I have a bunch of .png frames and a .mp3 audio file which I would like to convert into a video. Unfortunately, the frames do not correspond to a constant frame rate. For instance, one frame may need to be displayed for 1 second, whereas another may need to be displayed for 3 seconds.
Is there any open-source software (something like ffmpeg) which would help me accomplish this? Any feedback would be greatly appreciated.
Many thanks!
This is not an elegant solution, but it will do the trick: duplicate frames as necessary so that you end up with some resulting (fairly high) constant framerate, 30 or 60 fps (or higher if you need higher time resolution). You simply change which frame is duplicated at the closest new frame to the exact timestamp you want. Frames which are exact duplicates will be encoded to a tiny size (a few bytes) with any decent codec, so this is fairly compact. Then just encode with ffmpeg as usual.
If you have a whole lot of these and need to do it the "right" way: you can indicate the timing either in the container (such as mp4, mkv, etc) or in the codec. For example in an H.264 stream you will have to insert SEI messages of type pic_timing to specify the timing of each frame. Alternately you will have to write your own muxer relying on a container library such as Matroska (mkv) or GPAC (mp4) to indicate the timing in the container. Note that not all codecs/containers support arbitrarily variable frame rate. Only a few codecs support timing in the codec. Also, if timing is specified in both container and codec, the container timing is used (but if you are muxing a stream into a container, the muxer should pick up the individual frame timestamps from the codec).