// This is a port of the NTSC encode/decode shader pair in MAME and MESS, modified to use only
// one pass rather than an encode pass and a decode pass. It accurately emulates the sort of
// signal decimation one would see when viewing a composite signal, though it could benefit from a
// pre-pass to re-size the input content to more accurately reflect the actual size that would
// be incoming from a composite signal source.
//
// To encode the composite signal, I convert the RGB value to YIQ, then subsequently evaluate
// the standard NTSC composite equation. Four composite samples per RGB pixel are generated from
// the incoming linearly-interpolated texels.
//
// The decode pass implements a Fixed Impulse Response (FIR) filter designed by MAME/MESS contributor
// "austere" in matlab (if memory serves correctly) to mimic the behavior of a standard television set
// as closely as possible. The filter window is 83 composite samples wide, and there is an additional
// notch filter pass on the luminance (Y) values in order to strip the color signal from the luminance
// signal prior to processing.
//
// - UltraMoogleMan [8/2/2013]

uniform sampler2D color_texture;
uniform vec2      color_texture_sz;			// size of color_texture
uniform vec2      color_texture_pow2_sz;	// size of color texture rounded up to power of 2
uniform vec2      screen_texture_sz;		// size of output resolution
uniform vec2      screen_texture_pow2_sz;   // size of output resolution rounded up to power of 2

// Useful Constants
const vec4 Zero = vec4(0.0);
const vec4 Half = vec4(0.5);
const vec4 One = vec4(1.0);
const vec4 Two = vec4(2.0);
const float Pi = 3.1415926535;
const float Pi2 = 6.283185307;

// NTSC Constants
const vec4 A = vec4(0.5);
const vec4 B = vec4(0.5);
const float P = 1.0;
const float CCFrequency = 3.59754545;
const float YFrequency = 6.0;
const float IFrequency = 1.2;
const float QFrequency = 0.6;
const float NotchHalfWidth = 2.0;
const float ScanTime = 52.6;
const float MaxC = 2.1183;
const vec4 MinC = vec4(-1.1183);
const vec4 CRange = vec4(3.2366);

vec4 CompositeSample(vec2 UV) {
	vec2 InverseRes = 1.0 / screen_texture_sz.xy;
	vec2 InverseP = vec2(P, 0.0) * InverseRes;
	
	// UVs for four linearly-interpolated samples spaced 0.25 texels apart
	vec2 C0 = UV;
	vec2 C1 = UV + InverseP * 0.25;
	vec2 C2 = UV + InverseP * 0.50;
	vec2 C3 = UV + InverseP * 0.75;
	vec4 Cx = vec4(C0.x, C1.x, C2.x, C3.x);
	vec4 Cy = vec4(C0.y, C1.y, C2.y, C3.y);

	vec3 Texel0 = texture2D(color_texture, C0).rgb;
	vec3 Texel1 = texture2D(color_texture, C1).rgb;
	vec3 Texel2 = texture2D(color_texture, C2).rgb;
	vec3 Texel3 = texture2D(color_texture, C3).rgb;
	
	// Calculated the expected time of the sample.
	vec4 T = A * Cy * vec4(screen_texture_sz.x) * Two + B + Cx;

	const vec3 YTransform = vec3(0.299, 0.587, 0.114);
	const vec3 ITransform = vec3(0.595716, -0.274453, -0.321263);
	const vec3 QTransform = vec3(0.211456, -0.522591, 0.311135);

	float Y0 = dot(Texel0, YTransform);
	float Y1 = dot(Texel1, YTransform);
	float Y2 = dot(Texel2, YTransform);
	float Y3 = dot(Texel3, YTransform);
	vec4 Y = vec4(Y0, Y1, Y2, Y3);

	float I0 = dot(Texel0, ITransform);
	float I1 = dot(Texel1, ITransform);
	float I2 = dot(Texel2, ITransform);
	float I3 = dot(Texel3, ITransform);
	vec4 I = vec4(I0, I1, I2, I3);

	float Q0 = dot(Texel0, QTransform);
	float Q1 = dot(Texel1, QTransform);
	float Q2 = dot(Texel2, QTransform);
	float Q3 = dot(Texel3, QTransform);
	vec4 Q = vec4(Q0, Q1, Q2, Q3);

	vec4 W = vec4(Pi2 * CCFrequency * ScanTime);
	vec4 Encoded = Y + I * cos(T * W) + Q * sin(T * W);
	return (Encoded - MinC) / CRange;
}

vec4 NTSCCodec(vec2 UV)
{
	vec2 InverseRes = 1.0 / screen_texture_sz.xy;
	vec4 YAccum = Zero;
	vec4 IAccum = Zero;
	vec4 QAccum = Zero;
	float QuadXSize = screen_texture_sz.x * 4.0;
	float TimePerSample = ScanTime / QuadXSize;
	
