I recently got it into my head to compare the various popular video codecs in an effort to better understand how av1 works and looks compared to x264 and x265. I also had ideas of using a intel video card to compress a home video security setup, and what levels of compression I would need to get good results.
The Setup
I used the 4k 6.3gb blender project, tears of steel as a source. I downscaled the video to 1080p using all three codecs, and then attempted to compare the results using various crf levels.
To compare results I used imgsli, FFMetrics, and my own picture viewer to try and see what the differences are.
The Results
| crf | av1 KB | x265 KB | x264 KB |
|---|---|---|---|
| 18 | 419,261 | 632,079 | 685,217 – x246 visually lossless |
| 21 | 352,337 | 390,358 – x265 visually lossless | 411,439 |
| 24 | 301,517 – av1 VAMF visually lossless | 250,426 | 263,524 – x264 good enough |
| 27 | 245,685 | 165,079 – x265 good enough | 176,919 |
| 30 | 205,008 | 110,062 | 122,458 |
| 33 | 168,192 | 73,528 | 86,899 |
| 36 | 139,379 – av1 My visually lossless | 48,516 | 63,214 |
| 39 | 116,096 | 31,670 | 47,161 |
| 42 | 97,365 – av1 my good enough | 20,636 | 35,801 |
| 45 | 81,805 | 13,598 | 27,484 |
| 48 | 69,044 | 9,726 | 20,823 |
| 51 | 58,316 | 8,586 – worst possible | 16,120 – worst possible |
| 54 | 48,681 | - | - |
| 57 | 39,113 | - | - |
| 60 | 29,062 | - | - |
| 63 | 16,533 – worst possible | - | - |
I go into more detail with the hows and whys of my choices, in my journal-style blog post, as well as how i came to these conclusions, But in essence, if you want to lose practically no visual information, crf24 through 36 for av1, crf 21 for x265, and crf 18 for x264 will do the job.
If you are low on space, using my 'good enough' choices will get you practically the same visual results while using less space, depending on the codec.
Feels like certain information is missing. You get very different results both in encoding time and file size depending what preset you use.
CRF value also can't be translated 1:1 between codecs so comparing e.g. h265 CRF 21 to h264 CRF 21 doesn't mean much.
I consider the 'good enough' level to be, if I didn't pixel peep, I couldn't tell the difference. The visually lossless levels were the first crf levels where I couldn't tell a quality difference even when pixel peeping with imgsli. I also included VAMF results, which say that the quality loss levels are all the same at a pixel level.
I know that av1, x264, and x265 all have different ways of compressing video. Obviously, the whole point of this was to get a better idea of what that actually looked like. Everything on the visually lossless section is completely indistinguishable to my eyes, and everything on the good enough section has very minor bits of compression only noticed when i'm looking for it in a still image. This does not require the same codec to compare and contrast with.
Frankly, for anything other than real-time encoding, I don't actually consider encoding time to be a huge deal. None of my encodes were slower than 3fps on my 5800x3d, which is plenty for running on my media server as overnight job. For real-time encoding, I would just grab a Intel Arc card, and redo the whole thing since the bitrates will be different anyways.
Encoding speed heavily depends on your preset. Veryslow will give you better compression than medium or fast, but at a heavy expense of encoding speed. You're not gonna re-encode a movie overnight on slow preset. GPU encoding will also give you worse result than CPU encode so that's something one would have to take into consideration. It's not a big deal when you're streaming, but if it's for video files, I'd much prefer using the CPU.
I was mostly talking about how you organised your table by using CRF values as the rows. It implies that one should compare the results in each row, however that wouldn't be a comparison that makes much sense. E.g. looking at row "24" one might think that av1 is less effective than h264/5 due to greater file size, but the video quality is vastly different. A more "informative" way to present the data might have been to organise each row by their vmaf score.
Hopefully I don't come across as too cross or argumentative, just want to give some feedback on how to present the data in clearer way for people who aren't familiar with how encoding works.
Why is GPU encoding worse than CPU encoding?
GPU encoding uses (relatively) simpler fixed function encoders that do it much faster than the CPU which uses its general purpose transistors to run an encoding algorithm. End result is GPU encoding is speedy at the cost of visual quality per bitrate; the file size is bigger for same visual quality as a CPU encode. Importantly for storing your videos - CPU encoding, while much slower, will get your file size smaller at the same visual quality threshold you desire, so you can save more videos per drive!