`-I` of `image` does not invert the colors of a 1-bit image

[yvonnefroehlich]

Description of the problem

See also: GenericMappingTools/pygmt#4089 (comment)

Using -I of image to invert the colors of a 1-bit image does not work (anymore).

Full script that generated the error

(I)

gmt image @vader1.png -png image_gmt

(II)

gmt image @vader1.png -I -png image_I_gmt

Full error message

No error or warning messages occur.

Actual outcome
(I)

(II)

Expected outcome

The black and white pixels of the image should be flipped when using -I (second image).

System information

• Operating system: Windows 10
• GMT version (gmt --version): 6.4.0, 6.5.0, 6.7.0_7026382_2025.08.23