Custom PPSSPP shaders

[bdszoke]

Hey, I have a quick question about writing shaders. I am currently trying to create a reverse vignette shader, specifically to minimize the amount of vignetting in Final Fantasy Type 0. Instead of darkening the edges of the image, I want to lighten them. I was trying to modify the built in vignette shader, but while I have been able to increase the brightness of the center image, or actually invert image itself, I have not been able to figure out how to do this. I would assume that it would involve setting the blend mode to screen instead of multiply, to use photoshop terminology. Any help would be appreciated.