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> They may even have made it worse: from the "ladder" comparisons, it looks like they jump to higher resolutions at lower average bitrates now, so if you're st

by zbobet2012 6y ago

> They may even have made it worse: from the "ladder" comparisons, it looks like they jump to higher resolutions at lower average bitrates now, so if you're stuck at 1080p you're going to be getting worse quality after these changes than before. No they have not made it worse. 1080p encoded at lower bitrate can look better than 720p at a higher bitrate. Or vice versa. Even to the trained eye. It depends on the scene dynamics.* In the ladders they posted (called a RD chart) we can clearly see they have made a better selection of the resolution to encode at a given target bitrate increase the video quality. They have posted the BD rates for this stuff, the gains are substantial (30% or more visual quality at the same bitrate). > Also, something that's kind of weird about all this is that encoders (x264, x265, etc) already have their own rate control algorithms that decide on a frame or scene basis how many bits to use. Netflix taking an approach like this is equivalent to claiming that they're capable of doing a better job than these codecs in an automated way - so why not just contribute the code needed to achieve these improvements back to the projects they're using? Because they've built a split and stitch encoder on top of the codecs. That means the code is built around x264, not into it. > Last, it's very weird to select individual frames from an encode and not a selection of frames. The comparisons purport to show that the lower bitrate encode is better than the higher bitrate encode, but in fact (if I understand how they're using rate control correctly) what they are showing is a single frame in the lower bitrate encode that uses more bits than the same frame in the higher bitrate encode. So it's arguably not a fair comparison. And even with that, depending on how sensitive you are to artifacts, some of the "optimized ladder" encodes still look worse. This is extremely true. * High action scenes tend to benefit from lower resolution with more bits spent on inter compression while slower scenes benefit from tighter intra compression.

bscphil 6y ago
> No they have not made it worse. 1080p encoded at lower bitrate can look better than 720p at a higher bitrate. Or vice versa. Even to the trained eye. It depends on the scene dynamics.* Well, of course that's correct. Take 1080p encoded at 100 Mbps and 720 encoded at 300 Mbps, I guarantee you that 1080p wins every time. :-) > In the ladders they posted (called a RD chart) we can clearly see they have made a better selection of the resolution to encode at a given target bitrate increase the video quality. I understand how an RD chart works, but I disagree with your conclusions. Reread my original post carefully - my point is that if you're stuck with 1080p as the maximum resolution, than the quality you can get with the revised ladder is strictly worse, because Netflix will want to jump up to UHD resolutions, but your device won't support it. (And even assuming that the UHD will be better based on this chart is kind of a leap, because it depends on what artifacts you're most sensitive to. Objective metrics ≠ subjective experience. In several of their comparisons, I prefer the lower res version because the new one is simply too artifacted.) > Because they've built a split and stitch encoder on top of the codecs. That means the code is built around x264, not into it. I understand the technical difference, but I'm still not convinced. Fundamentally what you get at the end of the day is a single continuous h.264 stream. The quantizer is chosen based on some rate control algorithm, and IPB frame choices are made by another algorithm. Netflix's improvement amounts to having a better rate control, that understands scene differences and uses lower quantizers on more complex scenes. There's fundamentally no reason why you couldn't add such a rate control algorithm to x264. It might require using a 2-pass mode, but it could be done.
- zbobet2012 6y ago
  > Reread my original post carefully - my point is that if you're stuck with 1080p as the maximum resolution, than the quality you can get with the revised ladder is strictly worse, because Netflix will want to jump up to UHD resolutions, but your device won't support it. These are for their HEVC encodes. Apple (and most others) require H264 to have a separate ladder which will be 1080p limited and will be generated on different encode set and maximum video quality.* Almost any HEVC capable device can go to 4k, even if the screen is only 1080p. It will be used than down scaled. > (And even assuming that the UHD will be better based on this chart is kind of a leap, because it depends on what artifacts you're most sensitive to. Objective metrics ≠ subjective experience. In several of their comparisons, I prefer the lower res version because the new one is simply too artifacted.) Certainly objective and subjective metrics have differences, but VMAF does correlate pretty tightly with subjective metrics. This is a very detailed subject, I summed up many of my thoughts on the differences in my demuxed talk last year here: https://www.youtube.com/watch?v=nCUsXhSPyyw https://www.youtube.com/watch?v=nCUsXhSPyyw * https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices https://developer.apple.com/documentation/http_live_streamin... 1.23 iirc
  - bscphil 6y ago
    > These are for their HEVC encodes. Really? I wasn't aware Netflix even offered anything other than H.264 at <= 1080p. I know Youtube doesn't. If this is all HEVC maybe you're right and the resolution issue isn't such a big deal. If this is actually the case I wish they would have said so explicitly in the blog post. > VMAF does correlate pretty tightly with subjective metrics. This is a very detailed subject, I summed up many of my thoughts on the differences in my demuxed talk last year here: https://www.youtube.com/watch?v=nCUsXhSPyyw https://www.youtube.com/watch?v=nCUsXhSPyyw Thanks, I'll check that out when I have the time. I'd still emphasize that it's quite possible for objective metrics to be fooled (that's why codecs have psy tunings!), and I do prefer several "old-ladder" images over the newer versions in Netflix comparisons here. I suppose I've become rather cynical about claims made on the basis of metrics ("now 50% less bitrate for the same quality!"), when to my eye, web video has always been very bad and is getting worse as companies keep reducing the bitrate.

