Digitally remastering Old Masters to restore degraded colours?

Software darktable
12

We should always start with a high quality image, not a JPEG.

Here is an attempt to make the white lines transparent, and then Filling holes in priority order (“inpaint”). The priority order preserves edges that are continued into the holes. Windows BAT script:

set smAUTO=0
call %PICTBAT%slopeMag wolfPainting.jpeg x.tiff

magick ^
  x.tiff ^
  -level 10,50%% -morphology Dilate disk:3 ^
  +write x2.tiff ^
  ( wolfPainting.jpeg +write mpr:INP ^
    -colorspace Gray ^
  ) ^
  -compose Multiply -composite ^
  +write x3.tiff ^
  -threshold 50%% ^
  -negate ^
  +write x4.tiff ^
  mpr:INP ^
  +swap ^
  -alpha off -compose CopyOpacity -composite ^
  x5.tiff

magick ^
  x5.tiff ^
  -process 'fillholespri verbose hc off lsr 20 window_radius 2 st 0.01' ^
  x6.tiff

magick ^
  wolfPainting.jpeg ^
  -crop 500x400+1100+0 +repage ^
  wolfA.jpg

magick ^
  x6.tiff ^
  -crop 500x400+1100+0 +repage ^
  wolfB.jpg

Here is a wolfA.jpg, a crop from from your input:

wolfA

Here is a wolfB.jpg, a crop from from the cleaner output::

wolfB

This is not perfect. Tweaking the parameters can probably improve it.