@Austin Thanks for the tip about utvideo. I admit it has not even been on my radar. I should add an Export preset for that. Consider to use AC-3 instead of AAC for audio proxies as AAC introduces codec delay, which can make things more challenging and introduce minor latency to seek. Maybe there are other candidate audio codecs. If you ffmpeg -h encoder=aac | grep General you will see “delay” which means codec delay is at play.
I am reading through your posts even though I do not have a lot of time to have a discussion. Making mlt_rect parameters serialize and deserialize as a proportional (relative) value in conjunction with keyframes in MLT is very non-trivial. I tackled it once and had to scrap it. Obviously, I want to revisit it. It would help if you can take a look at trying to convert the crop filter, but do keep in mind that it needs to remain backwards compatible to render old projects. Thus, the proportional value handling should be done either by using new/different properties or looking for ‘%’ in the string value and then getting it as a double and multiplying it against the maximum value.