obscure it is! But not completely useless.
Its related to this problem of subpixel rendering. I have a 3 channel bitmap in “RGB” format, that’s actually not RGB. The values in each channel correspond to alpha intensities to be used against a given colour.
It’s easier to explain in terms of the normal rendering. When i have a bitmap glyph, i have a single channel bitmap that represents intensities of the text colour to use. ie a single alpha channel. But when subpixel rendering, the output bitmap is 3 times wider and corresponds to alpha channels for each of the R,G and B of the text colour.
This is how it gets its correct colour edges on the glyph pixels that work for any given text colour and against any background.
So im stumped on how to draw this effect because it requires blending with the current screen colour behind - whence the alpha blend.
To conclude, having a special draw mode for this would be too obscure. But it makes the case for user supplied blends to be given to the draw functions, or at least ones chosen from a palette of predetermined ROPs.