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  1. Ok, I tried googling this many times, still no answer and proper explanation how keyframe works, and which keyframe interval is the best for recording screen. I'm using bandicam using NVENC+AAC to record Netflix shows and also record games for youtube, the main thing that im trying to understand is what the heck is Keyframe Interval?
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  2. A Member since June, 2004 Keyser's Avatar
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    Your Google must be different from mine, because searching "keyframe" in MY Google, brings a lot of information.
    "The greatest trick the Devil ever pulled was convincing the world he didn't exist."
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    Originally Posted by kairukun View Post
    ....... the main thing that im trying to understand is what the heck is Keyframe Interval?
    It's the "distance" between two I-frames in a (compressed) video stream.

    http://en.wikipedia.org/wiki/Video_compression_picture_types

    http://en.wikipedia.org/wiki/Inter_frame

    which keyframe interval is the best for recording screen.
    1. Also, one should always use a secondary HDD for writing the capture file.
    Last edited by El Heggunte; 19th May 2015 at 13:16. Reason: edit
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    For best quality you can never have too many keyframes. But if you constrain your transcode to an average bitrate then too many keyframes will leave too little left for compression. Also it depends on the content.

    So, really it is a balancing act.

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  5. Keyframes are frames that don't rely on data from other frames to be displayed fully, so they're the most expensive in respect to bitrate and the highest quality, although if the encoder is doing a good job you shouldn't be able to see a huge difference between keyframes and other types of frames.
    Ever played a video where for a little bit blocks of the picture don't change from frame to frame, then suddenly the picture comes good? That'd be because some initial frames required data from other frames and the decoder wasn't getting it right, then you hit a keyframe which is a "standalone" frame and things goes back to normal.

    By default, the x264 encoder sets the maximum keyframe interval to 250, which would be 10 seconds for 25fps video. Changing it to 240 for 24fps wouldn't be unusual. Some GUI's do it automatically to keep the maximum keyint (keyframe interval) setting to 10 seconds. For x264 the default minimum keyint is 25 or one second. The x264 encoder tends to be very good at detecting scene changes and puts a keyframe at the beginning of a scene. Having a minimum keyint stops it from using a bunch of keyframes in a row when it thinks each new frame is a new scene (it might happen if there's a strobing effect in the video). If the first frame of each scene is a keyframe chances are the maximum keyint of ten seconds won't be reached all that often. The x264 encoder handles it all very well and probably should be left at the default settings. I'm not really sure about other encoders and how clever they may or may not be in respect to keyframes, scene changes and keyframe interval, although the only Nvenc encode I've seen appeared to put keyframes in the logical places (at the beginning of scenes).

    The Bluray spec requires a maximum keyframe interval of one second, so if you're encoding to create Bluray compliant video it might pay to change it to keyint 24 and minimum keyint 1, but other than that, what sort of keyframe options can you set? Anything you like? For a given quality smaller keyframe intervals will increase the biitrate. There's also a phenomenon called "keyframe pumping" where the quality is fine for a keyframe, then slowly deteriorates through the proceeding group of pictures, then suddenly improves again at the next keyframe and the cycle continues. That's another reason for an encoder to put keyframes at the beginning of scenes. If the quality changes at the same time as the scene, it's much less noticeable. For a decent encoder at decent bitrates though, keyframe pumping shouldn't be an issue no matter where the keyframes are.
    Last edited by hello_hello; 19th May 2015 at 17:26.
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