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]( 0001-avfilter-vf_curves-add-PCHIP-interpolator-and-interp.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)

[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