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Steinar H. Gunderson: 10-bit H.264 tests

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Following the post about 10-bit Y'CbCr earlier this week, I thought I'd make an actual test of 10-bit H.264 compression for live streaming. The basic question is; sure, it's better-per-bit, but it's also slower, so it is better-per-MHz? This is largely inspired by Ronald Bultje's post about streaming performance, where he largely showed that HEVC is currently useless for live streaming from software; unless you can encode at x264's “veryslow” preset (which, at 720p60, means basically rather simple content and 20 cores or so), the best x265 presets you can afford will give you worse quality than the best x264 presets you can afford. My results will maybe not be as scientific, but hopefully still enlightening.

I used the same test clip as Ronald, namely a two-minute clip of Tears of Steel. Note that this is an 8-bit input, so we're not testing the effects of 10-bit input; it's just testing the increased internal precision in the codec. Since my focus is practical streaming, I ran the last version of x264 at four threads (a typical desktop machine), using one-pass encoding at 4000 kbit/sec. Nageru's speed control has 26 presets to choose from, which gives pretty smooth steps between neighboring ones, but I've been sticking to the ten standard x264 presets (ultrafast, superfast, veryfast, faster, fast, medium, slow, slower, veryslow, placebo). Here's the graph:

The x-axis is seconds used for the encode (note the logarithmic scale; placebo takes 200–250 times as long as ultrafast). The y-axis is SSIM dB, so up and to the left is better. The blue line is 8-bit, and the red line is 10-bit. (I ran most encodes five times and averaged the results, but it doesn't really matter, due to the logarithmic scale.)

The results are actually much stronger than I assumed; if you run on (8-bit) ultrafast or superfast, you should stay with 8-bit, but from there on, 10-bit is on the Pareto frontier. Actually, 10-bit veryfast (18.187 dB) is better than 8-bit medium (18.111 dB), while being four times as fast!

But not all of us have a relation to dB quality, so I chose to also do a test that maybe is a bit more intuitive, centered around bitrate needed for constant quality. I locked quality to 18 dBm, ie., for each preset, I adjusted the bitrate until the SSIM showed 18.000 dB plus/minus 0.001 dB. (Note that this means faster presets get less of a speed advantage, because they need higher bitrate, which means more time spent entropy coding.) Then I measured the encoding time (again five times) and graphed the results:

x-axis is again seconds, and y-axis is bitrate needed in kbit/sec, so lower and to the left is better. Blue is again 8-bit and red is again 10-bit.

If the previous graph was enough to make me intrigued, this is enough to make me excited. In general, 10-bit gives 20-30% lower bitrate for the same quality and CPU usage! (Compare this with the supposed “up to 50%“ benefits of HEVC over H.264, given infinite CPU usage.) The most dramatic example is when comparing the “medium” presets directly, where 10-bit runs at 2648 kbit/sec versus 3715 kbit/sec (29% lower bitrate!) and is only 5% slower. As one progresses towards the slower presets, the gap is somewhat narrowed (placebo is 27% slower and “only” 24% lower bitrate), but in the realistic middle range, the difference is quite marked. If you run 3 Mbit/sec at 10-bit, you get the quality of 4 Mbit/sec at 8-bit.

So is 10-bit H.264 a no-brainer? Unfortunately, no; the client hardware support is nearly nil. Not even Skylake, which can do 10-bit HEVC encoding in hardware (and 10-bit VP9 decoding), can do 10-bit H.264 decoding in hardware. Worse still, mobile chipsets generally don't support it. There are rumors that iPhone 6s supports it, but these are unconfirmed; some Android chips support it, but most don't.

I guess this explains a lot of the limited uptake; since it's in some ways a new codec, implementers are more keen to get the full benefits of HEVC instead (even though the licensing situation is really icky). The only ones I know that have really picked it up as a distribution format is the anime scene, and they're feeling quite specific pains due to unique content (large gradients giving pronounced banding in undithered 8-bit).

So, 10-bit H.264: It's awesome, but you can't have it. Sorry :-)

Source: http://blog.sesse.net/blog/tech/2017-02-27-01-02_10_bit_h264_tests.html

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