I'm reading a bit on I-frames and their counterparts, P-frames and B-frames. I understand the usage. I-frames are the key to the compression and following P and B-frames can use the data in I-frames to save on bits.

However, what I would find interesting is knowing how the encoder determines which frames should be I-frames and which should be P's and B's and pull their data from them. How does the encoder decide which frames will be I-frames?

Side thoughts (you don't have to answer, but these are interesting and useful to me in the future): Are I-frames easier, as in faster, to find and extract than say extracting every 30th frame? If a video is a slide show with audio and no slide animations, are I-frames likely to coincide with slide changes? Would compression be better on a video made from slides images rather than the same images taken in via stream in real time?

  • Should I have used the keyframes tag? Should there be an i-frames tag or should it be a synonym? – user24601 Jan 23 '17 at 10:20
  • In addition to @Mulvya's fine answer, there is the case of older codecs and encoders that place I-frames at fixed intervals regardless of need. This was the era of the 'fixed-length GOP'. – Jim Mack Jan 23 '17 at 15:43
  • 'keyframe' is used in After Effects and other animation apps to describe a point on a timeline where a value is set. 'I-Frame' is more specific – stib Jan 23 '17 at 22:26
  • The same term is used within ffmpeg and x264 code, and I suspect, most/all other codecs. – Gyan Jan 24 '17 at 3:56

This is a complex topic, with the exact algorithm unique to each encoder.

Below is a pseudocode explanation from a x264 developer. B-frames aren't accounted for, but basic logic should be similar.

encode current frame as (a really fast approximation of) a P-frame and an I-frame.

if ((distance from previous keyframe) > keyint) then
    set IDR-frame
else if (1 - (bit size of P-frame) / (bit size of I-frame) < (scenecut / 100) * (distance from previous keyframe) / keyint) then
    if ((distance from previous keyframe) >= minkeyint) then
        set IDR-frame
        set I-frame
    set P-frame

encode frame for real.

scenecut is the scene change threshold value. 0 means current frame is identical to previous frame, and 100 means it is completely different.

keyint is the maximum permitted distance between two keyframes; minkeyint is the minimum.

IDR (instantaneous decoder refresh) frames are keyframes such that no future frame requires to refer to a frame earlier than the IDR-frame for decoding. Not necessarily true for plain I-frames.

| improve this answer | |

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.