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Work in progress. Possibly incorrect or incomplete.
There's a lot of jargon in the graphics stack. We try to maintain a list
of common words and acronyms `here <https://wiki.mozilla.org/Platform/GFX/Jargon>`__.
The graphics systems is responsible for rendering (painting, drawing)
the frame tree (rendering tree) elements as created by the layout
system. Each leaf in the tree has content, either bounded by a rectangle
(or perhaps another shape, in the case of SVG.)
The simple approach for producing the result would thus involve
traversing the frame tree, in a correct order, drawing each frame into
the resulting buffer and displaying (printing non-withstanding) that
buffer when the traversal is done. It is worth spending some time on the
“correct order” note above. If there are no overlapping frames, this is
fairly simple - any order will do, as long as there is no background. If
there is background, we just have to worry about drawing that first.
Since we do not control the content, chances are the page is more
complicated. There are overlapping frames, likely with transparency, so
we need to make sure the elements are draw “back to front”, in layers,
so to speak. Layers are an important concept, and we will revisit them
shortly, as they are central to fixing a major issue with the above
While the above simple approach will work, the performance will suffer.
Each time anything changes in any of the frames, the complete process
needs to be repeated, everything needs to be redrawn. Further, there is
very little space to take advantage of the modern graphics (GPU)
hardware, or multi-core computers. If you recall from the previous
sections, the frame tree is only accessible from the UI thread, so while
we’re doing all this work, the UI is basically blocked.
Layers framework was introduced to address the above performance issues,
by having a part of the design address each item. At the high level:
1. We create a layer tree. The leaf elements of the tree contain all
frames (possibly multiple frames per leaf).
2. We render each layer tree element and cache (retain) the result.
3. We composite (combine) all the leaf elements into the final result.
Let’s examine each of these steps, in reverse order.
We use the term composite as it implies that the order is important. If
the elements being composited overlap, whether there is transparency
involved or not, the order in which they are combined will effect the
result. Compositing is where we can use some of the power of the modern
graphics hardware. It is optimal for doing this job. In the scenarios
where only the position of individual frames changes, without the
content inside them changing, we see why caching each layer would be
advantageous - we only need to repeat the final compositing step,
completely skipping the layer tree creation and the rendering of each
leaf, thus speeding up the process considerably.
Another benefit is equally apparent in the context of the stated
deficiencies of the simple approach. We can use the available graphics
hardware accelerated APIs to do the compositing step. Direct3D, OpenGL
can be used on different platforms and are well suited to accelerate
Finally, we can now envision performing the compositing step on a
separate thread, unblocking the UI thread for other work, and doing more
work in parallel. More on this below.
It is important to note that the number of operations in this step is
proportional to the number of layer tree (leaf) elements, so there is
additional work and complexity involved, when the layer tree is large.
Render and retain layer elements
As we saw, the compositing step benefits from caching the intermediate
result. This does result in the extra memory usage, so needs to be
considered during the layer tree creation. Beyond the caching, we can
accelerate the rendering of each element by (indirectly) using the
available platform APIs (e.g., Direct2D, CoreGraphics, even some of the
3D APIs like OpenGL or Direct3D) as available. This is actually done
through a platform independent API (see Moz2D) below, but is important
to realize it does get accelerated appropriately.
Creating the layer tree
We need to create a layer tree (from the frames tree), which will give
us the correct result while striking the right balance between a layer
per frame element and a single layer for the complete frames tree. As
was mentioned above, there is an overhead in traversing the whole tree
and caching each of the elements, balanced by the performance
improvements. Some of the performance improvements are only noticed when
something changes (e.g., one element is moving, we only need to redo the
Rendering each layer
Tiling vs. Buffer Rotation vs. Full paint
Compositing for the final result
`Historical Documents <http://www.youtube.com/watch?v=lLZQz26-kms>`__
A number of posts and blogs that will give you more details or more
background, or reasoning that led to different solutions and approaches.
- 2010-01 `Layers: Cross Platform Acceleration <http://www.basschouten.com/blog1.php/layers-cross-platform-acceleration>`__
- 2010-04 `Layers <http://robert.ocallahan.org/2010/04/layers_01.html>`__
- 2010-07 `Retained Layers <http://robert.ocallahan.org/2010/07/retained-layers_16.html>`__
- 2011-04 `Introduction <https://web.archive.org/web/20140604005804/https://blog.mozilla.org/joe/2011/04/26/introducing-the-azure-project/>`__
- 2011-07 `Layers <http://chrislord.net/index.php/2011/07/25/shadow-layers-and-learning-by-failing/%20Shadow>`__
- 2011-09 `Graphics API Design <http://robert.ocallahan.org/2011/09/graphics-api-design.html>`__
- 2012-04 `Moz2D Canvas on OSX <http://muizelaar.blogspot.ca/2012/04/azure-canvas-on-os-x.html>`__
- 2012-05 `Mask Layers <http://featherweightmusings.blogspot.co.uk/2012/05/mask-layers_26.html>`__
- 2013-07 `Graphics related <http://www.basschouten.com/blog1.php>`__