VIPS

NIP2   (Compare Project)

Mostly written in Modula-2

License: gpl

Phatch   (Compare Project)

License: gpl3_or_later

G'MIC   (Compare Project)

License: GPL-1-0plus

CImg   (Compare Project)

Mostly written in C

License: gpl, lgpl

MyPaint   (Compare Project)

Mostly written in Python

License: gpl, lgpl21_or_later

grafx2   (Compare Project)

License: gpl

libpipi   (Compare Project)

Mostly written in C

License: WTFPL_2_0

GIMP   (Compare Project)

Mostly written in C

License: gpl3_or_later, lgpl21_or_later, lgpv3_or_later

Krita   (Compare Project)

Mostly written in C++

License: lgpl2_0

GraphicsMagick   (Compare Project)

Mostly written in C

License: mit