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One of the interesting things you can do with ffmpeg, if you have a LOT of scratch disk space, and you're trying to compare subjective encoder quality (not VMAF

One of the interesting things you can do with ffmpeg, if you have a LOT of scratch disk space, and you're trying to compare subjective encoder quality (not VMAF, but human eyeballs) on a certain video file: 1) take your raw uncompressed y4m file and write it out to a directory of PNG files, one png file per frame. this is your static image reference baseline for subjective eyeballs. 2) take your raw uncompressed y4m file and encode it to x265 or whatever codec you're testing, at various different bitrates and encoder settings 3) take your various encoded x265 files and also write those out to PNG files in separate directories 3) pick exactly the same frame number filename from your 'master' PNGs and your encoder-output PNGs, copy them and put them in the same directory so you can quickly flip back and forth between them in a image slideshow application. If you want to do this with 2160p24,p25 or p30 videos at several minute lengths, be prepared to have 150-160GB of scratch disk per y4m and per PNG-dump-directory.

leokennis 6y ago
Thing is, x264/265 are explicit designed to provide better subjective quality in moving images. For example, x264 will “allocate” more data to parts of the image that change less, as those remain on screen longer so artifacts on them are more visible. While fast moving parts can be compressed into a blurry blocky haze as they appear for maybe 20 frames so few people will really notice artifacts. (Of course the actual implementation is 100x more sophisticated and complex) You are doing the opposite with PNG’s: focusing on quality of 1 frame instead of on the perceived quality of the frames when viewed as video.
- walrus01 6y ago
  That's indeed true - and why for subjective eyeball moving picture tests you should also do things like cut out a 20 second piece of a raw/y4m file, encode it with various x265 settings, and then put those 4 or 5 pieces in a VLC playlist so you can quickly compare them in series. For objective comparisons there is of course VMAF, which is an essential tool for doing automated comparisons of videos vs their uncompressed original. One of the reasons why VMAF was created is that subjective eyeball evaluation of codecs (whether still or moving) will vary between person to person, and is very labor intensive. But still image PNGs of frames also serve a useful purpose when dealing with lower bitrate videos, for your personal opinion comparison of blockiness, color banding, blobs of color in areas that are mostly the same color, etc.
fomine3 6y ago
Theoretically, when comparing frames, frame type (I/P/B) should be considered and it's better to compare same frame type. How do you treat about frame types?

leokennis 6y ago

Thing is, x264/265 are explicit designed to provide better subjective quality in moving images. For example, x264 will “allocate” more data to parts of the image that change less, as those remain on screen longer so artifacts on them are more visible. While fast moving parts can be compressed into a blurry blocky haze as they appear for maybe 20 frames so few people will really notice artifacts. (Of course the actual implementation is 100x more sophisticated and complex) You are doing the opposite with PNG’s: focusing on quality of 1 frame instead of on the perceived quality of the frames when viewed as video.

walrus01 6y ago
That's indeed true - and why for subjective eyeball moving picture tests you should also do things like cut out a 20 second piece of a raw/y4m file, encode it with various x265 settings, and then put those 4 or 5 pieces in a VLC playlist so you can quickly compare them in series. For objective comparisons there is of course VMAF, which is an essential tool for doing automated comparisons of videos vs their uncompressed original. One of the reasons why VMAF was created is that subjective eyeball evaluation of codecs (whether still or moving) will vary between person to person, and is very labor intensive. But still image PNGs of frames also serve a useful purpose when dealing with lower bitrate videos, for your personal opinion comparison of blockiness, color banding, blobs of color in areas that are mostly the same color, etc.

fomine3 6y ago

Theoretically, when comparing frames, frame type (I/P/B) should be considered and it's better to compare same frame type. How do you treat about frame types?