// FXAA fragment shader by Timothy Lottes (public domain)
// http://timothylottes.blogspot.com/

uniform sampler2D tex;

float FXAA_SUBPIX_SHIFT = 1.0/4.0;

// posPos: Output of FxaaVertexShader interpolated across screen.
// tex: Input texture.
// rcpFrame: const vec2(1.0/frameWidth, 1.0/frameHeight).
vec3 FxaaPixelShader(vec4 posPos, sampler2D tex, vec2 rcpFrame) {
    #define FXAA_REDUCE_MIN   (1.0/128.0)
    #define FXAA_REDUCE_MUL   (1.0/8.0)
    #define FXAA_SPAN_MAX     8.0

    vec3 rgbNW = texture2DLod(tex, posPos.zw, 0.0).xyz;
    vec3 rgbNE = texture2DLod(tex, posPos.zw + vec2(1.0,0.0)*rcpFrame.xy, 0.0).xyz;
    vec3 rgbSW = texture2DLod(tex, posPos.zw + vec2(0.0,1.0)*rcpFrame.xy, 0.0).xyz;
    vec3 rgbSE = texture2DLod(tex, posPos.zw + vec2(1.0,1.0)*rcpFrame.xy, 0.0).xyz;
    vec3 rgbM  = texture2DLod(tex, posPos.xy,0.0).xyz;

    vec3 luma = vec3(0.299, 0.587, 0.114);
    float lumaNW = dot(rgbNW, luma);
    float lumaNE = dot(rgbNE, luma);
    float lumaSW = dot(rgbSW, luma);
    float lumaSE = dot(rgbSE, luma);
    float lumaM  = dot(rgbM,  luma);

    float lumaMin = min(lumaM, min(min(lumaNW, lumaNE), min(lumaSW, lumaSE)));
    float lumaMax = max(lumaM, max(max(lumaNW, lumaNE), max(lumaSW, lumaSE)));

    vec2 dir;
    dir.x = -((lumaNW + lumaNE) - (lumaSW + lumaSE));
    dir.y =  ((lumaNW + lumaSW) - (lumaNE + lumaSE));

    float dirReduce = max(
        (lumaNW + lumaNE + lumaSW + lumaSE) * (0.25 * FXAA_REDUCE_MUL),
        FXAA_REDUCE_MIN);
    float rcpDirMin = 1.0/(min(abs(dir.x), abs(dir.y)) + dirReduce);
    dir = min(vec2( FXAA_SPAN_MAX,  FXAA_SPAN_MAX),
          max(vec2(-FXAA_SPAN_MAX, -FXAA_SPAN_MAX),
          dir * rcpDirMin)) * rcpFrame.xy;

    vec3 rgbA = (1.0/2.0) * (
        texture2DLod(tex, posPos.xy + dir * (1.0/3.0 - 0.5),0.0).xyz +
        texture2DLod(tex, posPos.xy + dir * (2.0/3.0 - 0.5),0.0).xyz);
    vec3 rgbB = rgbA * (1.0/2.0) + (1.0/4.0) * (
        texture2DLod(tex, posPos.xy + dir * (0.0/3.0 - 0.5),0.0).xyz +
        texture2DLod(tex, posPos.xy + dir * (3.0/3.0 - 0.5),0.0).xyz);
    float lumaB = dot(rgbB, luma);
    if((lumaB < lumaMin) || (lumaB > lumaMax)) return rgbA;
    return rgbB;
}

vec4 FXAA(sampler2D tex, vec2 uv) {
    vec2 rcpFrame = vec2(1.0/iWidth, 1.0/iHeight);
    vec4 posPos = vec4(texcoord.st,texcoord.st -(rcpFrame * (0.5 + FXAA_SUBPIX_SHIFT)));
    vec4 c = vec4(0.0);
    c.rgb = FxaaPixelShader(posPos, tex, rcpFrame);
    // c.rgb = texture2D(tex, posPos.xy).rgb - c.rgb; // debug
    c.a = texture2D(tex, posPos.xy).a;
    return c;
}

void main() {
    FRAGCOLOR = FXAA(iChannel0, texcoord.st);
}