Android GL Architecture

（见WebCore/platform/graphics/android/GLWebViewState.h)

/////////////////////////////////////////////////////////////////////////////////

// GL Architecture
/////////////////////////////////////////////////////////////////////////////////
//
// To draw things, WebView use a tree of layers. The root of that tree is a
// BaseLayerAndroid, which may have numerous LayerAndroid over it. The content
// of those layers are SkPicture, the content of the BaseLayer is an PictureSet.
//
// When drawing, we therefore have one large "surface" that is the BaseLayer,
// and (possibly) additional surfaces (usually smaller), which are the
// LayerAndroids. The BaseLayer usually corresponds to the normal web page
// content, the Layers are used for some parts such as specific divs (e.g. fixed
// position divs, or elements using CSS3D transforms, or containing video,
// plugins, etc.).
//
// *** NOTE: The GL drawing architecture only paints the BaseLayer for now.
//
// The rendering model is to use tiles to display the BaseLayer (as obviously a
// BaseLayer's area can be arbitrarly large). The idea is to compute a set of
// tiles covering the viewport's area, paint those tiles using the webview's
// content (i.e. the BaseLayer's PictureSet), then display those tiles.
// We check which tile we should use at every frame.
//
// Overview
// ---------
//
// The tiles are grouped into a TiledPage -- basically a map of tiles covering
// the BaseLayer's surface. When drawing, we ask the TiledPage to prepare()
// itself then draw itself on screen. The prepare() function is the one
// that schedules tiles to be painted -- i.e. the subset of tiles that intersect
// with the current viewport. When they are ready, we can display
// the TiledPage.
//
// Note that BaseLayerAndroid::drawGL() will return true to the java side if
// there is a need to be called again (i.e. if we do not have up to date
// textures or a transition is going on).
//
// Tiles are implemented as a BaseTile. It knows how to paint itself with the
// PictureSet, and to display itself. A GL texture is usually associated to it.
//
// We also works with two TiledPages -- one to display the page at the
// current scale factor, and another we use to paint the page at a different
// scale factor. I.e. when we zoom, we use TiledPage A, with its tiles scaled
// accordingly (and therefore possible loss of quality): this is fast as it's
// purely a hardware operation. When the user is done zooming, we ask for
// TiledPage B to be painted at the new scale factor, covering the
// viewport's area. When B is ready, we swap it with A.
//
// Texture allocation
// ------------------
//
// Obviously we cannot have every BaseTile having a GL texture -- we need to
// get the GL textures from an existing pool, and reuse them.
//
// The way we do it is that when we call TiledPage::prepare(), we group the
// tiles we need (i.e. in the viewport and dirty) into a TilesSet and call
// BaseTile::reserveTexture() for each tile (which ensures there is a specific
// GL textures backing the BaseTiles).
//
// reserveTexture() will ask the TilesManager for a texture. The allocation
// mechanism goal is to (in order):
// - prefers to allocate the same texture as the previous time
// - prefers to allocate textures that are as far from the viewport as possible
// - prefers to allocate textures that are used by different TiledPages
//
// Note that to compute the distance of each tile from the viewport, each time
// we prepare() a TiledPage. Also during each prepare() we compute which tiles
// are dirty based on the info we have received from webkit.
//
// BaseTile Invalidation
// ------------------
//
// We do not want to redraw a tile if the tile is up-to-date. A tile is
// considered to be dirty an in need of redrawing in the following cases
//  - the tile has acquires a new texture
//  - webkit invalidates all or part of the tiles contents
//
// To handle the case of webkit invalidation we store two ids (counters) of the
// pictureSets in the tile.  The first id (A) represents the pictureSet used to
// paint the tile and the second id (B) represents the pictureSet in which the
// tile was invalidated by webkit. Thus, if A < B then tile is dirty.
//
// Since invalidates can occur faster than a full tiled page update, the tiled
// page is protected by a 'lock' (m_baseLayerUpdate) that is set to true to
// defer updates to the background layer, giving the foreground time to render
// content instead of constantly flushing with invalidates. See
// lockBaseLayerUpdate() & unlockBaseLayerUpdate().
//
// Painting scheduling
// -------------------
//
// The next operation is to schedule this TilesSet to be painted
// (TilesManager::schedulePaintForTilesSet()). TexturesGenerator
// will get the TilesSet and ask the BaseTiles in it to be painted.
//
// BaseTile::paintBitmap() will paint the texture using the BaseLayer's
// PictureSet (calling TiledPage::paintBaseLayerContent() which in turns
// calls GLWebViewState::paintBaseLayerContent()).
//
// Note that TexturesGenerator is running in a separate thread, the textures
// are shared using EGLImages (this is necessary to not slow down the rendering
// speed -- updating GL textures in the main GL thread would slow things down).
//
/////////////////////////////////////////////////////////////////////////////////