x264 crf HD vs SD

[SamNeal2001]

Why does:
--preset veryslow --tune film --level 4.1 --crf 20
look exceptionally great at HD resolutions, but terrible at SD resolutions?

[Dark Shikari]

Most likely because you're watching the SD version upscaled to your full screen resolution, so all the artifacts are going to be magnified dramatically.

[Bloax]

You could also think in another way, in SD - there are less pixels for the same amount of "space" compared to HD, so the artifacts at SD take up more of the actual space compared to HD.

Though I haven't experienced terrible results at --crf 20.

[SamNeal2001]

Maybe "terrible" is the wrong word. But it doesn't look NEARLY as good in SD as it does in HD. Results are only satisfactory.

[sneaker_ger]

As the others said: because of the resolution. x264 will use a much higher bitrate for the HD version at the same CRF and also artefacts would be less visible because of the smaller blocks (when displayed on the same screen).

[ajp_anton]

Same "per pixel" quality results in higher "per frame" quality when there are more pixels per frame.

[Sagittaire]

It's Simple:
- Pixel number for HD is 5X higher
- Size file for HD is 3-4X higher

If you want compare HD and SD advantage, don't make encoding at same quality/pixel level (aka quantizer aka crf mode) but at same size level.

[nm]

It's Simple:
- Pixel number for HD is 5X higher
- Size file for HD is 3-4X higher

If you want compare HD and SD advantage, don't make encoding at same quality/pixel level (aka quantizer aka crf mode) but at same size level.

If you mean encoding HD at 5x the bitrate of SD, that logic would only increase the quality of HD encodes compared to SD ones, which is the opposite that the original poster wanted.
If you mean encoding HD at the same bitrate as SD, that would lower the quality of HD videos way too much.

[Sagittaire]

If you mean encoding HD at the same bitrate as SD, that would lower the quality of HD videos way too much.

No, there are good compromise to find at each file size. absurdes exemples: HD encoding at 10 Mbps will be better in most case that SD encoding at 10 Mbps. HD encoding at 1 Mbps will be worst in most case that SD encoding at 1 Mbps. That mean that at each bitrate for each source, there are optimal definition.

With good filtering, with good source, with H264, you can make high quality encoding at 720p and 1 Mbps with really better visual quality than 480p at 1 Mbps.