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

[Alexei Podtelezhnikov]

Vitor,

Frankly, I can hardly relate your problem description to font rendering. Can 
you possibly rephrase it? In FreeType speak LCD rendering is just triplicated 
for 3 arbitrary color channels with slight differences depending how these 
channels are spatially shifted. Regular antialiasing is just a special case 
assuming that three channels are not shifted at all. Note that FreeType does 
not assign color to the channels. 

Alexei

 

> On Jun 6, 2025, at 09:35, VÍTOR RAMOS <vitor.ramos@ufrn.br> wrote:
> 
> All,
> 
> 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.
> 
> 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.
> 
> One factor that is not addressed (or even mentioned in discussions at
> all) is lossy color format and space transformations in display output
> pipelines.
> 
> 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.
> 
> 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.
> 
> 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.
> 
> To my knowledge, no libraries commercial or otherwise currently
> address this issue.
> 
> Vítor
>