The code in this directory converts the LayoutObject tree into an efficient rendering format for the compositor (a list of cc::Layers containing display item lists, and associated cc::PropertyTrees). For a high level overview, see the Overview section.
For information about how the display list and paint property trees are implemented, see the platform paint README file.
This code is owned by the rendering team.
This chapter is basically a clarification of CSS 2.1 appendix E. Elaborate description of Stacking Contexts.
Note: we use ‘element’ instead of ‘object’ in this chapter to keep consistency with the spec. We use ‘object’ in other places in this document.
According to the documentation, we can have the following types of elements that are treated in different ways during painting:
Stacked objects: objects that are z-ordered in stacking contexts, including:
Stacking contexts: elements with non-auto z-indices or other properties that affect stacking e.g. transform, opacity, blend-mode.
Replaced normal-flow stacking elements: replaced elements that do not have non-auto z-index but are stacking contexts for elements below them. Right now the only example is SVG
<foreignObject>. The difference between these elements and regular stacking contexts is that they paint in the foreground phase of the painting algorithm (as opposed to the positioned descendants phase).
Elements that are not real stacking contexts but are treated as stacking contexts but don't manage other stacked elements. Their z-ordering are managed by real stacking contexts. They are positioned elements with
z-index: auto (E.2.8 in the documentation).
They must be managed by the enclosing stacking context as stacked elements because
z-index:0 are considered equal for stacking context sorting and they may interleave by DOM order.
The difference of a stacked element of this type from a real stacking context is that it doesn't manage z-ordering of stacked descendants. These descendants are managed by the parent stacking context of this stacked element.
“Stacked element” is not defined as a formal term in the documentation, but we found it convenient to use this term to refer to any elements participating z-index ordering in stacking contexts.
A stacked element is represented by a
PaintLayerStackingNode associated with a
PaintLayer. It‘s painted as self-painting
PaintLayerPainter by executing all of the steps of the painting algorithm explained in the documentation for the element. When painting a stacked element of the second type, we don’t paint its stacked descendants which are managed by the parent stacking context.
Non-stacked pseudo stacking contexts: elements that are not stacked, but paint their descendants (excluding any stacked contents) as if they created stacking contexts. This includes
They are painted by
ObjectPainter::paintAllPhasesAtomically() which executes all of the steps of the painting algorithm explained in the documentation, except ignores any descendants which are positioned or have non-auto z-index (which is achieved by skipping descendants with self-painting layers).
Other normal elements.
Paint container: the parent of an object for painting, as defined by CSS2.1 spec for painting. For regular objects, this is the parent in the DOM. For stacked objects, it's the containing stacking context-inducing object.
Paint container chain: the chain of paint ancestors between an element and the root of the page.
Compositing container: an implementation detail of Blink, which uses
PaintLayers to represent some layout objects. It is the ancestor along the paint ancestor chain which has a PaintLayer. Implemented in
PaintLayer::compositingContainer(). Think of it as skipping intermediate normal objects and going directly to the containing stacked object.
Compositing container chain: same as paint chain, but for compositing container.
Visual rect: the bounding box of all pixels that will be painted by a for a display item It‘s in the space of the containing transform property node (see Building paint property trees). It’s calculated during paint for each display item.
Isolation nodes/boundary: In certain situations, it is possible to put in place a barrier that isolates a subtree from being affected by its ancestors. This barrier is called an isolation boundary and is implemented in the property trees as isolation nodes that serve as roots for any descendant property nodes. Currently, the
contain: paint css property establishes an isolation boundary.
Local property tree state: the
PropertyTreeState associated with each fragment. All fragments have a well-defined local property tree state. This is often cached in the
LocalBorderBoxProperties struct that belongs to
FragmentData objects don't have a
LocalBorderBoxProperties, but that is merely a memory optimization.
The primary responsibility of this directory is to convert the outputs from layout (the
LayoutObject tree) to the inputs of the compositor (the
cc::Layer list, which contains display items, and the associated
This process is done in the following document lifecycle phases:
Debugging blink objects has information about dumping the paint and compositing datastructures for debugging.
The process starts with pre-paint to generate property trees. During paint, each generated display item will be associated with a property tree state. Adjacent display items having the same property tree state will be grouped as
PaintChunk. The list of paint chunks then will be processed by
PaintArtifactCompositor for layerization. Property nodes that will be composited are converted into cc property nodes, while non-composited property nodes are converted into meta display items by
from layout | v +------------------------------+ | LayoutObject/PaintLayer tree | +------------------------------+ | | | | PrePaintTreeWalk::Walk() | | PaintPropertyTreeBuider::UpdatePropertiesForSelf() | v | +--------------------------------+ |<--| Property trees | | +--------------------------------+ | | | LocalFrameView::PaintTree() | | FramePainter::Paint() | | PaintLayerPainter::Paint() | | ObjectPainter::Paint() | v | +---------------------------------+ | | DisplayItemList/PaintChunk list | | +---------------------------------+ | | | |<---------------------------------+ | LocalFrameView::PushPaintArtifactToCompositor() | PaintArtifactCompositor::Update() | +---+---------------------------------+ | v | | +----------------------+ | | | Chunk list for layer | | | +----------------------+ | | | | | | PaintChunksToCcLayer::Convert() | v v v +----------------+ +-----------------------+ | cc::Layer list | | cc property trees | +----------------+ +-----------------------+ | | +------------------+ | to compositor v
Debugging blink objects has information about dumping these paint and compositing datastructures for debugging.
