- make sure that transparency and blending work sanely in all cases
- see what disabling channels does to it (honestly I'm not sure what it is supposed to do, it's not as clear cut as other blending modes since the channels represent directions that depend on each other for orientation)
- check how it works in non-8-bit color spaces
- break out logical blocks into methods
- run a profiler on an optimized build and see where the hot-spots are in a real-world test case
- optimize and SIMDize accordingly if there are hot-spots in the blending mode