Misbehaving Curves Filter

[kesh]

Summary of the bug:

The curves filter appears to produce a spline, which is far from that of the GIMP's curves' tool.

A failing case:

To illustrate, I have a 2-point curve in GIMP: [(7,152),(65,234)]

These points normalize to [(0.027450980392156862, 0.596078431372549), (0.2549019607843137, 0.9176470588235294)]

Accordingly, I run the following FFmpeg command on a single-row input image of increasing pixel values to get the curve (so I can plot it easily in Python, rather than getting gnuplot setup)

ffmpeg -report -nostdin -hide_banner -pix_fmt gray -i input.png \
  -vf "curves=m='0.0/0.0 0.027450980392156862/0.596078431372549 0.2549019607843137/0.9176470588235294 1.0/1.0'" \
  -f rawvideo -frames:v 1 -pix_fmt gray 

Then plotting the input vs output pixel values gave me this curve:

Descriptions of the other attachments:

• input.png: Input single-row image
• ffmpeg.log: FFmpeg log file generated by -report

This command was tested on ffmpeg version 5.1-essentials_build-www.gyan.dev

[kesh]

Changed links to attached files (if works

[kesh]

IMHO, it appears that ​PCHIP 1-D monotonic cubic interpolation produces a curve much like GIMP. Here is the PCHIP curve of the presented case over the FFmpeg curve:

Based on what I've seen online, this interpolation algorithm may very well be what Photoshop uses.

[Michael Koch]

The issue is also reproducible with a FFmpeg-made input file:

ffmpeg -f lavfi -i color=black:s=32x8,format=rgb24 -lavfi geq=r='X+32*Y':g='X+32*Y':b='X+32*Y',format=rgb24,datascope=s=960x288:mode=color2:format=dec,scale='2*iw':'2*ih' -frames 1 -y test_gray.png

ffmpeg -i test_gray.png -vf "curves=m='0.0/0.0 0.027450980392156862/0.596078431372549 0.2549019607843137/0.9176470588235294 1.0/1.0'" out.png

[kesh]

I submitted a patch last night to ffmpeg-devel list to add PCHIP interpolation to the curves filter option. Here are the input and output images of Michael Koch's test commands.

Input

Output (original natural cubic interpolation)

Output (patched PCHIP interpolation)

command:

ffmpeg -i test_gray.png \
  -vf curves=m='0.0/0.0 0.027450980392156862/0.596078431372549 0.2549019607843137/0.9176470588235294 1.0/1.0':interp=pchip \
  -loglevel debug out_pchip.png

[Michael Koch]

Are you sure that you uploaded the correct output image? It seems to be the same as the input image.

[kesh]

Oops, you're right. I must've renamed the test files incorrectly. I've fixed the image to the correct one above.

[Michael Koch]

65f96a965af415300f7533c75940eeae4e3a2287