ffmpeg psnr computation silently includes an extra frame

Question

I am trying to determine the PSNR between K frames of two image sequences. I get incorrect answers. I found using the stats_file=- option that psnr is analyzing an extra frame (sometimes beyond the end of the video).

Is there any way to avoid this extraneous frame?

Here is an example:

ffmpeg -f lavfi -i testsrc=duration=10:size=160x120:rate=1 v1.%03d.png
ffmpeg -f lavfi -i testsrc=duration=10:size=160x120:rate=1 v2.%03d.png
cp v1.001.png v2.004.png  # make the fourth frame differ

# We want the average PSNR over the first 3 frames; it should be infinity.
ffmpeg -i v1.%03d.png -i v2.%03d.png -frames:v 3 -lavfi "psnr=stats_file=-" -f null -

And its output:

n:1 mse_avg:0.00 mse_r:0.00 mse_g:0.00 mse_b:0.00 psnr_avg:inf psnr_r:inf psnr_g:inf psnr_b:inf 
n:2 mse_avg:0.00 mse_r:0.00 mse_g:0.00 mse_b:0.00 psnr_avg:inf psnr_r:inf psnr_g:inf psnr_b:inf 
n:3 mse_avg:0.00 mse_r:0.00 mse_g:0.00 mse_b:0.00 psnr_avg:inf psnr_r:inf psnr_g:inf psnr_b:inf 
n:4 mse_avg:6936.69 mse_r:6839.88 mse_g:6839.78 mse_b:7130.40 psnr_avg:9.72 psnr_r:9.78 psnr_g:9.78 psnr_b:9.60 
frame=    3 fps=0.0 q=-0.0 Lsize=N/A time=00:00:00.12 bitrate=N/A speed=41.3x    
video:2kB audio:0kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: unknown
[Parsed_psnr_0 @ 0x55dfcbe1d280] PSNR r:15.800917 g:15.800984 b:15.620263 average:15.739882 min:9.719282 max:inf

Even though I specified to use just 3 frames, it has included the fourth frame and reported an incorrect result. It is only thanks to stats_file that we see the inclusion of the fourth frame.

Answer 1

-vframes or -frames:v is evaluated on output frames before they're sent for encoding and is reactively enforced i.e. the first frame from the filtergraph that fails the criteria leads to EOF, so the PSNR filter will have processed an extra pair of frames. Trim the inputs beforehand.

Use

ffmpeg -i v1.%03d.png -i v2.%03d.png -lavfi "[0]trim=end_frame=3[a];[1]trim=end_frame=3[b];[a][b]psnr=stats_file=-" -f null -