Apply tranformation to pattern locally to fix some layout tests....

WebCore/platform/graphics/skia/PatternSkia.cpp

Index: WebCore/platform/graphics/skia/PatternSkia.cpp
===================================================================
--- WebCore/platform/graphics/skia/PatternSkia.cpp	(revision 11353)
+++ WebCore/platform/graphics/skia/PatternSkia.cpp	(working copy)
@@ -40,38 +40,63 @@
 
 PlatformPatternPtr Pattern::createPlatformPattern(const TransformationMatrix& patternTransform) const
 {
-    // Note: patternTransform is ignored since it seems to be applied elsewhere
-    // (when the pattern is used?). Applying it to the pattern (i.e.
-    // shader->setLocalMatrix) results in a double transformation. This can be
-    // seen, for instance, as an extra offset in:
-    // LayoutTests/fast/canvas/patternfill-repeat.html
-    // and expanded scale and skew in:
-    // LayoutTests/svg/W3C-SVG-1.1/pservers-grad-06-b.svg
+    // Note: |patternTransform| is ignored since it is a global transformation 
+    // applied by the GraphicsContext. Pattern-specific transformation is stored
+    // in |m_patternSpaceTransformation|.
 
     SkBitmap* bm = m_tileImage->nativeImageForCurrentFrame();
-    if (m_repeatX && m_repeatY)
-        return SkShader::CreateBitmapShader(*bm, SkShader::kRepeat_TileMode, SkShader::kRepeat_TileMode);
+    SkShader* shader = NULL;
 
-    // Skia does not have a "draw the tile only once" option. Clamp_TileMode
-    // repeats the last line of the image after drawing one tile. To avoid
-    // filling the space with arbitrary pixels, this workaround forces the
-    // image to have a line of transparent pixels on the "repeated" edge(s),
-    // thus causing extra space to be transparent filled.
-    SkShader::TileMode tileModeX = m_repeatX ? SkShader::kRepeat_TileMode : SkShader::kClamp_TileMode;
-    SkShader::TileMode tileModeY = m_repeatY ? SkShader::kRepeat_TileMode : SkShader::kClamp_TileMode;
-    int expandW = m_repeatX ? 0 : 1;
-    int expandH = m_repeatY ? 0 : 1;
+    if (m_repeatX && m_repeatY) {
+        shader = SkShader::CreateBitmapShader(*bm, SkShader::kRepeat_TileMode, SkShader::kRepeat_TileMode);
+    } else {
+        // Skia does not have a "draw the tile only once" option. Clamp_TileMode
+        // repeats the last line of the image after drawing one tile. To avoid
+        // filling the space with arbitrary pixels, this workaround forces the
+        // image to have a line of transparent pixels on the "repeated" edge(s),
+        // thus causing extra space to be transparent filled.
+        SkShader::TileMode tileModeX = m_repeatX ? SkShader::kRepeat_TileMode : SkShader::kClamp_TileMode;
+        SkShader::TileMode tileModeY = m_repeatY ? SkShader::kRepeat_TileMode : SkShader::kClamp_TileMode;
+        int expandW = m_repeatX ? 0 : 1;
+        int expandH = m_repeatY ? 0 : 1;
 
-    // Create a transparent bitmap 1 pixel wider and/or taller than the
-    // original, then copy the orignal into it.
-    // FIXME: Is there a better way to pad (not scale) an image in skia?
-    SkBitmap bm2;
-    bm2.setConfig(bm->config(), bm->width() + expandW, bm->height() + expandH);
-    bm2.allocPixels();
-    bm2.eraseARGB(0x00, 0x00, 0x00, 0x00);
-    SkCanvas canvas(bm2);
-    canvas.drawBitmap(*bm, 0, 0);
-    return SkShader::CreateBitmapShader(bm2, tileModeX, tileModeY);
+        // Create a transparent bitmap 1 pixel wider and/or taller than the
+        // original, then copy the orignal into it.
+        // FIXME: Is there a better way to pad (not scale) an image in skia?
+        SkBitmap bm2;
+        bm2.setConfig(bm->config(), bm->width() + expandW, bm->height() + expandH);
+        bm2.allocPixels();
+        bm2.eraseARGB(0x00, 0x00, 0x00, 0x00);
+        SkCanvas canvas(bm2);
+        canvas.drawBitmap(*bm, 0, 0);
+        shader = SkShader::CreateBitmapShader(bm2, tileModeX, tileModeY);
+    }
+
+    // The local transform is saved in m_patternSpaceTransformation, and we need
+    // to apply it to the pattern. But Skia seems to have a problem with
+    // positive translations in tile mode, so adjust the translation to an
+    // equivalent negative value if we are in tile mode.
+    // For example:
+    //   transform.e() == 56  (X translation)
+    //   bm->width() == 20    (Pattern width)
+    // To make it look the same, we can have a X translation of -6, -26, etc.
+    // An equivalent negative translation would be 56 less some multiple of 20,
+    // e.g. -6, -26, -46. We can obtain the first such number that satisfies
+    // this property by doing 56 - 3 * 20 = -6. The value of such multiple is
+    // obtained by ceil(56 / 20) = 3.
+    double translateX = 0.0;
+    double translateY = 0.0;
+    TransformationMatrix transform = m_patternSpaceTransformation;
+    if (m_repeatX && bm->width() && transform.e() > 0.0) {
+      translateX = -bm->width() * ceil(transform.e() / bm->width());
+    }
+    if (m_repeatY && bm->height() && transform.f() > 0.0) {
+      translateY = -bm->height() * ceil(transform.f() / bm->height());
+    }
+    // Note that transform.translate(x, y) is to apply translation to the
+    // current matrix, not setting the translation.
+    shader->setLocalMatrix(transform.translate(translateX, translateY));
+    return shader;
 }
 
 } // namespace WebCore