To be perfectly honest, the majority of the whatever-BR shaders were ported over piecemeal.
The majority of them, once they showed something resembling filtered output that wasn't outrageously out-of-whack, I moved on to the next shader. Some of them show a black screen due to them being some of the most complex of the BR series, and me not having the inclination to go back and fix them yet.
Some of them that do work are probably wrong, too - it's just that the libretro shader documentation is pretty vague as to what BR shaders suit which situations the best, so some of the ones with longer and more arcane names I tended to not bother digging too deeply into the visual quality, as I didn't know what they were supposed to look like anyway.
Porting Cg shaders to the minimalist GLSL-like subset that BGFX demands is actually quite a pain. BGFX only lets you use a bare minimum of shader features due to the fact that one of its supported platforms is WebGL, which doesn't get you a lot of niceties.
The main fun thing that you have to deal with is a lack of implicit casting between boolean and numeric types, a lack of non-square matrices, and some other fun things to do with implicit conversion between scalar and vector types. In short, as I said, it's a pain.
What would really help me would be if some kind soul would assemble an archive of "golden master" screenshots for every single libretro shader, consisting of the input screenshot at native resolution, and the final output from the shader. That would be a massive help for me in porting over the libretro shader library, and I would appreciate it if someone could do that. It would also get your preferred shaders working sooner (or at all).
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