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8th May 2013, 17:33 | #1 | Link |
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Updated Rule of ^3/4 for H.264 high profile?
So, a rule of thumb in compression has long been that changing frame size while maintaining similar subjective quality doesn't entail a linear increase/decrease it bitrate.
The classic rule of thumb had been that the change in pixel area should match a bitrate change to the power of 3/4ths of the pixel area. Thus going from a 640x480 1 Mbps to 1280x960 would entail 4^0.75=2.83 Mbps. Conversely, going down to 320x240 would allow a reduction to 0.25^0.75=0.35 Mbps This originally seems to have been calibrated around MPEG-4 pt 2, however. Zambelli did some extensive testing a few years ago and discovered 0.71 was a better value for VC-1. I imagine that in-loop deblocking would tend to reduce the value. Thus, I expect that the exponent would be even lower for H.264. I was wondering if anyone has come up with their own rule of thumb, experimentally or otherwise. I expect that High Profile is probably lower than Main, which is lower than Baseline. I expect the exponent would be even lower for HEVC with its huge block sizes and other features to very efficiently encode areas of lower detail. |
9th May 2013, 01:21 | #3 | Link | |
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But more broadly, I do think more advanced codecs will need fewer pixels per bit as frame size goes up. Hmmm. There's probably a similar heuristic for frame rate. It'll be lower since increasing frame rate reduces the motion between frames. |
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9th May 2013, 10:00 | #4 | Link | |
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A different question would be about the crossover for the resolution for similar subjective quality, given a fix bitrate and a fix display size (e.g. TV screen). |
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9th May 2013, 11:26 | #5 | Link | ||
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Since mpeg4p2 doesn't really have any intra prediction, and a very basic motion prediction, I think most of the explanation for the rule of thumb (instead of the expected linear relationship) lies in the interaction between 8x8 DCT transform, details & quantization. H264 has a good intra prediction, a better motion prediction (especially skip), an adaptive entropy coder, so it can handle large resolution a lot better. "Sadly", I think low resolution coding was improved even more (4x4 DCT, 4x4 partitions), so rule may not have changed that much. HEVC somewhat doesn't care about small resolutions (if I'm not mistaken, no more 4x4 inter partitions), and really improved the coding of large ones. There's no doubt in my mind that the exponent will be lower. Quote:
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10th May 2013, 20:03 | #6 | Link | |
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For example, comparing the extra bits to encode a 60p source at a full 60p instead of just 30p. If the frame rate exponent was 0.5 (arbitrary choice), that would mean that doubling frame rate would require an increase of 41% in bitrate. Comparing content shot at 24p with 1/48th of a second shutter versus 48p shot with 1/72nd shutter would require a higher exponent. The hierarchical B-frame example is interesting, but the differences of higher frame rate wouldn't be just that. For one, we could have maxed out the hierarchy possible at 30p already (I think x264 still only does one reference and one non-reference layer of B-frames). Second, the higher frame rate means that there are twice as many frames to pick between for reference frames, so we'd be able to pick better reference frames some of the time. We'd probably get longer B-frame chains on average as well, since there will be less new visual information per frame. |
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10th May 2013, 23:08 | #7 | Link | |||
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I've made a test encoding with x264 + 3B hierarchical, and average bitrate for all bframes ended at half the overall bitrate. With non reference bframes roughly twice as small as reference ones, non reference bframes have an average bitrate of 3/8th of the overall. That roughly means doubling the framerate by adding non reference bframes would increase the bitrate by 37.5%. Adding a layer to the pyramid would probably reduce the increase to 30%. Quote:
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13th May 2013, 18:50 | #8 | Link | ||||
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For example, The Hobbit was shot at 48p with 1/72nd exposure time. That's the average of the 1/48th you'd normally have with 24p and the 1/96th you'd expect with 48p. I think the 48p version would have looked a lot better with a 1/96th shutter, and that was probably one of the reasons it looked so weird to so many customers. Quote:
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Last edited by benwaggoner; 14th May 2013 at 20:28. Reason: Fix Quotes |
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14th May 2013, 20:32 | #10 | Link | |
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filmmakermagazine.com/60811-the-hobbit-arrives |
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18th March 2014, 19:02 | #11 | Link |
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Hello, I am developing my Master's dissertation on video quality for adaptive systems. I am very interested in exactly how the ^3/4 principle was developed in the first place and might be able to update it to H.264/x264 at 1080p. I understand this refers to subjective quality. Was this callibration done by a single individual's subjective perception, or were MOS tests conducted, or even a combination of these with objective metrics such as SSIM?
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x**3/4, x264 bitrate |
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