I don't know of a png size limit, but then again i've never closely examined the code. I do know that 32-bit png's cannot be displayed, you must use 8,16 or 24 bits. That might be your problem. Back to shaders, it looks like opengl only has a few switch settings and no sliders? is that correct? HLSL on the other hand has heaps of sliders, and I see no value in having them in the front-end where you can't see what the sliders are doing. If you can tell me more, and it sounds simple, I might be more interested in adding switches sooner.