Custom PPSSPP shaders

[Queen Fiona]

Simple request. Is it possible to port the sharp-bilinear-simple shader from libretro to PPSSPP? All it does is prescale the image by nearest neighbor integer scaling as high as possible under the current display resolution, and then uses hardware bilinear scaling to do the rest. Extremely useful for 2D games, and it has effectively no performance cost under modern hardware.

I'd do it myself, since the GLSL code doesn't seem to need rewriting, but I don't know how to split it into explicit fragment and vertex shaders as PPSSPP requires. (I also don't know if it'd be Vulkan compatible, and I'd like to use this with my Android phone as well.)

I can't link the shader because of account restrictions, but it's sharp-bilinear-simple.glsl under the libretro/glsl-shaders repo on GitHub. That or I'll just post the entire code here:

Code:

/*
   Author: rsn8887 (based on TheMaister)
   License: Public domain

   This is an integer prescale filter that should be combined
   with a bilinear hardware filtering (GL_BILINEAR filter or some such) to achieve
   a smooth scaling result with minimum blur. This is good for pixelgraphics
   that are scaled by non-integer factors.
  
   The prescale factor and texel coordinates are precalculated
   in the vertex shader for speed.
*/

#if defined(VERTEX)

#if __VERSION__ >= 130
#define COMPAT_VARYING out
#define COMPAT_ATTRIBUTE in
#define COMPAT_TEXTURE texture
#else
#define COMPAT_VARYING varying
#define COMPAT_ATTRIBUTE attribute
#define COMPAT_TEXTURE texture2D
#endif

#ifdef GL_ES
#define COMPAT_PRECISION mediump
#else
#define COMPAT_PRECISION
#endif

COMPAT_ATTRIBUTE vec4 VertexCoord;
COMPAT_ATTRIBUTE vec4 COLOR;
COMPAT_ATTRIBUTE vec4 TexCoord;
COMPAT_VARYING vec4 COL0;
COMPAT_VARYING vec4 TEX0;

uniform mat4 MVPMatrix;
uniform COMPAT_PRECISION int FrameDirection;
uniform COMPAT_PRECISION int FrameCount;
uniform COMPAT_PRECISION vec2 OutputSize;
uniform COMPAT_PRECISION vec2 TextureSize;
uniform COMPAT_PRECISION vec2 InputSize;

// vertex compatibility #defines
#define vTexCoord TEX0.xy
#define SourceSize vec4(TextureSize, 1.0 / TextureSize) //either TextureSize or InputSize
#define outsize vec4(OutputSize, 1.0 / OutputSize)

COMPAT_VARYING vec2 precalc_texel;
COMPAT_VARYING vec2 precalc_scale;

void main()
{
    gl_Position = MVPMatrix * VertexCoord;
    COL0 = COLOR;
    TEX0.xy = TexCoord.xy;

    precalc_texel = vTexCoord * SourceSize.xy;
    precalc_scale = max(floor(outsize.xy / InputSize.xy), vec2(1.0, 1.0));
}

#elif defined(FRAGMENT)

#if __VERSION__ >= 130
#define COMPAT_VARYING in
#define COMPAT_TEXTURE texture
out vec4 FragColor;
#else
#define COMPAT_VARYING varying
#define FragColor gl_FragColor
#define COMPAT_TEXTURE texture2D
#endif

#ifdef GL_ES
#ifdef GL_FRAGMENT_PRECISION_HIGH
precision highp float;
#else
precision mediump float;
#endif
#define COMPAT_PRECISION mediump
#else
#define COMPAT_PRECISION
#endif

uniform COMPAT_PRECISION int FrameDirection;
uniform COMPAT_PRECISION int FrameCount;
uniform COMPAT_PRECISION vec2 OutputSize;
uniform COMPAT_PRECISION vec2 TextureSize;
uniform COMPAT_PRECISION vec2 InputSize;
uniform sampler2D Texture;
COMPAT_VARYING vec4 TEX0;

// fragment compatibility #defines
#define Source Texture
#define vTexCoord TEX0.xy

#define SourceSize vec4(TextureSize, 1.0 / TextureSize) //either TextureSize or InputSize
#define outsize vec4(OutputSize, 1.0 / OutputSize)

COMPAT_VARYING vec2 precalc_texel;
COMPAT_VARYING vec2 precalc_scale;

void main()
{
   vec2 texel = precalc_texel;
   vec2 scale = precalc_scale;

   vec2 texel_floored = floor(texel);
   vec2 s = fract(texel);
   vec2 region_range = 0.5 - 0.5 / scale;

   // Figure out where in the texel to sample to get correct pre-scaled bilinear.
   // Uses the hardware bilinear interpolator to avoid having to sample 4 times manually.

   vec2 center_dist = s - 0.5;
   vec2 f = (center_dist - clamp(center_dist, -region_range, region_range)) * scale + 0.5;

   vec2 mod_texel = texel_floored + f;

   FragColor = vec4(COMPAT_TEXTURE(Source, mod_texel / SourceSize.xy).rgb, 1.0);
}
#endif