Bug 297706 – [TextureMapper] wrong drop-shadow position in high DPI

• Status: NEW
• Component: Platform

[Fujii Hironori]

[GTK] wrong drop-shadow position in high DPI

./Tools/Scripts/run-minibrowser --gtk --release LayoutTests/css3/filters/drop-shadow-current-color.html

EXPECTED: you should see 8 square boxes
ACTUAL: you see 4 rectangles

GTK WebKitTestRunner works as expected.

[Fujii Hironori]

Disabling the AC mode work around the problem. I can see 8 boxes. But, worng color, this is another bug.

WEBKIT_DISABLE_COMPOSITING_MODE=1 ./Tools/Scripts/run-minibrowser --gtk --release LayoutTests/css3/filters/drop-shadow-current-color.html

[Fujii Hironori]

Created attachment 476470 [details]
test case

[Fujii Hironori]

TextureMapper expects DIP (device independent pixel).
But, GraphicsLayerCoordinated uses DP (device pixel).

For example, there is 100px x 100px element in x2 device scale factor.

GraphicsLayerTextureMapper and GraphicsLayerWC creates a 100x100 TextureMapperLayer.
And, apply x2 scale transform to the root layer.

OTOH, GraphicsLayerCoordinated creates a 200x200 TextureMapperLayer.

TextureMapper::applyDropShadowFilter apply the drop-shadow filter.
https://github.com/WebKit/WebKit/blob/5701312dba4c15a1c46bf57662d1638eda398100/Source/WebCore/platform/graphics/texmap/TextureMapper.cpp#L1064

LayerTreeHost::applyDeviceScaleFactor and DrawingAreaWC::updateRootLayerDeviceScaleFactor apply a transform to the root layer.
https://github.com/WebKit/WebKit/blob/5701312dba4c15a1c46bf57662d1638eda398100/Source/WebKit/WebProcess/WebPage/CoordinatedGraphics/LayerTreeHostTextureMapper.cpp#L295
https://github.com/WebKit/WebKit/blob/5701312dba4c15a1c46bf57662d1638eda398100/Source/WebKit/WebProcess/WebPage/wc/DrawingAreaWC.cpp#L309

[Fujii Hironori]

I was wrong. GraphicsLayerCoordinated is using DIP.

[Fujii Hironori]

I confirmed Windows port has the same issue. This is a bug of TextureMapper.

[Fujii Hironori]

I think this is a the same problem with bug#218734.