> The way my scene change detection heuristic works like this: I trigger a > scene change (and therefore discard the frame averaging buffer) if the > distance between the current frame average brightness and the current > running average exceeds a threshold value, that threshold being (by > default) 20 cd/m². > > So we can divide the failures of this algorithm into two categories: > > 1. False negative (scene change without resetting the buffer): This can > only happen if there was an actual scene change but the average > brightness change did not exceed 20 cd/m², i.e. the scenes are > similar in brightness. I consider this a fairly harmful failure > because that also means there's no visual discontinuity since the > scenes are so similar to begin with. > > 2. False positive (buffer got reset without a scene change). This is the > more worrying failure of the algorithm, since it can happen in the > middle of a scene (e.g. as the result of a bright flash of light > on-screen), which will manifest itself as a sudden decrease in the > total frame brightness coinciding with the new source of light. (Or > vice versa, a sudden increase in brightness coinciding with the > sudden absence of a light source). > > The scene change threshold is a trade-off. Lowering the value decreases > the likelihood of #1 but increases the likelihood of #2. Increasing the > value decreases the likelihood of #2, but increases the likelihood (and > apparent effect) of #1. > > If you want to optimize or improve the algorithm, the case #2 is the one > I would be most interested in, i.e. reducing the rate of false > positives. This can surely be done in a smarter way, e.g. by comparing > more than just the scene average but also other easily obtained metrics. > > If you have access to low-level frame information, you could do > something like increasing the threshold for non-keyframes significantly, > since keyframes are likely to coincide with scene changes or cuts this > might help the algorithm out.
Thinking about this logic again, I came to realize that a different strategy might be to check instead for a minimum threshold brightness difference in a critical number of different areas of the screen. This way, a very bright light source appearing or becoming occluded in one local part of the frame will not trigger a scene change, while a sudden change in brightness of a large part of the frame will. I will try it if I get the opportunity to. _______________________________________________ ffmpeg-devel mailing list ffmpeg-devel@ffmpeg.org http://ffmpeg.org/mailman/listinfo/ffmpeg-devel
