This directory contains implementation of painters of layout objects. It covers the following document lifecycle states:
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.
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.
Paint invalidation container: the nearest object on the compositing container chain which is composited.
Paint invalidation marks anything that need to be painted differently from the original cached painting.
Though described in this document, most of the actual paint invalidation code is under
Paint invalidation is a document cycle stage after compositing update and before paint. During the previous stages, objects are marked for needing paint invalidation checking if needed by style change, layout change, compositing change, etc. In paint invalidation stage, we traverse the layout tree in pre-order, crossing frame boundaries, for marked subtrees and objects and send the following information to
PaintInvalidationState is an optimization used during the paint invalidation phase. Before the paint invalidation tree walk, a root
PaintInvalidationState is created for the root
LayoutView. During the tree walk, one
PaintInvalidationState is created for each visited object based on the
PaintInvalidationState passed from the parent object. It tracks the following information to provide O(1) complexity access to them if possible:
Paint invalidation container: Since as indicated by the definitions in [Glossaries](#Other glossaries), the paint invalidation container for stacked objects can differ from normal objects, we have to track both separately. Here is an example:
<div style="overflow: scroll"> <div id=A style="position: absolute"></div> <div id=B></div> </div>
If the scroller is composited (for high-DPI screens for example), it is the paint invalidation container for div B, but not A.
Paint offset and clip rect: if possible,
PaintInvalidationState accumulates paint offsets and overflow clipping rects from the paint invalidation container to provide O(1) complexity to map a point or a rect in current object‘s local space to paint invalidation container’s space. Because locations of objects are determined by their containing blocks, and the containing block for absolute-position objects differs from non-absolute, we track paint offsets and overflow clipping rects for absolute-position objects separately.
In cases that accurate accumulation of paint offsets and clipping rects is impossible, we will fall back to slow-path using
LayoutObject::mapToVisualRectInAncestorSpace(). This includes the following cases:
An object has transform related property, is multi-column or has flipped blocks writing-mode, causing we can't simply accumulate paint offset for mapping a local rect to paint invalidation container;
An object has has filter or reflection, which needs to expand paint invalidation rect for descendants, because currently we don't include and reflection extents into visual overflow;
For a fixed-position object we calculate its offset using
LayoutObject::localToAncestorPoint(), but map for its descendants in fast-path if no other things prevent us from doing this;
Because we track paint offset from the normal paint invalidation container only, if we are going to use
m_paintInvalidationContainerForStackedContents and it's different from the normal paint invalidation container, we have to force slow-path because the accumulated paint offset is not usable;
We also stop to track paint offset and clipping rect for absolute-position objects when
m_paintInvalidationContainerForStackedContents becomes different from
Texts are 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
FIRST_LINE_INHERITED pseudo ID.
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.
TODO(wangxianzhu): add details
PrePaintTreeWalk(Slimming paint v2 only)
InPrePaint document lifecycle state, this class is called to walk the whole layout tree, beginning from the root FrameView, across frame boundaries. We do the following during the tree walk:
Building paint property tree: creates paint property tree nodes for special things in the layout tree, including but not limit to: overflow clip, transform, fixed-pos, animation, mask, filter, etc.
Paint invalidation: Not implemented yet. TODO(wangxianzhu): add details after it's implemented.
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 enclose the display items that
PaintLayerPainter::paintContents() generates (including display items generated by sublayers) in a pair of
BeginSubsequence/EndSubsequence display items.
In a subsequence paint, if the layer would generate exactly the same display items, we'll get the whole subsequence from the cache instead of repainting them.
There are many conditions affecting
PaintLayerPainter.cppfor 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.
It‘s hard to clear a
needsPaintPhaseXXX flag when a layer no longer needs the paint phase, so we never clear the flags. Instead, we use another set of flags (
previousPaintPhaseXXXWasEmpty) to record if a painting of a phase actually produced nothing. We’ll skip the next painting of the phase if the flag is set, regardless of the corresponding
needsPaintPhaseXXX flag. We will clear the
previousPaintPhaseXXXWasEmpty flags when we paint with different clipping, scroll offset or interest rect from the previous paint.
We don‘t clear the
previousPaintPhaseXXXWasEmpty flags when the layer is marked
needsRepaint. Instead we clear the flag when the corresponding
needsPaintPhaseXXX is set. This ensures that we won’t clear
previousPaintPhaseXXXWasEmpty flags when unrelated things changed which won't cause the paint phases to become non-empty.
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.
We could update the
needsPaintPhaseXXX flags in a separate tree walk, but that would regress performance of the first paint. For slimming paint v2, we can update the flags during the pre-painting tree walk to simplify the logics.