/**
 * @author huwb / http://huwbowles.com/
 *
 * God-rays (crepuscular rays)
 *
 * Similar implementation to the one used by Crytek for CryEngine 2 [Sousa2008].
 * Blurs a mask generated from the depth map along radial lines emanating from the light
 * source. The blur repeatedly applies a blur filter of increasing support but constant
 * sample count to produce a blur filter with large support.
 *
 * My implementation performs 3 passes, similar to the implementation from Sousa. I found
 * just 6 samples per pass produced acceptible results. The blur is applied three times,
 * with decreasing filter support. The result is equivalent to a single pass with
 * 6*6*6 = 216 samples.
 *
 * References:
 *
 * Sousa2008 - Crysis Next Gen Effects, GDC2008, http://www.crytek.com/sites/default/files/GDC08_SousaT_CrysisEffects.ppt
 */

THREE.GodRaysDepthMaskShader = {

	uniforms: {

		tInput: {
			value: null
		}

	},

	vertexShader: [

		"varying vec2 vUv;",

		"void main() {",

		" vUv = uv;",
		" gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

		"}"

	].join( "\n" ),

	fragmentShader: [

		"varying vec2 vUv;",

		"uniform sampler2D tInput;",

		"void main() {",

		"	gl_FragColor = vec4( 1.0 ) - texture2D( tInput, vUv );",


		"}"

	].join( "\n" )

};


/**
 * The god-ray generation shader.
 *
 * First pass:
 *
 * The depth map is blurred along radial lines towards the "sun". The
 * output is written to a temporary render target (I used a 1/4 sized
 * target).
 *
 * Pass two & three:
 *
 * The results of the previous pass are re-blurred, each time with a
 * decreased distance between samples.
 */

THREE.GodRaysGenerateShader = {

	uniforms: {

		tInput: {
			value: null
		},
		fStepSize: {
			value: 1.0
		},
		vSunPositionScreenSpace: {
			value: new THREE.Vector2( 0.5, 0.5 )
		}

	},

	vertexShader: [

		"varying vec2 vUv;",

		"void main() {",

		" vUv = uv;",
		" gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

		"}"

	].join( "\n" ),

	fragmentShader: [

		"#define TAPS_PER_PASS 6.0",

		"varying vec2 vUv;",

		"uniform sampler2D tInput;",

		"uniform vec2 vSunPositionScreenSpace;",
		"uniform float fStepSize;", // filter step size

		"void main() {",

		// delta from current pixel to "sun" position

		"	vec2 delta = vSunPositionScreenSpace - vUv;",
		"	float dist = length( delta );",

		// Step vector (uv space)

		"	vec2 stepv = fStepSize * delta / dist;",

		// Number of iterations between pixel and sun

		"	float iters = dist/fStepSize;",

		"	vec2 uv = vUv.xy;",
		"	float col = 0.0;",

		// This breaks ANGLE in Chrome 22
		//	- see http://code.google.com/p/chromium/issues/detail?id=153105

		/*
		// Unrolling didnt do much on my hardware (ATI Mobility Radeon 3450),
		// so i've just left the loop

		"for ( float i = 0.0; i < TAPS_PER_PASS; i += 1.0 ) {",

		// Accumulate samples, making sure we dont walk past the light source.

		// The check for uv.y < 1 would not be necessary with "border" UV wrap
		// mode, with a black border color. I don't think this is currently
		// exposed by three.js. As a result there might be artifacts when the
		// sun is to the left, right or bottom of screen as these cases are
		// not specifically handled.

