Extracting a screenshot from a video using ffmpeg

Question

I feel like this should be so simple but I just can't find any way to do it. I simply want to grab the opening frame of a video file and output it as a PNG within a filter-complex command.

I can do this using the following command in isolation although I get a warning:

ffmpeg -y -i infile -frames:v 1 output.png

[image2 @ 0000020bbf053640] The specified filename 'output.png' does not contain an image sequence pattern or a pattern is invalid.

[image2 @ 0000020bbf053640] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.

How do I incorporate the same outcome in a filter-complex function?

I have used the following crop command:

ffmpeg -y -i input.mp4 -filter_complex "[0]crop=iw:ih[grab]" -map [grab] "out.png"

and although I do get the required output, I get warning/error which I'd rather not get:

[image2 @ 000002a9089c1280] The specified filename 'out.png' does not contain an image sequence pattern or a pattern is invalid.

[image2 @ 000002a9089c1280] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.

[image2 @ 000002a9089c1280] Cannot write more than one file with the same name. Are you missing the -update option or a sequence pattern? av_interleaved_write_frame(): Invalid argument

Error muxing a packet for output file #0

Answer 1

Just drop the output label "[grab]" so you don't have to -map it.

$ ffmpeg-4.4-amd64-static/ffmpeg -y -i input.mp4 -filter_complex "[v:0]crop=iw:ih" -frames:v 1 -f image2 out.png

On newer versions of FFmpeg, you'll get a warning that tells you that you have to put a number pattern in your output filename. You can ignore this warning, since you're using the -frames:v 1 option to write only a single frame to that file.

However, if you drop the -frame parameter, you'll get an error, because FFmpeg tries to write to the same file again (as indicated by the frame counter at the end):

$ ffmpeg-6.0-amd64-static/ffmpeg -y -i input.mp4 -filter_complex "[v:0]crop=iw:ih" -frames:v 1 -update 1 -f image2 out3.png
[...]
[image2 @ 0x5f5b280] The specified filename 'out3.png' does not contain an image sequence pattern or a pattern is invalid.
[image2 @ 0x5f5b280] Use a pattern such as %03d for an image sequence or use the -update option (with -frames:v 1 if needed) to write a single image.
[image2 @ 0x5f5b280] Cannot write more than one file with the same name. Are you missing the -update option or a sequence pattern?
av_interleaved_write_frame(): Invalid argument
[out#0/image2 @ 0x5f5b140] Error muxing a packet
frame=    2 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.40 bitrate=N/A speed= 1.1x
video:2023kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
Conversion failed!

Despite the error, the first frame is still written to disk, and that's what you wanted anyways.

To avoid these warnings and errors, pass the -update 1 option which tells the image2 muxer that it's okay to overwrite the same image file over and over again. Since you're writing a single file only with the frames:v 1 option, the -update 1 option is unnecessary, but doesn't hurt.

Of course, if you're using number patterns in your filename, you won't encounter this overwrite issue at all, e.g.: out_%03d.png.

Answer 2

As the 2nd warning says, use -frames along with -update.

ffmpeg -y -i input.mp4 -filter_complex "[0]crop=iw:ih[grab]" -map [grab] -frames:v 1 -update true "out.png"