Source/core/paintThis directory contains implementation of painters of layout objects.
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.
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:auto and 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 PaintLayers by 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 Source/core/layout.
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 GraphicsLayers and PaintControllers:
PaintInvalidationStatePaintInvalidationState 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::localToAncestorPoint() or 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 m_paintInvalidationContainer.
Texts are painted by InlineTextBoxPainter using InlineTextBox as display item client. Text backgrounds and masks are painted by InlineTextFlowPainter using InlineFlowBox as display item client. We should invalidate these display item clients when their painting will change.
LayoutInlines and 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 LayoutInline in LayoutInline::invalidateDisplayItemClients() and InlineTextBoxs contained by the LayoutText in LayoutText::invalidateDisplayItemClients(). We don't need to traverse into the subtree of InlineFlowBoxs in LayoutInline::invalidateDisplayItemClients() because the descendant InlineFlowBoxs and InlineTextBoxs will be handled by their owning LayoutInlines and LayoutTexts, respectively, when changed style is propagated.
::first-line::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
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.
We'll create a CachedDisplayItem when a painter would create a DrawingDisplayItem exactly the same as the display item created in the previous painting. After painting, PaintController will replace CachedDisplayItem with the corresponding display item from the cache.
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 simply output a CachedSubsequence display item in place of the display items, and skip all paintings of the layer and its descendants in painting order. After painting, PaintController will replace CacheSubsequence with cached display items created in previous paintings.
There are many conditions affecting
shouldCreateSubsequence() and shouldRepaintSubsequence() in PaintLayerPainter.cpp for the conditions.