InPrePaint document lifecycle state, this class is called to walk the whole layout tree, beginning from the root FrameView, and across frame boundaries. This is an in-order tree traversal which is important for efficiently computing DOM-order hierarchy such as the parent containing block.
Paint invalidator marks anything that need to be painted differently from the original cached painting.
During the document lifecycle stages prior to PrePaint, objects are marked for needing paint invalidation checking if needed by style change, layout change, compositing change, etc. In PrePaint stage, we traverse the layout tree in pre-order, crossing frame boundaries, for marked subtrees and objects and invalidate display item clients that will generate different display items.
At the beginning of the PrePaint tree walk, a root
PaintInvalidatorContext is created for the root
LayoutView. During the tree walk, one
PaintInvalidatorContext is created for each visited object based on the
PaintInvalidatorContext passed from the parent object. It tracks the painting layer which will initiate painting of the current object.
PaintInvalidatorContext for the current object, then calls
LayoutObject::InvalidatePaint() which calls the object's paint invalidator (e.g.
BoxPaintInvalidator) to complete paint invalidation of the object.
Text is painted by
InlineTextBox as display item client. Text backgrounds and masks are painted by
InlineFlowBox as display item client. We should invalidate these display item clients when their painting will change.
LayoutTexts are marked for full paint invalidation if needed when new style is set on them. During paint invalidation, we invalidate the
InlineFlowBoxs directly contained by the
InlineTextBoxs contained by the
LayoutText::InvalidateDisplayItemClients(). We don't need to traverse into the subtree of
LayoutInline::InvalidateDisplayItemClients() because the descendant
InlineTextBoxs will be handled by their owning
LayoutTexts, respectively, when changed style is propagated.
::first-line pseudo style dynamically applies to all
InlineBox's in the first line in the block having
::first-line style. The actual applied style is computed from the
::first-line style and other applicable styles.
If the first line contains any
LayoutInline, we compute the style from the
::first-line style and the style of the
LayoutInline and apply the computed style to the first line part of the
LayoutInline. In Blink's style implementation, the combined first line style of
LayoutInline is identified with
The normal paint invalidation of texts doesn't work for first line because:
ComputedStyle::VisualInvalidationDiff()can't detect first line style changes;
We have a special path for first line style change: the style system informs the layout system when the computed first-line style changes through
LayoutObject::FirstLineStyleDidChange(). When this happens, we invalidate all
InlineBoxes in the first line.
This class is responsible for building property trees (see platform/paint/README.md for information about what property trees are).
LayoutObject has one or more
FragmentData objects (see below for more on fragments). Every
FragmentData has an
ObjectPaintProperties object if any property nodes are induced by it. For example, if the object has a transform, its
ObjectPaintProperties::Transform() field points at the
TransformPaintPropertyNode representing that transform.
DescendantNeedsPaintPropertyUpdate dirty bits on
LayoutObject control how much of the layout tree is traversed during each
Additionally, some dirty bits are cleared at an isolation boundary. For example if the paint property tree topology has changed by adding or removing nodes for an element, we typically force a subtree walk for all descendants since the descendant nodes may now refer to new parent nodes. However, at an isolation boundary, we can reason that none of the descendants of an isolation element would be affected, since the highest node that the paint property nodes of an isolation element's subtree can reference are the isolation nodes established at this element itself.
Implementation note: the isolation boundary is achieved using alias nodes, which are nodes that are put in place on an isolated element for clip, transform, and effect trees. These nodes do not themselves contribute to any painted output, but serve as parents to the subtree nodes. The alias nodes and isolation nodes are synonymous and are used interchangeably. Also note that these nodes are placed as children of the regular nodes of the element. This means that the element itself is not isolated against ancestor mutations; it only isolates the element's subtree.
+----------------------+ | 1. Root LayoutObject | +----------------------+ / \ +-----------------+ +-----------------+ | 2. LayoutObject | | 3. LayoutObject | +-----------------+ +-----------------+ / / \ +-----------------+ +-----------------+ +-----------------+ | 4. LayoutObject | | 5. LayoutObject | | 6. LayoutObject | +-----------------+ +-----------------+ +-----------------+ / \ +-----------------+ +-----------------+ | 7. LayoutObject | | 8. LayoutObject | +-----------------+ +-----------------+
Suppose that element 3's style changes to include a transform (e.g.
