[FFmpeg-user] Question about tmix filter

[Michael Koch]

Am 06.03.2019 um 20:45 schrieb Paul B Mahol:
>
>>>> Your interpretation of test results is invalid.
>>>> You should count frames.
>>> Well, I did run the video in VLC and measure the duration with a
>>> stopwatch, and it is 5 seconds. Below is the FFprobe output.
>>> How can I count the frames?
>> Old fashioned way - by hand.
> -f null -
>
> AKA null muxer counts frames for you.

ok, it's 76 frames and 5 seconds. Lesson learned, don't trust the video 
length.

Next question. I want to make a dark subtract for my videos of the night 
sky. I capture a few seconds of video with the cap on the lens. From 
this video I want to average 100 frames and extract the result as a PNG 
image, which will then be subtracted from the video of the night sky.
Do you think this is the correct command for extracting the average of 
the first 100 frames?

ffmpeg -i dark.mov -vf tmix=frames=100 -frames 1 dark.png

Thanks,
Michael