		"	col += ( i <= iters && uv.y < 1.0 ? texture2D( tInput, uv ).r : 0.0 );",
		"	uv += stepv;",

		"}",
		*/

		// Unrolling loop manually makes it work in ANGLE

		"	if ( 0.0 <= iters && uv.y < 1.0 ) col += texture2D( tInput, uv ).r;",
		"	uv += stepv;",

		"	if ( 1.0 <= iters && uv.y < 1.0 ) col += texture2D( tInput, uv ).r;",
		"	uv += stepv;",

		"	if ( 2.0 <= iters && uv.y < 1.0 ) col += texture2D( tInput, uv ).r;",
		"	uv += stepv;",

		"	if ( 3.0 <= iters && uv.y < 1.0 ) col += texture2D( tInput, uv ).r;",
		"	uv += stepv;",

		"	if ( 4.0 <= iters && uv.y < 1.0 ) col += texture2D( tInput, uv ).r;",
		"	uv += stepv;",

		"	if ( 5.0 <= iters && uv.y < 1.0 ) col += texture2D( tInput, uv ).r;",
		"	uv += stepv;",

		// Should technically be dividing by 'iters', but 'TAPS_PER_PASS' smooths out
		// objectionable artifacts, in particular near the sun position. The side
		// effect is that the result is darker than it should be around the sun, as
		// TAPS_PER_PASS is greater than the number of samples actually accumulated.
		// When the result is inverted (in the shader 'godrays_combine', this produces
		// a slight bright spot at the position of the sun, even when it is occluded.

		"	gl_FragColor = vec4( col/TAPS_PER_PASS );",
		"	gl_FragColor.a = 1.0;",

		"}"

	].join( "\n" )

};

/**
 * Additively applies god rays from texture tGodRays to a background (tColors).
 * fGodRayIntensity attenuates the god rays.
 */

THREE.GodRaysCombineShader = {

	uniforms: {

		tColors: {
			value: null
		},

		tGodRays: {
			value: null
		},

		fGodRayIntensity: {
			value: 0.69
		},

		vSunPositionScreenSpace: {
			value: new THREE.Vector2( 0.5, 0.5 )
		}

	},

	vertexShader: [

		"varying vec2 vUv;",

		"void main() {",

		"	vUv = uv;",
		"	gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

		"}"

	].join( "\n" ),

	fragmentShader: [

		"varying vec2 vUv;",

		"uniform sampler2D tColors;",
		"uniform sampler2D tGodRays;",

		"uniform vec2 vSunPositionScreenSpace;",
		"uniform float fGodRayIntensity;",

		"void main() {",

		// Since THREE.MeshDepthMaterial renders foreground objects white and background
		// objects black, the god-rays will be white streaks. Therefore value is inverted
		// before being combined with tColors

		"	gl_FragColor = texture2D( tColors, vUv ) + fGodRayIntensity * vec4( 1.0 - texture2D( tGodRays, vUv ).r );",
		"	gl_FragColor.a = 1.0;",

		"}"

	].join( "\n" )

};


/**
 * A dodgy sun/sky shader. Makes a bright spot at the sun location. Would be
 * cheaper/faster/simpler to implement this as a simple sun sprite.
 */

THREE.GodRaysFakeSunShader = {

	uniforms: {

		vSunPositionScreenSpace: {
			value: new THREE.Vector2( 0.5, 0.5 )
		},

		fAspect: {
			value: 1.0
		},

		sunColor: {
			value: new THREE.Color( 0xffee00 )
		},

		bgColor: {
			value: new THREE.Color( 0x000000 )
		}

	},

	vertexShader: [

		"varying vec2 vUv;",

		"void main() {",

		"	vUv = uv;",
		"	gl_Position = projectionMatrix * modelViewMatrix * vec4( position, 1.0 );",

		"}"

	].join( "\n" ),

	fragmentShader: [

		"varying vec2 vUv;",

		"uniform vec2 vSunPositionScreenSpace;",
		"uniform float fAspect;",

		"uniform vec3 sunColor;",
		"uniform vec3 bgColor;",

		"void main() {",

		"	vec2 diff = vUv - vSunPositionScreenSpace;",

		// Correct for aspect ratio

		"	diff.x *= fAspect;",

		"	float prop = clamp( length( diff ) / 0.5, 0.0, 1.0 );",
		"	prop = 0.35 * pow( 1.0 - prop, 3.0 );",

		"	gl_FragColor.xyz = mix( sunColor, bgColor, 1.0 - prop );",
		"	gl_FragColor.w = 1.0;",

		"}"

	].join( "\n" )

};
