/////////////////////////////////////////////////////////////////////////
// Pixel shader for the final pass
//
// Copyright 2013 DigiPen Institute of Technology
////////////////////////////////////////////////////////////////////////
#version 330

uniform int mode, WIDTH, HEIGHT,camNear,camFar;
uniform bool useTexture;

uniform vec3 cameraPos,lightPos,lightForward;


//shadow stuff
uniform sampler2D ldepth;
vec4 vShdwDepthAndTexCoord;
uniform vec2 clipDistance;
uniform mat4 ShadowViewMatrix;
uniform mat4 ShadowProjMatrix;


uniform sampler2D gBufferPos;
uniform sampler2D gBufferNormal;
uniform sampler2D gBufferDiffuse;
uniform mat4 lightProj;

out vec4 fragColor;

in vec2 texCoord;

#define MAX_STEPS 2000



vec3 GetPos()
{

	vec2 coords;
	coords.x = texCoord.s * WIDTH;
	coords.y = texCoord.t * HEIGHT;
	ivec2 iCoords = ivec2(coords.x,coords.y);
	// index into the gBufferPos texture correctly to grab the appropiate texel for final rendering
	vec3 pos = texelFetch(gBufferPos,iCoords,0).xyz;
	return pos;
}



float VolumeLight()
{
	vec3 vertexPosWS = GetPos();

	// gets initial vector for screen trace
	vec3 forwardVec = normalize(vertexPosWS - cameraPos);
	// scale vector to nearPlane(to start trace)
	float traceDistance = dot(vertexPosWS - (cameraPos + forwardVec * camNear),forwardVec);
	traceDistance = clamp(traceDistance, 0.0,2500.0); // clamp at max traceDistance

	// get first trace position.
	vertexPosWS = cameraPos + forwardVec * camNear;
	float samplingRate = 1.0f;
	// NOTE: make sampling rate a modifable variable, pass it in from cpu, maybe make a gui
	forwardVec *= samplingRate * 2.0f;
	float numberOfSteps = min(traceDistance / length(forwardVec),MAX_STEPS);

	//add jitter, this wil benefit from having a noise texture, but a rand might do
	float seed = 1234.0f;
	float jitter = noise1(seed) * 256.0f;
	//float jitter = 1.0f;

	float step = length(forwardVec);
	float baseBrightness = 0.0009 ; // minimum brightness for samples???
	float scale = step * baseBrightness;


	vec3 curPos = vertexPosWS + forwardVec * jitter;
	vec4 coordinates = ShadowProjMatrix*ShadowViewMatrix*vec4(curPos.x,curPos.y,curPos.z,1.0f);
	coordinates.z = curPos.z;

	curPos = vertexPosWS + forwardVec * (1.0f + jitter);
	vec4 coordinatesEnd  = ShadowProjMatrix*ShadowViewMatrix*vec4(curPos.x,curPos.y,curPos.z,1.0f);
	coordinatesEnd.z = curPos.z;


	vec4 coordinateDelta = coordinatesEnd - coordinates;


	float sample;
	float light = 0;

	for(float i = 0; i < numberOfSteps; ++i)
	{

		vec2 tc     = coordinates.st / coordinates.q;
		sample = float(texture2D(ldepth, tc).x < coordinates.z); // float() is a cast from bool to float
		if(sample > 0)
		light += scale * sample;
		coordinates += coordinateDelta;
	}


	return light;
}

void main()
{

    vec3 worldVertex = GetPos();

    float light = VolumeLight(); // calculates volume light value of this pixel by tracing the screen

    fragColor.xyz = vec3(light,light,light);
    fragColor.w = 1.0;

}