tree c33759b5ebc2b09fdbc999e68e0ebc304fa32e48
parent cf0505a8612456178aa4e9b8321314a23e52f7cd
author Vignesh Venkatasubramanian <vigneshv@google.com> 1684362275 -0700
committer Vignesh Venkatasubramanian <vigneshvg@users.noreply.github.com> 1684789112 -0700

Use global configs for keyframe interval

Instead of computing the required keyframe interval in libavif
code and passing the "force keyframe" flag on individual frames,
use each encoder's global configuration parameter to set the
maximum keyframe interval.

For libaom: `kf_max_dist`.
For rav1e: `key_frame_interval`.
For svt-av1: `intra_period_length` and `force_key_frames`.

Here's the output obtained from various encoders for encoding an
animated file with 10 frames with different values for
--keyframe. Each line is read as follows:
--keyframe value: list of key frames before => list of keyframes after.

aom:
 --keyframe 0: same before and after.
 --keyframe 1: same before and after.
 --keyframe 2: 1,2,3,4,5,6,7,8,9,10 => 1,3,5,7,9 (the old behavior was not incorrect, but new one is exact).
 --keyframe 5: 1,2,6,7 =>1,6 (the old behavior was not incorrect, but new one is exact).

rav1e:
  * no changes. same output before and after.

svt-av1:
 --keyframe 0: same before and after.
 --keyframe 1: 1 => 1,2,3,4,5,6,7,8,9,10 (old behavior was incorrect).
 --keyframe 2: 1 => 1,3,5,7,9 (old behavior was incorrect).
 --keyframe 5: 1 => 1,6 (old behavior was incorrect).

To sum up:
 * svt-av1 was not producing the intended output with --keyframe
   before this change.
 * libaom had some weirdness in keyframe spacing when using the
   force keyframe flag on each frame and that weirdness goes away
   when using the kf_max_dist configuration (see:
   https://bugs.chromium.org/p/aomedia/issues/detail?id=3445).

The `AVIF_ADD_IMAGE_FLAG_FORCE_KEYFRAME` flag is retained in the
codebase since there are some follow-up changes that will make
use of the flag to synchronize keyframe placement between the
color and alpha plane encoders (Issue #841).