Hello! I'm trying to implement this CSS value on the Cairo backend and I'm stuck with a problem regarding the Porter-Duff composition operators (why is CAIRO_OPERATOR_DEST_IN unbound?). I've made the following changes to reach this point:
1. I implemented a version of GraphicsContext::clipToImageBuffer starting from this patch: https://bugs.webkit.org/show_bug.cgi?id=23526 , but saving the image mask and the masking rect on a stack and pushing only a group for the masked content. 2. In GraphicsContext::restorePlatformState when the save count is 0 for the current mask, I recover the masked content as a pattern, I clip it using the mask from the stack (I used a new group and operator CAIRO_OPERATOR_IN) and then I should apply the result on the destination surface using CAIRO_OPERATOR_DEST_IN, but bounded to the current mask. The problem here is that CAIRO_OPERATOR_DEST_IN is unbounded so it doesn't take into account the mask, thus affecting all the surface. 3. I modify RenderBoxModelObject::paintFillLayerExtended so that when drawing the background image to use the bgLayer->composite() operator if the clip is TextFillBox. Using CompositeSourceIn can't work because clipToImageBuffer started a new group which uses a different surface for drawing (so there is no destination text to composite with). This way the composition will be done in restorePlatformState. I know this is ugly and that it's highly unlikely to work in this form with SVG masking. For now I just need a hook to make this work. Do you have any other suggestions to make this CSS value work with Cairo? Thanks, Andrei.
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