How to Use the FFmpeg PSNR Filter

This article provides a quick guide on how to use the Peak Signal-to-Noise Ratio (PSNR) filter in FFmpeg to measure video quality. You will learn the basic command-line syntax required to compare a compressed or processed video against its original reference, how to log the results to a text file, and how to interpret the final decibel (dB) scores output by the tool.

Basic command syntax

The psnr filter in FFmpeg requires two video inputs: the modified (distorted) video and the original (reference) video. Because it compares two distinct video streams, you must use FFmpeg’s -filter_complex option to map both inputs into the filter.

Here is the standard command to calculate the PSNR:

ffmpeg -i distorted.mp4 -i original.mp4 -filter_complex "[0:v][1:v]psnr" -f null -

Command breakdown:

• -i distorted.mp4: The first input (0:v), which is the compressed or processed video you want to evaluate.
• -i original.mp4: The second input (1:v), which serves as the high-quality reference video.
• -filter_complex "[0:v][1:v]psnr": Takes the video streams from both inputs and passes them into the psnr filter.
• -f null -: Instructs FFmpeg to process the video and calculate the metrics without exporting a new video file.

Saving PSNR results to a log file

If you need to analyze the quality of the video frame-by-frame, you can export the PSNR statistics to a log file using the stats_file parameter.

ffmpeg -i distorted.mp4 -i original.mp4 -filter_complex "[0:v][1:v]psnr=stats_file=psnr_results.log" -f null -

This command generates a file named psnr_results.log in your working directory, containing the frame number, loss metric, and individual PSNR values for the Y, U, and V color channels of every single frame.

Understanding the output

Once the process finishes, FFmpeg will display a summary line in the terminal that looks similar to this:

[Parsed_psnr_0 @ 0x55bfca609bc0] PSNR y:38.45 u:42.15 v:41.80 average:39.12 min:32.40 max:inf

To interpret these results: * y, u, v: Represents the PSNR values for the luminance (Y) and chrominance (U, V) channels. The Y (luminance) channel is the most critical for human visual perception. * average: The overall average PSNR value across the entire video. * min / max: The lowest and highest PSNR values recorded. A value of inf (infinite) means the compared frames are identical.

Evaluating the scores

PSNR is measured on a logarithmic decibel (dB) scale: * Above 40 dB: Excellent quality; the compressed video is virtually indistinguishable from the original. * 30 dB to 40 dB: Good quality; distortions are minimal and generally hard to notice. * Below 30 dB: Poor quality; visible compression artifacts, pixelation, or blurriness are present.