Detail from www.getty.edu/art/collection/objects/1502/simon-marmion-and-david-aubert-les-visions-du-chevalier-tondal-franco-flemish-1475

Color to grey conversion:
GIMP 2.9.3
Desaturate -> Color to Grey tool
"uses envelopes formed with the STRESS aproach used perform local color-difference preserving local greyscale generation"
# GIMP yes tool settings
(time 0)
(radius 300)
(samples 4)
(iterations 10)
# end of yes tool settings


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Title of the source painting:
Les Visions du chevalier Tondal

Artist/Maker:
Simon Marmion (Flemish, active 1450 - 1489)
and David Aubert (Flemish, active 1453 - 1479)

Culture:
Franco-Flemish

Place:
Ghent Belgium Valenciennes France (Place created)

Date:
1475

Medium:
Tempera colors, gold leaf, gold paint, and ink on parchment tipped into a binding of wood boards covered with brown calf
Dimensions:

Leaf: 36.3 × 26.2 cm (14 5/16 × 10 5/16 in.)