Some people were discussing x264 CRF. The fact is you cannot compare same CRF at different settings because they yield different quality. CRF does not mean same quality; it' s just a method of rate control
Here is one of the older examples - It's the Planet Earth trailer. I couldn't find the older encodes on my archived HDD's, so I just redid them with the current x264. Yadif(order=1, mode=0) to single rate deinterlace, spline36resize to 720x400 in this example. Ok you can also argue that it's a "trailer" and not "typical" content either... But do enough of these tests, and I assure you will see differences 99 times out of 100
Typically, you will see lower b-frame quality, and problems where there is motion, expecially with fine details like grass, grain etc....(open the pics in different tabs and flip back & forth) . Using one of the quicker presets , those types details won't be preserved as well. These screenshots below are b-frame comparisons. I'm uploading the source clip and encodes and will update once they are finished
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Another example from a consumer camcorder a few years ago. I couldn't find the tests, so I redid them with current x264 as above. These aren't new findings - this has been established years ago with dozens of tests posted (and thousands probably not posted). I just can't seem to find the threads (maybe they were at doom10 which is down now of course...)
The MTS original clip can be downloaded from vimeo if you are signed in
Again - b-frame comparions: Notice the fine fur details are obliterated in the superfast encode, also that the file size "trend" is switched (slower encode is actually larger). If you look on other frames, the backround details are reduced as well, the moss is over smoothed for example. This detail loss is typical 99 times out of 100 when looking at quicker settings vs. slower settings for the vast majority of source scenarios. It's not a "cherry picked" example. The only time you might not see differences if source had few details to begin with, or there is very little motion, e.g. maybe some low quality animation
Interesting, you were right the whole time. The one with weak settings does look worse despite the higher bitrate. But what did I do in my limited tests that was so different? The only major difference I see is that I had mb-tree enabled with both which might play a part. But I realize complaining about this invalidates what I originally thought anyway because a CRF was supposed to maintain the quality regardless of the settings.
I also expected the ultraslow one to be lower quality. I vaguely remember around 2009 how I had to lower the CRF if I used very strong settings because x264 would under-allocate bits, the video would be twice as smaller than medium preset and looked slightly worse.
In order to see differences, you need more complex motion and/or details . I think the main difference was the source - the one you posted looked to be low in detail (it looked like a multi generation encode, oversmoothed), so the resulting differences were fairly minor to the human eye (you'd still be able to measure the differences with metrics). For the same reasons, you wouldn't see large differences on "simple" cartoons either, but you would see it to a larger extent on modern high detail CG
You can repeat the tests with --preset veryfast, which has mb-tree, and you will still see differences. The point is that there are differences in quality, both measureable and visible at the same CRF