ultrafast vs slow preset in x264, and the resulting visual quality

[codemaster]

I want to transcode 1080i video to 720p, relatively fast, using x264. The content of the video is sports, and it has alot of rapid movement between frames.

I used the preset "Slow", and CRF 24. On my computer, with these settings, the transcoding speed is about 3 frames per second.

After that, I tried to transcode again, but this time, I used the preset "Ultra Fast" and CRF 24. The transcoding speed was 8 frames per second. Then I opened the transcoded 720p video to check if the visual quality is lower than the one transcoded with preset "Slow", but I didn't notice any difference in visual quality. To me, both transcodes had the same visual quality.

The only difference that I noticed is that the h264 stream that was encoded with preset slow, has ~10 MB, and the other one, encoded with preset ultrafast, has ~30 MB.

So the x264 presets, like slow, slower, superfast, ultrafast, placebo, etc, are just for the level of compressibility ? About how much the video will be compressed ? Or does it also have an impact on the visual quality, and I didn't notice it ?

I'm asking this because I would prefer to use the preset ultrafast to transcode as fast as possible, but without trading visual quality for speed of transcode (if possible), since the video that I'm transcoding will be sitting on my hdd temporarily, meaning I will delete it after watching it, therefore I don't mind if it has 3 GB instead of 1 GB.

[fritzi93]

You discovered the answer yourself. If you're using CRF settings, the faster preset will result in larger file size. I presume because slower settings can be more efficient in allocating bits while still respecting the quality setting.

Now, as to whether the result is identical visually, not necessarily. At least according to a few posts on Doom9 by the developer of the x264 encoder. But that's the idea anyway, a given quality setting should give you approximately the same visual quality.

[jagabo]

When using CRF encoding, generally the slower presets generate smaller files. But they also generate slightly better visual quality. If you look closely at moving edges you'll see they're rougher at the faster settings. You also get better retention of small low contrast detail (film grain), especially in dark areas, that helps keep away posterization artifacts (banding, macroblocking).

By the way, you can make up for most of the loss of grain and macroblocking by adding "--aq-mode=2 --aq-strength=2.0" after the ultrafast preset (set aq-strength somewhere between 1.0 and 2.0). It will eat up even more bitrate though. You can also smooth out moving edges a bit by adding "--subme=6". That will slow it down a bit though.

[MedConv]

xxxxx

[hello_hello]

I'm asking this because I would prefer to use the preset ultrafast to transcode as fast as possible, but without trading visual quality for speed of transcode (if possible), since the video that I'm transcoding will be sitting on my hdd temporarily, meaning I will delete it after watching it, therefore I don't mind if it has 3 GB instead of 1 GB.

That does beg the question.... why re-encode it at all?

3fps for the Slow x264 preset at 720p sounds a little err..... slow, and 8fps for UltraFast sounds a lot slow, but it might depend on your CPU. What filtering are you using, if any? I assume you're de-interlacing?
I'd check your CPU usage while the video is being converted. You may find not all cores are running flat out which would probably mean there's a bottleneck somewhere other than the encoder speed. It might even be the decoding speed. If there is a bottleneck, you could try running more than one encode simultaneously to speed things up that way.

If it helps.... I just tried a couple of quick 1080p to 720p encodes using this fairly old E6750 dual core. MeGUI/Avisynth.
Well it was 1920x800 to 1280x720 as it was just a test and that's the closest thing I had handy. No de-interlacing. CRF24. For the x264 Slow preset it seemed to settle down to about 5fps, but for the UltraFast preset it sat on a fairly steady 28fps.