crt.frag

/*  CRT shader
 *
 *  Copyright (C) 2010, 2011 cgwg, Themaister and DOLLS
 *
 *  This program is free software; you can redistribute it and/or modify it
 *  under the terms of the GNU General Public License as published by the Free
 *  Software Foundation; either version 2 of the License, or (at your option)
 *  any later version.
 */

uniform sampler2D rubyTexture;
uniform vec2 rubyInputSize;
uniform vec2 rubyOutputSize;
uniform vec2 rubyTextureSize;

varying vec2 texCoord;
varying vec2 one;
varying float mod_factor;

// Enable screen curvature.
#define CURVATURE

// Controls the intensity of the barrel distortion used to emulate the
// curvature of a CRT. 0.0 is perfectly flat, 1.0 is annoyingly
// distorted, higher values are increasingly ridiculous.
#define distortion 0.08

// Simulate a CRT gamma of 2.4.
#define inputGamma  2.4

// Compensate for the standard sRGB gamma of 2.2.
#define outputGamma 2.2

// Macros.
#define TEX2D(c) pow((texture2D(rubyTexture, (c)) * gl_Color), vec4(inputGamma))
#define PI 3.141592653589

// Apply radial distortion to the given coordinate.
vec2 radialDistortion(vec2 coord)
{
	coord *= rubyTextureSize / rubyInputSize;
	vec2 cc = coord - 0.5;
	float dist = dot(cc, cc) * distortion;
	return (coord + cc * (1.0 + dist) * dist) * rubyInputSize / rubyTextureSize;
}

// Calculate the influence of a scanline on the current pixel.
//
// 'distance' is the distance in texture coordinates from the current
// pixel to the scanline in question.
// 'color' is the colour of the scanline at the horizontal location of
// the current pixel.
vec4 scanlineWeights(float distance, vec4 color)
{
	// The "width" of the scanline beam is set as 2*(1 + x^4) for
	// each RGB channel.
	vec4 wid = 2.0 + 2.0 * pow(color, vec4(4.0));

	// The "weights" lines basically specify the formula that gives
	// you the profile of the beam, i.e. the intensity as
	// a function of distance from the vertical center of the
	// scanline. In this case, it is gaussian if width=2, and
	// becomes nongaussian for larger widths. Ideally this should
	// be normalized so that the integral across the beam is
	// independent of its width. That is, for a narrower beam
	// "weights" should have a higher peak at the center of the
	// scanline than for a wider beam.
	vec4 weights = vec4(distance / 0.3);
	return 1.4 * exp(-pow(weights * inversesqrt(0.5 * wid), wid)) / (0.6 + 0.2 * wid);
}

void main()
{
	// Here's a helpful diagram to keep in mind while trying to
	// understand the code:
	//
	//  |      |      |      |      |
	// -------------------------------
	//  |      |      |      |      |
	//  |  01  |  11  |  21  |  31  | <-- current scanline
	//  |      | @    |      |      |
	// -------------------------------
	//  |      |      |      |      |
	//  |  02  |  12  |  22  |  32  | <-- next scanline
	//  |      |      |      |      |
	// -------------------------------
	//  |      |      |      |      |
	//
	// Each character-cell represents a pixel on the output
	// surface, "@" represents the current pixel (always somewhere
	// in the bottom half of the current scan-line, or the top-half
	// of the next scanline). The grid of lines represents the
	// edges of the texels of the underlying texture.

	// Texture coordinates of the texel containing the active pixel.
#ifdef CURVATURE
	vec2 xy = radialDistortion(texCoord);
#else
	vec2 xy = texCoord;
#endif

	// Of all the pixels that are mapped onto the texel we are
	// currently rendering, which pixel are we currently rendering?
	vec2 ratio_scale = xy * rubyTextureSize - vec2(0.5);
	vec2 uv_ratio = fract(ratio_scale);

	// Snap to the center of the underlying texel.
	xy.y = (floor(ratio_scale.y) + 0.5) / rubyTextureSize.y;

	// Calculate the effective colour of the current and next
	// scanlines at the horizontal location of the current pixel.
	vec4 col  = TEX2D(xy);
	vec4 col2 = TEX2D(xy + vec2(0.0, one.y));

	// Calculate the influence of the current and next scanlines on
	// the current pixel.
	vec4 weights  = scanlineWeights(uv_ratio.y, col);
	vec4 weights2 = scanlineWeights(1.0 - uv_ratio.y, col2);
	vec3 mul_res  = (col * weights + col2 * weights2).rgb;

	// dot-mask emulation:
	// Output pixels are alternately tinted green and magenta.
	vec3 dotMaskWeights = mix(
		vec3(1.0, 0.7, 1.0),
		vec3(0.7, 1.0, 0.7),
		floor(mod(mod_factor, 2.0))
		);

	mul_res *= dotMaskWeights;

	gl_FragColor = vec4(pow(mul_res, vec3(1.0 / outputGamma)), 1.0);
}