chromium-paint

blink Display item types - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h?q=blink::DisplayItem&sq=package:chromium&l=61
Display Lists seem to be overused in both chromium and blink. There are blink display items and cc display items.

blink web items are just skia items? - https://cs.chromium.org/chromium/src/cc/blink/web_display_item_list_impl.h?q=appendDrawingItem&sq=package:chromium&l=46 - appendDisplayItem is an SkPicture? Seems odd.

The skia commands that we can actually record - https://cs.chromium.org/chromium/src/third_party/skia/include/private/SkRecords.h?q=SK_RECORD_TYPES&sq=package:chromium&dr=C&l=44

Plays commands through SkCanvas which does some analysis on the canvas as a whole it seems - https://cs.chromium.org/chromium/src/skia/ext/analysis_canvas.h?q=AnalysisCanvas&sq=package:chromium&l=21
Current implementation still seems to not use display list and goes from frame tree -> SkCanvas commands. - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/GraphicsContext.cpp?q=GraphicsContext&sq=package:chromium&l=238 . This implies this is mostly still true - https://www.chromium.org/developers/design-documents/gpu-accelerated-compositing-in-chrome
Slimming paint display items are wrappers around skia draw calls that existed before slimming paint. eg. DrawingDisplayItem is just a wrapper around SkPicture

Slimming paint part 2 isn't finished yet ,so current build (Sept 2016), still has graphics layers and blink isn't shipping display items directly to cc.
Right now

invalidate based on layout objects
compute partial display lists, but those get converted back to a single SkPicture for cc::Layer consumption to raster. - Converting WebContentDisplayList to CC::display lists - https://cs.chromium.org/chromium/src/cc/blink/web_content_layer_impl.cc?q=web_content_layer_&sq=package:chromium&l=75

Frame Tree in Chrome is LayoutObject (https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutObject.h?q=LayoutObject&sq=package:chromium&l=209)
They have two kinds of layers (Both of these seem to be under the process of rewriting via Slimming paint v2)
    PaintLayer - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/PaintLayer.h?q=PaintLayer&sq=package:chromium&l=142 - PaintLayers group layout objects into one place depending on paint order
    GraphicsLayer - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/GraphicsLayer.h?q=GraphicsLayer&sq=package:chromium&dr=CSs&l=27 - Things that need their own texture / surface. Not 1:1 with PaintLayer

    Multiple PaintLayers can go into 1 graphics layer. GraphicsLayer just means it gets it's own backing store.


Where are these mini SkPictures coming from via the display lists?

Are these the actual display items - https://cs.chromium.org/chromium/src/cc/blink/web_display_item_list_impl.cc?sq=package:chromium&l=111 ? Compositign items are a wrapper around SkPictures?
All the CC display items are here -https://cs.chromium.org/chromium/src/cc/playback/compositing_display_item.h?q=CompositingDisplay&sq=package:chromium&l=26 in this directory, but these are the CC ones
All the webkit display items are here - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/paint/?q=CompositingDisplayItem&sq=package:chromium
https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h?l=7

What are DisplayItemClients - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/paint/DisplayItemClient.h?q=DisplayItemClient&sq=package:chromium&l=23 - they actually look like display items in our terms as part of the display list dump

Current theory
GraphicsLayers record paints which output SkPictures - GraphicsLayers know what to paint since they are off the Layout tree. GrahpicsLayers paint into WebKit display items
These SkPictures get grouped up into SlimmingPaint type DisplayItemLists - https://cs.chromium.org/chromium/src/cc/playback/display_item_list.h?sq=package:chromium&rcl=1475680623&l=205
The DisplayItemLists then get converted into CC Display Lists - https://cs.chromium.org/chromium/src/cc/blink/web_display_item_list_impl.cc?sq=package:chromium&rcl=1475695348&l=53 (I'm fairly certain CC Display Items are just wrappers around SkPictures)
CC Display Lists then tile render replay the SkPictures / commands to an actual SkCanvas - These are the slimming paint type of display lists

What are WebKit display items used for then? - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/paint/DisplayItem.h?l=70
Conversion from Webkit display items to slimming paint display items - https://cs.chromium.org/chromium/src/cc/blink/web_content_layer_impl.cc?q=web_content_layer_impl&sq=package:chromium&l=83 - DrawDisplayItem is the same name

