I have a video that is a timelapse (generated by a GoPro, if that matters) with images every 5 seconds. I want to encode a timestamp onto it. Previously I've used a command like this one to get a timestamp burned in
# Convert the date to EPOCH. This will be used to set the time for the draw text
# method.
EPOCH=$(date --date="${STARTDATE}" +%s)
# we assume that the STARTDATE is in UTC 0000, Zulu time, GMT and that we want
# to convert it to the local time on the computer.
ffmpeg -i "${INPUT}" -vf drawtext="fontsize=30:fontcolor=yellow:text='%{pts\:localtime\:${EPOCH}}':x=(w-text_w) - 10:y=(h-text_h) - 10" -vcodec libx265 -crf 28 "${OUTPUT}"
The issue is that the timestamps generated by this progress as if it is a normal video, stamping a 30 minute timelapse as if it were 25 seconds. What I want are timestamps that match the timelapse.
I've looked at the drawtext docs. I thought rate might be the key, but 1/5 and 150 both produce errors like this one:
Parsed_drawtext_0 @ 0x10c607370] Failed to parse expression: (h-text_h) - 10 r=1/5
I figure I might need to multiply the current frame value to get the correct time, but I don't know how to do that.
The pts function will just use the stored timestamp. You will have to modify the timestamp before the drawtext filter and restore the original value afterwards i.e.
setpts=PTS*10,drawtext=...,setpts=PTS/10
where the factor 10 is the ratio of realtime between frames and their display interval in the timelapse video.