Re: [Discussion] Subpixel rendering issues with compressed video output formats

Vitor,

Frankly, I can hardly relate your problem description to font rendering. Can=
 you possibly rephrase it? In FreeType speak LCD rendering is just triplicat=
ed for 3 arbitrary color channels with slight differences depending how thes=
e channels are spatially shifted. Regular antialiasing is just a special cas=
e assuming that three channels are not shifted at all. Note that FreeType do=
es not assign color to the channels.=20

Alexei

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> On Jun 6, 2025, at 09:35, V=C3=8DTOR RAMOS <[email protected]> wrote:
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> =EF=BB=BFAll,
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> I have been studying the subpixel rendering code and I could spot a
> few improvement areas and I would like to hear your opinion about it.
> In this message I discuss one of them.
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> There are a few issues people face with subpixel antialiasing that
> yield artifacts on their setup. The most common culprit is gamma
> mismatches, but quite a few other less obvious factors are involved,
> such as geometry which is partially addressed.
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> One factor that is not addressed (or even mentioned in discussions at
> all) is lossy color format and space transformations in display output
> pipelines.
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> It has become increasingly common that display adapters may
> autoconfigure the output to use compressed video streams under a lossy
> (chroma-subsampled) YUV output color format. This is usually done in
> the context of a trade-off to achieve higher resolutions, refresh
> rates or bit depths. As an example, many high-DPI panels only offer
> UHD at 60 Hz under a chroma-subsampled YUV 4:2:0 stream. As another
> example, many high-refresh rate panels only offer their maximum rates
> under a likewise compressed format. As yet another example, higher bit
> depth may also only be available under a chroma-subsampled format. In
> general, video throughput may be limited by panels, adapters and even
> physical channels, and the display output may be automatically
> configured to use a compressed format.
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> As is, the project implicitly assumes an RGB 4:4:4 uncompressed output
> video format and performs excellently, but if the output has chroma
> subsampling then subpixel antialiased rendering can exhibit color
> fringe artifacts. Indeed, several complaints about fringing may very
> well be caused by chroma subsampling rather than gamma issues.
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> Exposing a subpixel antialiasing filter that addresses cases where the
> output format has subsampled chroma is quite a large undertaking but
> is feasible. Though the simpler solution in this case is to switch to
> grayscale antialiasing.
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> To my knowledge, no libraries commercial or otherwise currently
> address this issue.
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> V=C3=ADtor
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