Andrew just tried your colorize layer branch and I love it. The "Color (YCbCr & clip S)" blending mode is exactly what's needed for artist like myself that love to sketch concept art in Mypaint in grayscale and use layers to color. The colorize brush blending option with good, but this is better imo. Hopefully some form of this will make it into the main branch. Thank you.
Thanks! I need to to update it to integrate the latest changes from master, which is changing very rapidly currently.
I'm considering settling on what Cairo calls "CAIRO_OPERATOR_HSL_LUMINOSITY" and "CAIRO_OPERATOR_HSL_COLOR" - it's the blend mode used by the brush colourise, and supposedly the same thing as older versions of photoshop. The Cairo docs on it are excellent, which should make it easier for other programs to implement.I made some notes on the 3 methods I'm considering
a while back. I wonder if you could take a look at them and let me know whether you think the hot pixels coming from the "Y assignment and scale" operation in the sample image are acceptable? Arguably this is giving the user what they asked for and avoids the under-saturation you often get with lightness assignments... by preserving the saturation in a way that looks really odd with low-brightness coloured pixels in the source image!