CLIPPING_AND_POSITIONING.md

Original Design

To understand the current design for clipping and positioning (transformations and scrolling) in WebRender it can be useful to have a little background about the original design for these features. The most important thing to remember is that originally clipping, scrolling regions, and transformations were properties of stacking contexts and they were completely hierarchical. This goes a long way toward representing the majority of CSS content on the web, but fails when dealing with important edges cases and features including:

1. Support for sticky positioned content
2. Scrolling areas that include content that is ordered both above and below intersecting content from outside the scroll area.
3. Items in the same scrolling root, clipped by different clips one or more of which are defined outside the scrolling root itself.
4. Completely non-hierarchical clipping situations, such as when items are clipped by some clips in the hierarchy, but not others.

Design changes have been a step by step path from the original design to one that can handle all CSS content.

Current Design

All positioning and clipping is handled by the ClipScrollTree. The name is a holdover from when this tree was a tree of Layers which handled both positioning and clipping. Currently the ClipScrollTree holds:

1. A hierarchical collection of SpatialNodes, with the final screen transformation of each node depending on the relative transformation of the node combined with the transformations of all of its ancestors. These nodes are responsible for positioning display list items and clips.
2. A collection of ClipNodes which specify a rectangular clip and, optionally, a set of rounded rectangle clips and a masking image.
3. A collection of ClipChains. Each ClipChain is a list of ClipNode elements. Every display list item has an assigned ClipChain which specifies what ClipNodes are applied to that item.

The SpatialNode of each clip applied to an item is completely independent of the SpatialNode applied to the item itself.

One holdover from the previous design is that both ClipNode and SpatialNodes have a parent node, which is either a SpatialNode or a ClipNode. From this node WebRender can determine both a parent ClipNode and a parent SpatialNode by finding the first ancestor of that type. This is handled by the DisplayListFlattener.

SpatialNode

There are three types of SpatialNodes:

1. Reference frames which are created when content needs to apply transformation or perspective properties to display list items. Reference frames establish a new coordinate system, so internally all coordinates on display list items are relative to the reference frame origin. Later any non-reference frame positioning nodes that display list items belong to can adjust this position relative to the reference frame origin.
2. Scrolling nodes are used to define scrolling areas. These nodes have scroll offsets which are a 2D translation relative to ancestor nodes and, ultimately, the reference frame origin.
3. Sticky frames are responsible for implementing position:sticky behavior. This is also an 2D translation.

SpatialNodes are defined as items in the display list. After display list "flattening" each node is traversed hierarchically during the ClipScrollTree::update() step. Once reference frame transforms and relative offsets are calculated, a to screen space transformation can be calculated for each SpatialNode. This transformation is added the TransformPalette and becomes directly available to WebRender shaders.

In addition to screen space transformation calculation, the SpatialNode tree is divided up into compatible coordinate systems. These are coordinate systems which differ only by 2D translations from their parent system. These compatible coordinate systems may even cross reference frame boundaries. The goal here is to allow the application clipping rectangles from different compatible coordinate systems without generating mask images.

ClipNode

Each clip node holds a clip rectangle along with an optional collection of rounded clip rectangles and a mask image. The fact that ClipNodes all have a clip rectangle is important because it means that all content clipped by a clip node has a bounding rectangle, which can be converted into a bounding screen space rectangle. This rectangle is called the outer rectangle of the clip. ClipNodes may also have an inner rectangle, which is an area within the boundaries of the outer rectangle that is completely unclipped.

These rectangles are calculated during the ClipScrollTree::update() phase. In addition, each ClipNode produces a template ClipChainNode used to build the ClipChains which use that node.

ClipChains

There are two ways that ClipChains are defined in WebRender. The first is through using the API for manually specifying ClipChains via a parent ClipChain and a list of ClipNodes. The second is through the hierarchy of a ClipNode established by its parent node. Every ClipNode has a chain of ancestor SpatialNodes and ClipNodes. The creation of a ClipNode automatically defines a ClipChain for this hierarchy. This behavior is a compatibility feature with the old completely hierarchical clipping architecture and is still how Gecko and Servo create most of their ClipChains. These hierarchical ClipChains are constructed during the ClipNode::update() step.

During ClipChain construction, WebRender tries to eliminate clips that will not affect rendering, by looking at the combined outer rectangle and inner rectangle of a ClipChain and the outer rectangle and inner rectangle of any ClipNode appended to the chain. An example of the goal of this process is to avoid having to render a mask for a large rounded rectangle when the rest of the clip chain constrains the content to an area completely inside that rectangle. Avoiding mask rasterization in this case and others has large performance impacts on WebRender.

Clipping and Positioning in the Display List

Each non-structural WebRender display list item has

• A SpatialId of a SpatialNode for positioning
• A ClipId of a ClipNode or a ClipChain for clipping
• An item-specific rectangular clip rectangle

The positioning node determines how that item is positioned. It's assumed that the positioning node and the item are children of the same reference frame. The clipping node determines how that item is clipped. This should be fully independent of how the node is positioned and items can be clipped by any ClipChain regardless of the reference frame of their member clips. Finally, the item-specific clipping rectangle is applied directly to the item and should never result in the creation of a clip mask itself.

Converting user-exposed ClipId/SpatialId to internal indices

WebRender must access ClipNodes and SpatialNodes quite a bit when building scenes and frames, so it tries to convert ClipId/SpatialId, which are already per-pipeline indices, to global scene-wide indices. Internally this is a conversion from ClipId into ClipNodeIndex or ClipChainIndex, and from SpatialId into SpatialNodeIndex. In order to make this conversion cheaper, the DisplayListFlattner assigns offsets for each pipeline and node type in the scene-wide ClipScrollTree.

Nodes are added to their respective arrays sequentially as the display list is processed during "flattening." When encountering an iframe, the DisplayListFlattener must start processing the nodes for that iframe's pipeline, meaning that nodes are now being added out of order to the node arrays of the ClipScrollTree. In this case, the ClipScrollTree fills in the gaps in the node arrays with placeholder nodes.

Hit Testing

Hit testing is the responsibility of the HitTester data structure. This structure copies information necessary for hit testing from the ClipScrollTree. This is done so that hit testing can still take place while a new ClipScrollTree is under construction.

Ideas for the Future

1. Expose the difference between ClipId and ClipChainId in the API.
2. Prevent having to duplicate the ClipScrollTree for hit testing.
3. Avoid having to create placeholder nodes in the ClipScrollTree while processing iframes.