Featherweight Musings

Tuesday, May 29, 2012

Building a mask layer

Mask layers are built in layout/base/FrameLayerBuilder.cpp. Most of the work constructing the mask layer is done in SetupMaskLayer (which I guess should actually be called SetupMaskLayerForRoundedRectClip or something). But before that, we have to figure out if making a mask layer makes sense or not.

This is easy for image, colour, and container layers. But for Thebes layers it is more complicated, there can be multiple display items in a single Thebes layer, and we can only create a mask for the rounded rect clips that all items have in common. In practice, this is usually one clip, and often when there is only one item in the layer, but we cope with more. As items are added to a Thebes layer we keep track of the number of rounded rect clips in common using ThebesLayerData::mCommonClipCount, mostly in ThebesLayerData::UpdateCommonClipCount. We keep the clip for some item in the layer in UpdateCommonClipCount::mItemClip, the first mCommonClipCount clips in this clip are common to all items in the layer (we only find common clips if they are the first for each item). As we add items to the layer, their clips are compared to mItemClip, and mCommonClipCount is updated as necessary.

When Thebes layers are popped from the stack (i.e., we are done building), then mCommonClipCount is moved to ThebesLayerEntry::mCommonClipCount (it must be initialised, i.e., greater than zero by now). We create a mask layer for the first mCommonClipCount clipping rects. When the thebes layer is drawn, we only clip for clipping rounded rects other than the common ones.

FrameLayerBuilder has a neat way of recycling layers, this is a combination of caching and memory management: we recycle and create layers in order and if we get back the same layer as we had last time round (common case) we don't bother to rebuild it, but leave the layer as is. We do a similar thing with mask layers. This is a bit complicated because we only guarantee that layers of the same type are recycled in order, so we need to keep lists of mask layers for each type of layer. We store information about the clips we are masking for in the user data on the mask layer, this is checked in SetupMaskLayer to see if we have to build the mask fresh, or if we can get away with re-using the old version. Thus, we avoid building the mask too often.

When the mask is built in FrameLayerBuilder.cpp, we create a mask surface which is the same size as the masked layer's visible region. We paint the clip into this surface, but only the relevant bits of the clip will be inside the surface. So, the mask is in it's own coordinate system, which is relative to the masked layer's visible region. We create a transform for the mask which transforms the mask's coordinate system to the masked layer's coordinate system. (Note, I'm currently changing this stuff, so it might not be true for long). After the layer tree is built, it is walked to compute the effective transform of each layer (this means different things for the different backends). We updated this walk to also calculate an effective transform for mask layers. So, when we come to render the layer tree, the effective transform for the mask layer is the one that we need for drawing the mask layer 'on top of' the masked layer's visible region, however the masked layer is rendered (I'll describe the transform in more detail in later posts).

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