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General question about video codecs: why are codecs tied to resolutions? Why can't some codecs be sued for resolutions larger than some maximum? This is probabl

General question about video codecs: why are codecs tied to resolutions? Why can't some codecs be sued for resolutions larger than some maximum? This is probably the one thing I never understood about codecs.

cesarb 7y ago
Not a codec expert, but I can see two reasons. The first is that some fields in the encoding are not big enough, for instance a 12-bit field for the resolution in some header can represent only up to 4095 pixels (or 4096 if "zero pixels" is not allowed). The second is that all video codecs I know of subdivide the image into small blocks and compress each block individually (mostly; there's some prediction from nearby blocks, and some mixing of the blocks at the end to smooth the edges between them), and older codecs only have blocks which are too small for higher resolutions (imagine for instance a 8x8 block in a 320x240 video, and then in the same video at 1280x960; the block clearly covers much less of the later video, so there's less variation for it to work with. Increase the resolution too much, and small blocks would start acting more like individual pixels, and you'd lose nearly all the compression.)
- mehrdadn 7y ago
  The first reason seems possible I guess, but not likely as being the only reasons. The second reason though should only imply that you lose the compression, not that you're unable to produce the file in the given format in the first place? What I'm referring is that I recall encoder tools outright rejecting high resolutions for older formats.
opencl 7y ago
Mostly it's just because the standards say so. There's nothing about how MPEG-2 works that inherently makes it impossible to encode 4K video with it, the standard just says that MPEG-2 video shall not exceed 80Mbit/s, 1920 horizontal pixels, 1152 vertical pixels, or 62,668,800 luminance samples per second. The limits are just there for interoperability. As is, if you encode a video that complies with MPEG-2 High Level, then you can be pretty confident that any MPEG-2 High Level decoder will decode it. Without the limit everything becomes a mess. You're writing a decoder, what max res should you support? 4K? 8K? The people writing decoders don't agree and then people trying to distribute these high resolution files find that they work in some decoders but not others.
- mehrdadn 7y ago
  Thanks! What do you mean by "not being able to decode" something though? Do you mean time-wise (not enough bandwidth to keep up) or do you mean the algorithm would fail in a different way? The latter doesn't make sense to me, and the former seems like it's entirely system-dependent? Like isn't it the exact same problem if I have an underpowered computer?
  - Wowfunhappy 7y ago
    It means hardware decoders won't be able to decode the videos. (Some software decoders that are overly rigid about following the spec may refuse as well, but most won't, and none actually have to from my understanding.) You can absolutely use h.264 to encode 8K@120fps if you want to.
    - mehrdadn 7y ago
      Ohh hardware decoders! Makes sense.
cardiffspaceman 7y ago
A looser tie to definition (which is what I interpret your use of resolution as) is computation. Representing an 1080i stream in MPEG2 takes more bits per second than representing that same stream in MPEG4 or MPEG5. So if you keep the bandwidth constant, each generation has a rough maximum definition. MPEG 4 is great for HD, but MPEG 5 is probably what you want if you want to do UHD because the bandwidth won't increase. I don't negate the idea that a 10-bit field for the horizontal pixel count would be a significant barrier, but if it were super hard to come up with more efficient compression, some sort of extension would be defined to allow bigger horizontal pixel counts.

cesarb 7y ago

Not a codec expert, but I can see two reasons. The first is that some fields in the encoding are not big enough, for instance a 12-bit field for the resolution in some header can represent only up to 4095 pixels (or 4096 if "zero pixels" is not allowed). The second is that all video codecs I know of subdivide the image into small blocks and compress each block individually (mostly; there's some prediction from nearby blocks, and some mixing of the blocks at the end to smooth the edges between them), and older codecs only have blocks which are too small for higher resolutions (imagine for instance a 8x8 block in a 320x240 video, and then in the same video at 1280x960; the block clearly covers much less of the later video, so there's less variation for it to work with. Increase the resolution too much, and small blocks would start acting more like individual pixels, and you'd lose nearly all the compression.)

mehrdadn 7y ago
The first reason seems possible I guess, but not likely as being the only reasons. The second reason though should only imply that you lose the compression, not that you're unable to produce the file in the given format in the first place? What I'm referring is that I recall encoder tools outright rejecting high resolutions for older formats.

opencl 7y ago

Mostly it's just because the standards say so. There's nothing about how MPEG-2 works that inherently makes it impossible to encode 4K video with it, the standard just says that MPEG-2 video shall not exceed 80Mbit/s, 1920 horizontal pixels, 1152 vertical pixels, or 62,668,800 luminance samples per second. The limits are just there for interoperability. As is, if you encode a video that complies with MPEG-2 High Level, then you can be pretty confident that any MPEG-2 High Level decoder will decode it. Without the limit everything becomes a mess. You're writing a decoder, what max res should you support? 4K? 8K? The people writing decoders don't agree and then people trying to distribute these high resolution files find that they work in some decoders but not others.

mehrdadn 7y ago
Thanks! What do you mean by "not being able to decode" something though? Do you mean time-wise (not enough bandwidth to keep up) or do you mean the algorithm would fail in a different way? The latter doesn't make sense to me, and the former seems like it's entirely system-dependent? Like isn't it the exact same problem if I have an underpowered computer?
- Wowfunhappy 7y ago
  It means hardware decoders won't be able to decode the videos. (Some software decoders that are overly rigid about following the spec may refuse as well, but most won't, and none actually have to from my understanding.) You can absolutely use h.264 to encode 8K@120fps if you want to.
  - mehrdadn 7y ago
    Ohh hardware decoders! Makes sense.

cardiffspaceman 7y ago

A looser tie to definition (which is what I interpret your use of resolution as) is computation. Representing an 1080i stream in MPEG2 takes more bits per second than representing that same stream in MPEG4 or MPEG5. So if you keep the bandwidth constant, each generation has a rough maximum definition. MPEG 4 is great for HD, but MPEG 5 is probably what you want if you want to do UHD because the bandwidth won't increase. I don't negate the idea that a 10-bit field for the horizontal pixel count would be a significant barrier, but if it were super hard to come up with more efficient compression, some sort of extension would be defined to allow bigger horizontal pixel counts.