Typically, here is the order of the walk (depth first) and updates:
Now suppose that element 5 has “contain: paint” style, which establishes an isolation boundary. The walk changes in the following way:
Note that there are subtleties when deciding whether we can skip the subtree walk. Specifically, not all subtree walks can be stopped at an isolation boundary. For more information, see
PaintPropertyTreeBuilder and its use of IsolationPiercing vs IsolationBlocked subtree update reasons.
In the absence of multicolumn/pagination, there is a 1:1 correspondence between
FragmentData. If there is multicolumn/pagination, there may be more
FragmentDatas. If a
LayoutObject has a property node, each of its fragments will have one. The parent of a fragment‘s property node is the property node that belongs to the ancestor
LayoutObject which is part of the same column. For example, if there are 3 columns and both a parent and child
LayoutObject have a transform, there will be 3
FragmentData objects for the parent, 3 for the child, each
FragmentData will have its own
TransformPaintPropertyNode, and the child’s ith fragment‘s transform will point to the ith parent’s transform.
FragmentData receives its own
ClipPaintPropertyNode. They also store a unique
LayoutMultiColumnFlowThread.h for a much more detail about multicolumn/pagination.
Paint walks the LayoutObject tree in paint-order and produces a list of display items. This is implemented using static painter classes (e.g.,
BlockPainter) and appends display items to a
PaintController. There is only one
PaintController for the entire
LocalFrameView. During this treewalk, the current property tree state is maintained (see:
PaintController segments the display item list into
PaintChunks which are sequential display items that share a common property tree state.
PaintController holds the previous painting result as a cache of display items. If some painter would generate results same as those of the previous painting, we'll skip the painting and reuse the display items from cache.
When a painter would create a
DrawingDisplayItem exactly the same as the display item created in the previous painting, we'll reuse the previous one instead of repainting it.
When possible, we create a scoped
PaintLayerPainter::PaintContents() to record all display items generated in the scope as a “subsequence”. Before painting a layer, if we are sure that the layer will generate exactly the same display items as the previous paint, we'll get the whole subsequence from the cache instead of repainting them.
There are many conditions affecting whether we need to generate subsequence for a PaintLayer and whether we can use cached subsequence for a PaintLayer. See
PaintLayerPainter.cpp for the conditions.
During painting, we walk the layout tree multiple times for multiple paint phases. Sometimes a layer contain nothing needing a certain paint phase and we can skip tree walk for such empty phases. Now we have optimized
PaintPhaseFloat for empty paint phases.
During paint invalidation, we set the containing self-painting layer's
NeedsPaintPhaseXXX flag if the object has something needing to be painted in the paint phase.
During painting, we check the flag before painting a paint phase and skip the tree walk if the flag is not set.
When layer structure changes, and we are not invalidate paint of the changed subtree, we need to manually update the
NeedsPaintPhaseXXX flags. For example, if an object changes style and creates a self-painting-layer, we copy the flags from its containing self-painting layer to this layer, assuming that this layer needs all paint phases that its container self-painting layer needs.
Hit testing is done in paint-order, and to preserve this information the paint system is re-used to record hit test information when painting the background. This information is then used in the compositor to implement cc-side hit testing. Hit test information is recorded even if there is no painted content.
We record different types of hit test information in the following data structures:
Paint chunk bounds
The bounds of the current paint chunk are expanded to ensure the bounds contain the hit testable area.
Used for touch action rects which are areas of the page that allow certain gesture effects, as well as areas of the page that disallow touch events due to blocking touch event handlers.
Used for wheel event handler regions which are areas of the page that disallow default wheel event processing due to blocking wheel event handlers.
Used to create non-fast scrollable regions to prevent compositor scrolling of non-composited scrollers, plugins with blocking scroll event handlers, and resize handles.
scroll_translation is not null, this is also used to force a special cc::Layer that is marked as being scrollable when composited scrolling is needed for the scroller.
During painting, for a non-custom scrollbar we create a ScrollbarDisplayItem which contains a cc::Scrollbar and other information that are needed to actually paint the scrollbar into a paint record or to create a cc scrollbar layer. During PaintArtifactCompositor update, we decide whether to composite the scrollbar and, if not composited, actually paint the scrollbar as a paint record, otherwise create a cc scrollbar layer of type cc::SolidColorScrollbarLayer, cc::PaintedScrollbarLayer or cc::PaintedOverlayScrollbarLayer depending on the type of the scrollbar.
Custom scrollbars are still painted into drawing display items directly.
LayoutNG is a project that will change how Layout generates geometry/style information for painting. Instead of modifying LayoutObjects, LayoutNG will generate an NGFragment tree.
The goal is for PaintNG to eventually paint from NGFragment tree, and not see LayoutObjects at all. Until this goal is reached, LegacyPaint, and NGPaint will coexist.
When a particular LayoutObject subclass fully migrates to NG, its LayoutObject geometry information might no longer be updated(*), and its painter needs to be rewritten to paint NGFragments. For example, see how BlockPainter is being rewritten as NGBoxFragmentPainter.