,6 +274,9 @@ protected:
     // for handing out to people
     void UpdateTempTransformMatrix();
     nsTransform2D mTempTransform;
+
+    // keeping track of pushgroup/popgroup opacities
+    nsTArray<float> mOpacityArray;
 };
 
 #endif  // NSCAIRORENDERINGCONTEXT__H__

gfx/thebes/public/gfxContext.h

@@ -477,6 +477,13 @@ public:
     void Clip(gfxRect rect); // will clip to a rect
     void Clip(const gfxRegion& region); // will clip to a region
 
+    /**
+     * Groups
+     */
+    void PushGroup();
+    gfxPattern *PopGroup();
+    void PopGroupToSource();
+
     /**
      ** Filters/Group Rendering
      ** XXX these aren't really "filters" and should be renamed properly.

gfx/thebes/public/gfxPattern.h

@@ -45,7 +45,11 @@
 #include "gfxMatrix.h"
 #include "gfxTypes.h"
 
+class gfxContext;
+
 class NS_EXPORT gfxPattern {
+    friend class gfxContext;
+
     THEBES_DECL_REFCOUNTING
 
 public:

gfx/thebes/src/gfxContext.cpp

@@ -497,6 +497,24 @@ void gfxContext::Paint(gfxFloat alpha)
     cairo_paint_with_alpha(mCairo, alpha);
 }
 
+// groups
+
+void gfxContext::PushGroup()
+{
+    cairo_push_group(mCairo);
+}
+
+gfxPattern *gfxContext::PopGroup()
+{
+    cairo_pattern_t *pat = cairo_pop_group(mCairo);
+    return new gfxPattern(pat);
+}
+
+void gfxContext::PopGroupToSource()
+{
+    cairo_pop_group_to_source(mCairo);
+}
+
 // filters
 void gfxContext::PushFilter(gfxFilter& filter, FilterHints hints, gfxRect& maxArea)
 {