//
// PUBLIC DOMAIN CRT STYLED SCAN-LINE SHADER
//
//   by Timothy Lottes
//
// This is more along the style of a really good CGA arcade monitor.
// With RGB inputs instead of NTSC.
// The shadow mask example has the mask rotated 90 degrees for less chromatic aberration.
//
// Left it unoptimized to show the theory behind the algorithm.
//
// It is an example what I personally would want as a display option for pixel art games.
// Please take and use, change, or whatever.
//

#ifdef GL_ES
precision mediump float;
#endif

#define hardScan -8.0
#define hardPix -3.0
#define warpX 0.031
#define warpY 0.041
#define maskDark 0.5
#define maskLight 1.5
#define scaleInLinearGamma 1
#define shadowMask 1.0
#define brightboost 1.0

struct inputdata
{
   vec2 video_size;
   vec2 texture_size;
   vec2 output_size;
   float frame_count;
   float frame_direction;
   float frame_rotation;
};

uniform sampler2D Sample;
varying vec2 v_texCoord;
uniform inputdata IN;

#define warp vec2(warpX,warpY)

//------------------------------------------------------------------------

// sRGB to Linear.
// Assuing using sRGB typed textures this should not be needed.
float ToLinear1(float c)
{
   if (scaleInLinearGamma==0) return c;
   return(c<=0.04045)?c/12.92:pow((c+0.055)/1.055,2.4);
}
vec3 ToLinear(vec3 c)
{
   if (scaleInLinearGamma==0) return c;
   return vec3(ToLinear1(c.r),ToLinear1(c.g),ToLinear1(c.b));
}

// Linear to sRGB.
// Assuing using sRGB typed textures this should not be needed.
float ToSrgb1(float c)
{
   if (scaleInLinearGamma==0) return c;
   return(c<0.0031308?c*12.92:1.055*pow(c,0.41666)-0.055);
}

vec3 ToSrgb(vec3 c)
{
   if (scaleInLinearGamma==0) return c;
   return vec3(ToSrgb1(c.r),ToSrgb1(c.g),ToSrgb1(c.b));
}

// Nearest emulated sample given floating point position and texel offset.
// Also zero's off screen.
vec3 Fetch(vec2 pos,vec2 off){
  pos=(floor(pos*IN.texture_size.xy+off)+vec2(0.5,0.5))/IN.texture_size.xy;
  return ToLinear(brightboost * texture2D(Sample,pos.xy).rgb);
}

// Distance in emulated pixels to nearest texel.
vec2 Dist(vec2 pos){pos=pos*IN.texture_size.xy;return -((pos-floor(pos))-vec2(0.5));}
    
// 1D Gaussian.
float Gaus(float pos,float scale){return exp2(scale*pos*pos);}

// 3-tap Gaussian filter along horz line.
vec3 Horz3(vec2 pos,float off){
  vec3 b=Fetch(pos,vec2(-1.0,off));
  vec3 c=Fetch(pos,vec2( 0.0,off));
  vec3 d=Fetch(pos,vec2( 1.0,off));
  float dst=Dist(pos).x;
  // Convert distance to weight.
  float scale=hardPix;
  float wb=Gaus(dst-1.0,scale);
  float wc=Gaus(dst+0.0,scale);
  float wd=Gaus(dst+1.0,scale);
  // Return filtered sample.
  return (b*wb+c*wc+d*wd)/(wb+wc+wd);}

// 5-tap Gaussian filter along horz line.
vec3 Horz5(vec2 pos,float off){
  vec3 a=Fetch(pos,vec2(-2.0,off));
  vec3 b=Fetch(pos,vec2(-1.0,off));
  vec3 c=Fetch(pos,vec2( 0.0,off));
  vec3 d=Fetch(pos,vec2( 1.0,off));
  vec3 e=Fetch(pos,vec2( 2.0,off));
  float dst=Dist(pos).x;
  // Convert distance to weight.
  float scale=hardPix;
  float wa=Gaus(dst-2.0,scale);
  float wb=Gaus(dst-1.0,scale);
  float wc=Gaus(dst+0.0,scale);
  float wd=Gaus(dst+1.0,scale);
  float we=Gaus(dst+2.0,scale);
  // Return filtered sample.
  return (a*wa+b*wb+c*wc+d*wd+e*we)/(wa+wb+wc+wd+we);}

// Return scanline weight.
float Scan(vec2 pos,float off){
  float dst=Dist(pos).y;
  return Gaus(dst+off,hardScan);}

// Allow nearest three lines to effect pixel.
vec3 Tri(vec2 pos){
  vec3 a=Horz3(pos,-1.0);
  vec3 b=Horz5(pos, 0.0);
  vec3 c=Horz3(pos, 1.0);
  float wa=Scan(pos,-1.0);
  float wb=Scan(pos, 0.0);
  float wc=Scan(pos, 1.0);
  return a*wa+b*wb+c*wc;}

// Distortion of scanlines, and end of screen alpha.
vec2 Warp(vec2 pos){
  pos=pos*2.0-1.0;    
  pos*=vec2(1.0+(pos.y*pos.y)*warp.x,1.0+(pos.x*pos.x)*warp.y);
  return pos*0.5+0.5;}

// Shadow mask.
vec3 Mask(vec2 pos){
  pos.x+=pos.y*3.0;
  vec3 mask=vec3(maskDark,maskDark,maskDark);
  pos.x=fract(pos.x/6.0);
  if(pos.x<0.333)mask.r=maskLight;
  else if(pos.x<0.666)mask.g=maskLight;
  else mask.b=maskLight;
  return mask;}    

void main()
{
   vec2 pos=Warp(v_texCoord.xy*(IN.texture_size.xy/IN.video_size.xy))*(IN.video_size.xy/IN.texture_size.xy);
   vec3 outColor = Tri(pos);

   if(shadowMask != 0.0)
      outColor.rgb*=Mask(floor(v_texCoord.xy*(IN.texture_size.xy/IN.video_size.xy)*IN.output_size.xy)+vec2(0.5,0.5));
	  
   gl_FragColor = vec4(ToSrgb(outColor.rgb),1.0);
}