Key Frames and Key Frame Distance

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Aug 22, 2015 Aug 22, 2015

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I posted this is the Premiere forum two months ago, but no response.

I’m trying to understand where Key Frames are positioned, but the info I have found on the web is unclear. Here is Adobe’s explanation from the Adobe Media Encoder PDF:

Explanation 1

Key Frames are complete video frames (or images) that are inserted at consistent intervals in a video clip. The frames between the Key Frames contain information on changes that occurs between Key Krames.

By default, AME automatically determines the key frame interval (Key Frame Distance) to use based on the frame rate of the video clip.

If your footage has a lot of scene changes or rapidly moving motion or animation, then the overall image quality may benefit from a lower Key Frame Distance. A smaller key frame distance corresponds to a larger output file.

In another Adobe document it states that a Key Frame Distance is the:

Explanation 2

Number of frames after which the codec will create a key frame when exporting video.

Somewhere else it states (re Key Frame Distance):

Explanation 3

This sets the maximum # of frames between key frames. If this # of frames without a keyframe occurs, the compressor will insert a keyframe regardless of whether the scene has changed.

There are differences between the explanations:

  1. the first says that Key Frames are created at a constant separation;
  2. the second implies that Key Frames may not be at a constant separation (because a new Key Frame will be created a certain number of frames after the previous Key Frame, but doesn’t say anything about the Key Frame before that one);
  3. the last explanation is quite clear (but at variance with the first): The Key Frame distance is the maximum number of frames allowed between Key Frames.

My interest in Key Frame Distance is because I have noticed a very slight, brief shimmering at one-second intervals in my slide shows exported from Premiere in h.264 (Blu-ray), 23.976 fps, 20-30 Mbps bit rate, 2-pass, Key Frame Distance unticked (with “23” appearing dimmed). It’s a very subtle effect, not always visible. I assume the shimmering is when a new Key Frame has been inserted during a still. Whether or not that really is the cause, I'd still like to know more about Key Frames. Most of my Premiere work is with stills, with the occasional video clip from a Lumix GH3 (1920 x 1080, 23.976 fps, AVCHD) thrown in. The stills have mainly simple dissolve transitions.

Q1: I assume the clips out of my GH3 have, as per Explanation 1, a constant distance between Key Frames. If such a clip is cropped inside Premiere at a frame that is not a Key Frame, does Premiere, when exporting, insert a new Key Frame at the crop point, using information contained in the original clip to create the image at that particular frame?

Q2: In general, where two unrelated video clips abut (but not at Key Frames), what does Premiere do about Key Frames? Add one at the last frame of the first clip, and at the first frame of the second clip? Then apply the Key Frame Distance parameter to ensure that within each clip there are a suitable number of Key Frames?

Q3: For stills, does Premiere insert Key Frames at the Key Frame Distance? Or is it clever enough to know that: “Hey, this is a still. No Key Frames are needed.”

Q4: Where are Key Frames created for a still that is dissolving? Assume a Key Frame Distance much greater than the dissolve time. If I was designing Key Frame inserting software, I would create a Key Frame at the start of the dissolve, one in the centre, another at the end of the dissolve, and then no more until the next clip.

Q5: Can Premiere (or AME or After Effects) tell me where Key Frames are located? Can I move them?

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