VideoCapture.retrieve(Mat, driver specific flag) -> driver specific flag as milliseconds possible?

due to different frequencies I try to grab every frame by its position in milliseconds.

Right now I use the methods set and read to do this.
At first I set the position in the videocapture instance like this:

cap.set(CAP_PROP_POS_MSEC, time_in_ms);

after that I grab the “next” frame with read and store it in Mat_at_time_in_ms:;

This works fine but is just very slow.

Therefore I am looking for another possibilty. (I do not want to use set().

I was looking and found the method retrieve(). In the documentation I found that you can pass a driver specific flag (other than the frame index) to the function.

I was wondering wether I could use this function and pass a time in milliseconds to retrieve (instead of an index).
I couldnt find a helpful post on this problem yet. Maybe you can help.
Thanks in advance.

you’re on the wrong path here, there is no such flag implemented, also you would have to call grab() before a retrieve(), and the combination of it is exactly, what read() does.

then, think about it, what makes “random access” (no matter if by millis or index) so expensive ?
to get your frame, most codecs have to rewind to some keyframe, and successively decode up to the desired position

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thanks for your quick help