bscphil 6y ago

> No they have not made it worse. 1080p encoded at lower bitrate can look better than 720p at a higher bitrate. Or vice versa. Even to the trained eye. It depends on the scene dynamics.* Well, of course that's correct. Take 1080p encoded at 100 Mbps and 720 encoded at 300 Mbps, I guarantee you that 1080p wins every time. :-) > In the ladders they posted (called a RD chart) we can clearly see they have made a better selection of the resolution to encode at a given target bitrate increase the video quality. I understand how an RD chart works, but I disagree with your conclusions. Reread my original post carefully - my point is that if you're stuck with 1080p as the maximum resolution, than the quality you can get with the revised ladder is strictly worse, because Netflix will want to jump up to UHD resolutions, but your device won't support it. (And even assuming that the UHD will be better based on this chart is kind of a leap, because it depends on what artifacts you're most sensitive to. Objective metrics ≠ subjective experience. In several of their comparisons, I prefer the lower res version because the new one is simply too artifacted.) > Because they've built a split and stitch encoder on top of the codecs. That means the code is built around x264, not into it. I understand the technical difference, but I'm still not convinced. Fundamentally what you get at the end of the day is a single continuous h.264 stream. The quantizer is chosen based on some rate control algorithm, and IPB frame choices are made by another algorithm. Netflix's improvement amounts to having a better rate control, that understands scene differences and uses lower quantizers on more complex scenes. There's fundamentally no reason why you couldn't add such a rate control algorithm to x264. It might require using a 2-pass mode, but it could be done.

zbobet2012 6y ago
> Reread my original post carefully - my point is that if you're stuck with 1080p as the maximum resolution, than the quality you can get with the revised ladder is strictly worse, because Netflix will want to jump up to UHD resolutions, but your device won't support it. These are for their HEVC encodes. Apple (and most others) require H264 to have a separate ladder which will be 1080p limited and will be generated on different encode set and maximum video quality.* Almost any HEVC capable device can go to 4k, even if the screen is only 1080p. It will be used than down scaled. > (And even assuming that the UHD will be better based on this chart is kind of a leap, because it depends on what artifacts you're most sensitive to. Objective metrics ≠ subjective experience. In several of their comparisons, I prefer the lower res version because the new one is simply too artifacted.) Certainly objective and subjective metrics have differences, but VMAF does correlate pretty tightly with subjective metrics. This is a very detailed subject, I summed up many of my thoughts on the differences in my demuxed talk last year here: https://www.youtube.com/watch?v=nCUsXhSPyyw https://www.youtube.com/watch?v=nCUsXhSPyyw * https://developer.apple.com/documentation/http_live_streaming/hls_authoring_specification_for_apple_devices https://developer.apple.com/documentation/http_live_streamin... 1.23 iirc
- bscphil 6y ago
  > These are for their HEVC encodes. Really? I wasn't aware Netflix even offered anything other than H.264 at <= 1080p. I know Youtube doesn't. If this is all HEVC maybe you're right and the resolution issue isn't such a big deal. If this is actually the case I wish they would have said so explicitly in the blog post. > VMAF does correlate pretty tightly with subjective metrics. This is a very detailed subject, I summed up many of my thoughts on the differences in my demuxed talk last year here: https://www.youtube.com/watch?v=nCUsXhSPyyw https://www.youtube.com/watch?v=nCUsXhSPyyw Thanks, I'll check that out when I have the time. I'd still emphasize that it's quite possible for objective metrics to be fooled (that's why codecs have psy tunings!), and I do prefer several "old-ladder" images over the newer versions in Netflix comparisons here. I suppose I've become rather cynical about claims made on the basis of metrics ("now 50% less bitrate for the same quality!"), when to my eye, web video has always been very bad and is getting worse as companies keep reducing the bitrate.