Bug 917703 - Avoid copying to a sub image in CreateSamplingRestrictedDrawable if possible. r=roc

gfx/thebes/gfxDrawable.cpp

@@ -134,6 +134,12 @@ gfxSurfaceDrawable::Draw(gfxContext* aContext,
     return true;
 }
 
+already_AddRefed<gfxImageSurface>
+gfxSurfaceDrawable::GetAsImageSurface()
+{
+    return mSurface->GetAsImageSurface();
+}
+
 gfxCallbackDrawable::gfxCallbackDrawable(gfxDrawingCallback* aCallback,
                                          const gfxIntSize aSize)
  : gfxDrawable(aSize)

gfx/thebes/gfxDrawable.h

@@ -15,6 +15,7 @@
 #include "gfxPattern.h"
 
 class gfxASurface;
+class gfxImageSurface;
 class gfxContext;
 
 /**
@@ -41,6 +42,7 @@ public:
                         bool aRepeat,
                         const gfxPattern::GraphicsFilter& aFilter,
                         const gfxMatrix& aTransform = gfxMatrix()) = 0;
+    virtual already_AddRefed<gfxImageSurface> GetAsImageSurface() { return nullptr; }
     virtual gfxIntSize Size() { return mSize; }
 
 protected:
@@ -62,6 +64,8 @@ public:
                         bool aRepeat,
                         const gfxPattern::GraphicsFilter& aFilter,
                         const gfxMatrix& aTransform = gfxMatrix());
+    
+    virtual already_AddRefed<gfxImageSurface> GetAsImageSurface();
 
 protected:
     nsRefPtr<gfxASurface> mSurface;

gfx/thebes/gfxImageSurface.cpp

@@ -297,6 +297,8 @@ gfxImageSurface::GetSubimage(const gfxRect& aRect)
 {
     gfxRect r(aRect);
     r.Round();
+    MOZ_ASSERT(gfxRect(0, 0, mSize.width, mSize.height).Contains(r));
+
     unsigned char* subData = Data() +
         (Stride() * (int)r.Y()) +
         (int)r.X() * gfxASurface::BytePerPixelFromFormat(Format());

gfx/thebes/gfxUtils.cpp

@@ -245,16 +245,23 @@ CreateSamplingRestrictedDrawable(gfxDrawable* aDrawable,
     if (needed.IsEmpty())
         return nullptr;
 
+    nsRefPtr<gfxASurface> temp;
     gfxIntSize size(int32_t(needed.Width()), int32_t(needed.Height()));
-    nsRefPtr<gfxASurface> temp =
-        gfxPlatform::GetPlatform()->CreateOffscreenSurface(size, gfxASurface::ContentFromFormat(aFormat));
-    if (!temp || temp->CairoStatus())
-        return nullptr;
 
-    nsRefPtr<gfxContext> tmpCtx = new gfxContext(temp);
-    tmpCtx->SetOperator(OptimalFillOperator());
-    aDrawable->Draw(tmpCtx, needed - needed.TopLeft(), true,
-                    gfxPattern::FILTER_FAST, gfxMatrix().Translate(needed.TopLeft()));
+    nsRefPtr<gfxImageSurface> image = aDrawable->GetAsImageSurface();
+    if (image && gfxRect(0, 0, image->GetSize().width, image->GetSize().height).Contains(needed)) {
+      temp = image->GetSubimage(needed);
+    } else {
+      temp =
+          gfxPlatform::GetPlatform()->CreateOffscreenSurface(size, gfxASurface::ContentFromFormat(aFormat));
+      if (!temp || temp->CairoStatus())
+          return nullptr;
+
+      nsRefPtr<gfxContext> tmpCtx = new gfxContext(temp);
+      tmpCtx->SetOperator(OptimalFillOperator());
+      aDrawable->Draw(tmpCtx, needed - needed.TopLeft(), true,
+                      gfxPattern::FILTER_FAST, gfxMatrix().Translate(needed.TopLeft()));
+    }
 
     nsRefPtr<gfxDrawable> drawable = 
         new gfxSurfaceDrawable(temp, size, gfxMatrix().Translate(-needed.TopLeft()));