Filter Effects in Chrome can be somewhat confusing. The SVG and CSS specs give us three different ways of specifying filters (SVG-on-SVG, SVG-in-CSS, shorthand CSS), Chrome has two rendering modes (GPU-accelerated or CPU), and two filter implementations (Blink and Skia). This is an attempt to document the current situation, and where we're headed.
One important thing to note is that we currently only trigger the GPU filter path for source elements that are already in a composited layer, e.g., 2D <canvas>, WebGL, <video>, 3D CSS. We don't really have a choice but to apply these filters in the compositor, since only the compositor contains the source textures to which the filters are to be applied. We do trigger the GPU path for filters which are animated, but only for the duration of the animation, after which we switch back to software (same as is done for animated opacity, 2D transforms, etc). All other rendering of filters on web content uses the CPU. Currently, SVG-on-SVG filters are only rendered using a CPU path.
Ways to author filters (in chronological order):
SVG-on-SVG filters: the oldest (spec-wise, and blink-wise). Syntax: filter="url(#foo)" on the target SVG element, where "foo" refers to an SVG filter graph. Applies a SVG filter to an SVG element, via a DOM attribute. Animatable via SVG. First shipped in Chrome M6.
SVG-in-CSS filters (aka reference filters, aka url() filters). Specified by CSS property -webkit-filter: url(#foo), where foo refers to the ID of an SVG filter node. Renders an HTML element to a buffer, then applies an SVG filter DAG to it. First appeared in Firefox 5 or so. Implemented in Chrome M25 on the software path, and M31 on the GPU path (disabled in M32 due to ubercomp; re-enabled in M33). Animatable via SVG.
CSS shorthand filters. Specified by (e.g.,) CSS property -webkit-filter: grayscale(50%) hue-rotate(90deg). A simple linear chain of filters. This is essentially a simplified subset of what is possible with the SVG DAG. Animatable via CSS. Proposed by Apple (but first shipped in Chrome M19), shipped in Safari 6.
The Blink implementation uses a graph of FilterEffect nodes (in platform/graphics/filters/FilterEffect.*, and subclasses FE*.) This renders only on the CPU.
The Skia implementation uses a graph of SkImageFilter nodes (in src/core/SkImageFilter.*, and subclasses *ImageFilter.cpp). This has CPU and GPU modes.
(*there was actually a prototype implementation under the "Accelerated SVG Filters" flag, but it ran in the renderer process, and the resulting filtered element had to incur a readback from the GPU to be (CPU) composited. It was useful for testing the individual Skia GPU filters before we had the accelerated SVG-in-CSS path, but it's now old and busted. I've removed the flag in M36.)
Future (paths which will change are in bold):
The GPU path for SVG-on-SVG filters (***) can forced on via --force-gpu-rasterization (the Ganesh GPU rasterization path, essentially).
Now that these changes are done, all software rasterization of filters is be performed on the impl-thread using the Skia implementation, and compatible with multi-threaded and GPU rasterization. The Blink implementation of filter processing can be removed (including its bespoke ParallelJobs threading library). As a side note, this will also make filters compatible with non-tiled GPU-accelerated rendering of web content too (a la IE9+), if that proves promising.