A while back I made a tool for monochromising images. A discussion in another thread made me realise I’d never released it publicly, so I’m releasing it now as a standalone open source tool.
→ 
(Space Cab and Turtle Bridge banners used with permission from the artist - @Vampirics.)
→ 
→ 
That looks interesting … how does it compare to (say) Gimp’s conversion?
I haven’t the foggiest.
The main benefit here is ease of use - you can just drag & drop images onto the executable and get a decent result without too much fiddling around.
If you wanted to be more perfectionist then you could give one of GIMP’s filters a go and tweak it until you get something closer to what you want.
GIMP’s threshold tool is a good one to try. There’s quite a bit you can fiddle with but only adjusting the two slider’s is often enough.
I have another tool that does mostly the same thing (implemented with a Powershell script), though obviously it must operate slightly differently to how GIMP does it:
(I’m presuming that, rather than using the average of the three channels, GIMP uses the value or saturation.)
At any rate, a lot of the time I can’t be bothered with messing around with sliders, I just want to figure out the operation I want and then bulk process all my images.
I could probably find out how to do it with GIMP’s command line, but most of the time it’s easier to just write something by hand for the simpler effects.
You can select what GIMP’s threshold tool uses for threshold values from:
Value, Luminance, RGB, or one of Red, Green, Blue or Alpha channel.
For the image in my reply above, I used the blue channel.
