I gotta say, as the guy who brought up DSC, that last Reddit post especially had me doubting. That is not what DSC artifacts look like. DSC subsamples the chroma, which causes distinct color bleeding issues. That is luma bloom, which doesn't happen with DSC.

So I took my Mac Mini, hooked up to a 4K monitor, verified there were no DSC artifacts at native resolution, set it to "2560x1440" and sure enough the same artifacts appeared for me too, but still no telltale signs of DSC. So yeah, I gotta say, this is on Apple. Between this and dropping subpixel antialiasing support for text, it's pretty clear that their only properly supported configuration is 2x scaling on high-DPI displays.

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crazygringo 2 hours ago | parent [-]

Huh, very interesting.

OK, I just grabbed my loupe to make sure I'm not missing anything, and pulled up an app in dark mode (so ringing should be more visible) on my MBA M4. I'm using its built-in display. I've cycled through all 4 available resolution settings in Display, and absolutely zero artifacts or ringing. Then tried connecting to my LG UltraFine 4K which connects over Thunderbolt, that gives 5 resolution settings instead of 4, and zero artifacts/ringing on any of those either.

So I have no idea what's going on. I don't doubt that you're seeing it, and it's there in that Reddit photo. But maybe it's something specific to external monitors over a certain connection type or something? Seems very strange that Apple would use a different downsampling algorithm under different circumstances though.

I'd normally assume the most likely culprit would be some kind of sharpening setting on a monitor, as that can absolutely cause the type of ringing seen in that Reddit photo. But on the other hand, if you're testing it right now and not seeing it at native 2x, then that would seem to be ruled out, at least in your case. Maybe it's some kind of resolution negotiation mismatch where it's actually the monitor applying a second scaling that has ringing, since monitors can accept signals that don't match their native hardware resolution?

	▲	kbolino 44 minutes ago \| parent [-]
		I can get a mild form of it on my M4 MBP's built-in display at "1800x1125"* but it's not nearly as noticeable as it was on the 4K external display at "2560x1440" and honestly I needed my cell phone camera zoomed in to definitively identify it, so that was more of a fishing expedition than a real problem. However, I have tried 2 different Macs, 2 different 4K monitors (both LG UltraFine also, though they differ in firmware version and color reproduction because of course they do), and 2 different interfaces (HDMI, Thunderbolt), and I can reliably replicate it under all of those combinations. I think that exact scaling factor probably has a bad interaction with the scaling algorithm. I do agree that a lot of other scalings do not produce the ringing/halo/bloom effect. * = You have to go click "Advanced...", enable "Show resolutions as list", then when back on the main Displays page, enable "Show all resolutions", to get this and many other options -- but this is only necessary on the internal display, the external display offers "2560x1440" as a non-advanced choice