Explanation of x264 tune

Question

Running this command:

ffmpeg -i xr.mp4 -tune ya zu.mp4

reveals the possible tune values for x264 and in turn FFmpeg:

[libx264 @ 0000000002167100] Possible tunes: film animation grain stillimage psnr ssim
                                             fastdecode zerolatency

However I could not find a reference explaining what these values actually do.

Answer 1

This is what the modes do:

• film – intended for high-bitrate/high-quality movie content. Lower deblocking is used here.
• animation – intended for cartoons, etc., where deblocking is boosted to compensate for larger, flat areas. More reference frames are used.
• grain – this should be used for material that is already grainy. Here, the grain won't be filtered out as much.
• stillimage – like the name says, it optimizes for still image encoding by lowering the deblocking filter.
• psnr and ssim – these are debugging modes to optimize for good PSNR and SSIM values only. Better metrics don't necessarily mean better quality though.
• fastdecode – disables CABAC and the in-loop deblocking filter to allow for faster decoding on devices with lower computational power.
• zerolatency – optimization for fast encoding and low latency streaming

You can see the detailed options applied with each tune with x264 --fullhelp:

--tune <string>         Tune the settings for a particular type of source
                          or situation
                              Overridden by user settings.
                              Multiple tunings are separated by commas.
                              Only one psy tuning can be used at a time.
                              - film (psy tuning):
                                --deblock -1:-1 --psy-rd <unset>:0.15
                              - animation (psy tuning):
                                --bframes {+2} --deblock 1:1
                                --psy-rd 0.4:<unset> --aq-strength 0.6
                                --ref {Double if >1 else 1}
                              - grain (psy tuning):
                                --aq-strength 0.5 --no-dct-decimate
                                --deadzone-inter 6 --deadzone-intra 6
                                --deblock -2:-2 --ipratio 1.1 
                                --pbratio 1.1 --psy-rd <unset>:0.25
                                --qcomp 0.8
                              - stillimage (psy tuning):
                                --aq-strength 1.2 --deblock -3:-3
                                --psy-rd 2.0:0.7
                              - psnr (psy tuning):
                                --aq-mode 0 --no-psy
                              - ssim (psy tuning):
                                --aq-mode 2 --no-psy
                              - fastdecode:
                                --no-cabac --no-deblock --no-weightb
                                --weightp 0
                              - zerolatency:
                                --bframes 0 --force-cfr --no-mbtree
                                --sync-lookahead 0 --sliced-threads
                                --rc-lookahead 0