Anti-aliasing techniques used in Skia GPU Implementation

[Tarun Gehlaut]

Hello everyone.

The skia website mentions about AAA as the anti-aliasing technique in the CPU pipeline(https://skia.org/dev/design/aaa).

I was interested to know what techniques have been used in Ganesh.

From the code I could see that the technique varies depending on the Rasterizer being used (GrTessellatingPathRenderer, GrCoverageCountingPathRenderer, etc). Is there any document that could be useful for me?

Is the coverage anti-aliasing (which is what I found in most of the rasterizers) similar to (http://www.nvidia.com/object/coverage-sampled-aa.html) or (http://research.nvidia.com/sites/default/files/pubs/2015-08_Decoupled-Coverage-Anti-Aliasing/hpg15_DCAA.pdf).

Thanks.

[Brian Osman]

Hi,

The GPU implementation uses a few different AA techniques, depending on your surface configuration & the type of primitive being drawn. Until recently, most of the techniques would fall into one of two buckets:

1. MSAA: If your surface was constructed as an MSAA framebuffer, Skia computes binary coverage per sample, and relies on MSAA resolve for AA.
2. Geometric / analytic: I think this is more common among our clients. Here, Skia tries to compute an exact coverage value per pixel using some geometric or analytic technique. For simple primitives like circles or round-rects, there are shaders that inflate the bounds by a pixel, then use the fragment position to compute the exact distance to the edge of the shape. For more complex paths, the different path renderers use a variety of approaches, but one common idea is to inflate the path geometry with a skirt of triangles, and interpolate coverage across that skirt.

More recently, a new path rendering algorithm has been added - CCPR, which is a particular GPU implementation of the idea from the AAA slides you linked above.

-Brian

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[Corey Lucier]

Brian,

What GL caps/features are required for the CCPR approach currently?

[Brian Salomon]

ES 3.0 is the minimum but it helps performance to have geometry shaders.

On Wed, Feb 21, 2018 at 9:50 AM Corey Lucier <corey....@gmail.com> wrote:

Brian,

What GL caps/features are required for the CCPR approach currently?

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[Brian Salomon]

Oh, and half-float alpha as a color-renderable format.

[Corey Lucier]

So no hope of cross compilation to Web GL 1.0 then. ;)

[Brian Salomon]

Nope :(

WebGL 2.0 + EXT_color_buffer_half_float is almost enough. The WebGL extension strips out single channel half-float rendering from the ES extension, though. We could probably find a way to make it work with another texture format if that was useful.

On Wed, Feb 21, 2018 at 10:01 AM Corey Lucier <corey....@gmail.com> wrote:

So no hope of cross compilation to Web GL 1.0 then. ;)

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