	// Frequency cutoffs for the individual portions of the signal that we extract.
	// Y1 and Y2 are the positive and negative frequency limits of the notch filter on Y.
	// 
	float Fc_y1 = (CCFrequency - NotchHalfWidth) * TimePerSample;
	float Fc_y2 = (CCFrequency + NotchHalfWidth) * TimePerSample;
	float Fc_y3 = YFrequency * TimePerSample;
	float Fc_i = IFrequency * TimePerSample;
	float Fc_q = QFrequency * TimePerSample;
	float Pi2Length = Pi2 / 82.0;
	vec4 NotchOffset = vec4(0.0, 1.0, 2.0, 3.0);
	vec4 W = vec4(Pi2 * CCFrequency * ScanTime);
	for(float n = -41.0; n < 42.0; n += 4.0)
	{
		vec4 n4 = n + NotchOffset;
		vec4 CoordX = UV.x + InverseRes.x * n4 * 0.25;
		vec4 CoordY = vec4(UV.y);
		vec2 TexCoord = vec2(CoordX.r, CoordY.r);
		vec4 C = CompositeSample(TexCoord) * CRange + MinC;
		vec4 WT = W * (CoordX  + A * CoordY * Two * screen_texture_sz.x + B);

		vec4 SincYIn1 = Pi2 * Fc_y1 * n4;
		vec4 SincYIn2 = Pi2 * Fc_y2 * n4;
		vec4 SincYIn3 = Pi2 * Fc_y3 * n4;
		bvec4 notEqual = notEqual(SincYIn1, Zero);
		vec4 SincY1 = sin(SincYIn1) / SincYIn1;
		vec4 SincY2 = sin(SincYIn2) / SincYIn2;
		vec4 SincY3 = sin(SincYIn3) / SincYIn3;
		if(SincYIn1.x == 0.0) SincY1.x = 1.0;
		if(SincYIn1.y == 0.0) SincY1.y = 1.0;
		if(SincYIn1.z == 0.0) SincY1.z = 1.0;
		if(SincYIn1.w == 0.0) SincY1.w = 1.0;
		if(SincYIn2.x == 0.0) SincY2.x = 1.0;
		if(SincYIn2.y == 0.0) SincY2.y = 1.0;
		if(SincYIn2.z == 0.0) SincY2.z = 1.0;
		if(SincYIn2.w == 0.0) SincY2.w = 1.0;
		if(SincYIn3.x == 0.0) SincY3.x = 1.0;
		if(SincYIn3.y == 0.0) SincY3.y = 1.0;
		if(SincYIn3.z == 0.0) SincY3.z = 1.0;
		if(SincYIn3.w == 0.0) SincY3.w = 1.0;
		//vec4 IdealY = (2.0 * Fc_y1 * SincY1 - 2.0 * Fc_y2 * SincY2) + 2.0 * Fc_y3 * SincY3;
		vec4 IdealY = (2.0 * Fc_y1 * SincY1 - 2.0 * Fc_y2 * SincY2) + 2.0 * Fc_y3 * SincY3;
		vec4 FilterY = (0.54 + 0.46 * cos(Pi2Length * n4)) * IdealY;		
		
		vec4 SincIIn = Pi2 * Fc_i * n4;
		vec4 SincI = sin(SincIIn) / SincIIn;
		if (SincIIn.x == 0.0) SincI.x = 1.0;
		if (SincIIn.y == 0.0) SincI.y = 1.0;
		if (SincIIn.z == 0.0) SincI.z = 1.0;
		if (SincIIn.w == 0.0) SincI.w = 1.0;
		vec4 IdealI = 2.0 * Fc_i * SincI;
		vec4 FilterI = (0.54 + 0.46 * cos(Pi2Length * n4)) * IdealI;
		
		vec4 SincQIn = Pi2 * Fc_q * n4;
		vec4 SincQ = sin(SincQIn) / SincQIn;
		if (SincQIn.x == 0.0) SincQ.x = 1.0;
		if (SincQIn.y == 0.0) SincQ.y = 1.0;
		if (SincQIn.z == 0.0) SincQ.z = 1.0;
		if (SincQIn.w == 0.0) SincQ.w = 1.0;
		vec4 IdealQ = 2.0 * Fc_q * SincQ;
		vec4 FilterQ = (0.54 + 0.46 * cos(Pi2Length * n4)) * IdealQ;
		
		YAccum = YAccum + C * FilterY;
		IAccum = IAccum + C * cos(WT) * FilterI;
		QAccum = QAccum + C * sin(WT) * FilterQ;
	}
	
	float Y = YAccum.r + YAccum.g + YAccum.b + YAccum.a;
	float I = (IAccum.r + IAccum.g + IAccum.b + IAccum.a) * 2.0;
	float Q = (QAccum.r + QAccum.g + QAccum.b + QAccum.a) * 2.0;
	
	vec3 YIQ = vec3(Y, I, Q);

	vec3 OutRGB = vec3(dot(YIQ, vec3(1.0, 0.956, 0.621)), dot(YIQ, vec3(1.0, -0.272, -0.647)), dot(YIQ, vec3(1.0, -1.106, 1.703)));		
	
	return vec4(OutRGB, 1.0);
}

void main(void) {
	vec2 InverseRes = 1.0 / screen_texture_sz.xy;
	vec2 UV = gl_FragCoord.xy;

	// compensate for MAME
	UV.y = screen_texture_sz.y - gl_FragCoord.y;
	UV = UV * InverseRes;

	vec4 OutPixel = NTSCCodec(UV);
	gl_FragColor = OutPixel;
}