Webkit display list dump for reddit.com - https://drive.google.com/a/mozilla.com/file/d/0B880pG1iaq2eSFFBLTItY0RmRVE/view?usp=sharing - Basically looks like a frame tree.
Blink Display Items are wrappers around LayoutObjects grouped by their paint phase - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/PaintPhase.h?sq=package:chromium&dr=C&rcl=1475839200
Simpler look at what CC display items are - https://cs.chromium.org/chromium/src/cc/playback/display_item_proto_factory.cc?sq=package:chromium&l=29
cc::DrawingDisplayItem - Just a wrapper around an SKPicture
Also seems to do some basic global analysis of drawing commands for an SkPicture with analysis_canvas - https://cs.chromium.org/chromium/src/skia/ext/analysis_canvas.cc?sq=package:chromium&rcl=1475752711&l=117

How does multi threaded painting happen?
- Multiple raster tasks that go into a work pool, then the threads just rasterize whatever work is in the pool via tiling

Walkthrough

Basic example of Hello World text
Starts with a LayoutTextObject - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/LayoutText.cpp?sq=package:chromium&l=161 - Essentially a text dom node
Assigns RenderLayers to backing store layers ( https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/compositing/CompositingLayerAssigner.cpp?sq=package:chromium&l=271)
CompositedLayerMapping - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/layout/compositing/CompositedLayerMapping.h?q=CompositedLayerMapping&sq=package:chromium&l=66 - maps PaintLayers -> GraphicsLayers
Main thread recording tells compositor thread to paint - https://cs.chromium.org/chromium/src/cc/trees/proxy_main.cc?q=proxy_main.cc&sq=package:chromium&dr&l=223
Assign tiles - https://cs.chromium.org/chromium/src/cc/tiles/tile_manager.cc?sq=package:chromium&rcl=1475680623&l=634
Start running tile rendering - https://cs.chromium.org/chromium/src/cc/tiles/tile_manager.cc?q=tile_manager&sq=package:chromium&l=949
Actual rasterization of things happens on the compositor tile worker thread - https://cs.chromium.org/chromium/src/cc/playback/display_item_list.cc?q=display_item_list.cc&sq=package:chromium&l=142 - This is called on the main, compositor, and tile worker threads. Only difference is on main/compositor thread, it's an SkAnalysisCanvas, not a real SkCanvas we paint on.
"Paint" which is really starting to record from GraphicsLayers to DisplayItems - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/GraphicsLayer.cpp?q=GraphicsLayer.cp&sq=package:chromium&l=277
GraphicsContext are what actually do the drawing - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/platform/graphics/GraphicsContext.cpp?q=GraphicsContext.cpp&sq=package:chromium&dr&l=720

Cleaner walkthrough:

LayoutObjects get wrapped around blink display lists / items - grouped based on paint phase https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/PaintPhase.h?sq=package:chromium&dr=C&rcl=1475839200
Go down the LayoutObjects and layers and record each LayoutObject as it gets translated into skia calls that gets painted
Take the objects and convert them into an blink::DisplayItem. At this point, they are mostly just wrappers around recorded SkPictures that were there from the layout being painted
Run the playback through an analysis phase (might be invalidation)
Once that is done, convert them to cc::DisplayLists, which are basically still wrappers around SkPictures but without references to the DOM tree suitable for consumption on the compositor thread
Get painted / recorded via SkPicture - https://cs.chromium.org/chromium/src/cc/playback/display_item_list.cc?q=DisplayItemList::Ra&sq=package:chromium&l=141 - Why does each display item have it's own SkPicture? When do they get merged? They get merged during the cc::DisplayList::Raster - https://cs.chromium.org/chromium/src/cc/playback/drawing_display_item.cc?sq=package:chromium&l=84

Text getting painted - https://cs.chromium.org/chromium/src/third_party/WebKit/Source/core/paint/TextPainter.cpp?q=TextPainter.cpp&sq=package:chromium&l=65 which calls into a skia recording - https://cs.chromium.org/chromium/src/third_party/skia/include/core/SkCanvas.h?q=SkCanvas&sq=package:chromium&dr=CSs&l=1097

From blink::Display List -> CC::DisplayList which are SkPictures - https://cs.chromium.org/chromium/src/cc/layers/picture_layer.cc?q=picture_layer.cc&sq=package:chromium&l=127
Playback on compositor tile worker of cc::display items - https://cs.chromium.org/chromium/src/cc/playback/raster_source.cc?q=raster_source.cc&sq=package:chromium&l=199
tile compositing - https://cs.chromium.org/chromium/src/cc/layers/picture_layer_impl.cc?q=picture_layer_impl.cc&sq=package:chromium&l=331

good sources
https://www.youtube.com/watch?v=5Xv2A7aqJ9Y
https://www.youtube.com/watch?v=Sc-vQDHAFtU
tiling -https://www.chromium.org/developers/design-documents/impl-side-painting
https://www.chromium.org/blink/getting-started-with-blink-debugging
https://www.chromium.org/developers/design-documents/impl-side-painting