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FRAME_TYPE_SELECTION.md

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Frame Type Selection

Table of Content

Current Features/Process

  • The first frame of the video is always a key frame.
  • rav1e looks ahead up to the maximum number of frames in a sub-GOP to detect "flashes" of content, which are then marked ineligible for a scenecut.
  • If the user has forced the current frame to be a key frame, it is marked as a key frame. This overrides all other criteria for frame type selection. (TODO: How does a user do this?)
  • If there have been fewer than the number of frames specified by --min-keyint since the last key frame, the current frame is marked as an inter frame.
  • If there have been equal to the number of frames specified by --keyint (i.e. the max keyint) since the last key frame, the current frame is marked as a key frame.
  • If no other criteria have been met, the current frame is compared with the previous frame to see if it is a scenecut. If it is a scenecut, it is marked as a key frame, otherwise it is marked as an inter frame.

Detection Algorithm

  • On speeds 0-9, the algorithm compares frame intra cost vs. inter cost. This is better for compression, but slower.
  • On speed 10, the algorithm compares the amount of difference between frames.

Desired Improvements

  • If the max keyint length is in the middle of a flash of content, the key frame should be placed at either the start or end of the flash, instead of in the middle (exactly on the max keyint).