Sunday, September 09, 2012

More on the Layers refactoring

The refactoring is coming along nicely, I think the hard work is done - I have a set of abstractions I'm happy with, and most of the code has been refactored into the new architecture. There are a lot of rough edges still to iron out, and a lot of things marked TODO. There are also some of the Compositor related refactorings, which are not related to texture passing, still to do.

In this blog post, I want to outline the new abstractions for dealing with graphics and why I've chosen them, I'll mostly describe them as if they are new, but of course, Layers have been there all along, I'm mostly just breaking them up. The main abstractions are: shadowable layers, shadow layers, buffers, and textures. The latter two are divided up into host and client versions, where shadowable ~= client and shadow ~= host. The terminology should settle down as the refactoring finishes off.

There are different kinds of layers for different kinds of content, in particular image, canvas, and Thebes (these last are badly named and we should change the name to ContentLayers soon, thus Thebes ~= Content for Buffers, but more on that later). Colour layers don't transfer any kind of rendered graphics to the compositor, so don't matter here. Container layers are just for organising the layer tree, so also don't matter here. Shadowable layers are always drawn without HWA, so there is only the software backend. They should work with any compositing backend - Shadowable layers should be backend-independent.

A texture represents a lump of graphics data and is responsible for its memory and how that data is transmitted from drawing to compositing processes. Textures are backend dependent. Theoretically any texture can be used by any kind of layer, but in practice each kind of layer only uses a subset of kinds of texture.

A buffer abstracts how a layer uses its textures. A layer has a single buffer, and that buffer has one or more textures. In the simplest case, a buffer just has a single texture. Examples of more complex buffers are YUV images which have a texture for each channel, and tiled images which have a texture for each tile. The buffer defines how the textures are organised: when drawn, when transmitted to the compositor, and when (and how) they are composited. A layer only interacts with textures via a buffer, in fact layers do not even know that textures exist. Buffers are backend independent and (theoretically) can work with any kind of Layer and any kind of texture. In practice, each layer uses different buffers, and the names reflect this (CanvasClient, ImageClient, ContentClient are the different kinds of BufferClient). Also, the buffers are kind of picky about which textures they use. There is some re-use, for example canvas and image layers use the same kind of BufferHosts, and some kinds of texture are used by multiple kinds of buffer. Hopefully re-use will increase with time.

As an example of how this ties together, lets use a tiled Thebes layer as an example (in fact, I haven't converted tiled layers yet): some part of the layer is invalidated, the layer is responsible for knowing which parts are visible and valid, and therefore which regions must be updated. It tells its buffer to update this region, the buffer works out which textures must be repainted and repaints them (by calling back into the layer, because the repainting code is independent of buffers, textures, etc.). The buffer then tells the changed texture to update the compositor, which they do in their own way. On the shadow side, the invalidated textures are updated. When the screen needs updating, the shadow layer knows which region to draw, the buffer host knows how to map that region into textures and how to draw it to the screen.

Communication between renderer and compositor is on a per-layer basis, and I do not change this protocol. But instead of just sending a layer and some data, we now send a layer, a texture identifier, and some data. The texture client sends the message, a reference to the layer is passed in, and it knows its own identifier. On the compositing side, the message is received by the shadow layer (via ShadowLayersParent) and passed directly to its buffer host. The buffer host can then use the identifier as it chooses. The identifier includes the type of buffer and texture and this is verified. If the buffer has only one texture host, then it can be updated with the data from the message. If there is more than one, then the buffer host uses the texture identifier to work out which texture host should be updated. Buffer hosts/clients are free to use the identifier however they like.

Buffer and texture hosts and clients are not created directly, hosts are created by the compositor and clients are created by a specific factory (name TBC). A layer asks for a buffer client of a certain kind, the factory will create that buffer client if possible, or might return some other kind of client if not. Buffer hosts are created in a similar fashion when they are required (when a texture host is added, see later). Texture clients are usually created by a buffer client's constructor (but sometimes not, and sometimes later as well). Texture clients are also created with a call to the factory, the buffer client should know what kind of texture client it wants, and whether it can make do with some other kind if it is not available. When a texture client is created, a message is sent to the compositing side, and the compositor creates a corresponding texture host and adds it to the corresponding shadow layer, the layer should pass the texture host straight to its buffer host, creating one if necessary (what kind of buffer host is included with the message to create the texture host). Likewise destroying a texture client alerts the compositor to destroy the corresponding texture host. But, texture hosts can be created and destroyed at will, without alerting the drawing side. So, host/client pairs (buffers and textures) are loosely associated - there will 'always' be a pair, related by the combination of a layer reference and an identifier, but the exact objects in the pairing might change.

That's enough for one blog post, probably too much. Next time - types!


Mike Ratcliffe said...

Very interesting ... are you planning on presenting this at the next fx-team hack week?

It would be useful, or at least interesting for us to understand exactly how the layout engine works.

Anonymous said...

Nice design, but it's better to have a diagram.

Anonymous said...

Buffer hosts are created in a similar fashion when they are required (when a texture host is added, see later).

Ding-ding, you win the passive voice award (I mean "It has been found that the passive voice award was won"). Passive voice sucks the life out of sentences and obfuscates who is requesting or doing the work. (Clients? a compositor? ???)

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