// Copyright (C) 2009-2017, Panagiotis Christopoulos Charitos and contributors.
// All rights reserved.
// Code licensed under the BSD License.
// http://www.anki3d.org/LICENSE

// Screen space lens flare. Used the technique from here
// http://john-chapman-graphics.blogspot.no/2013/02/pseudo-lens-flare.html

#include "shaders/Common.glsl"

#define MAX_GHOSTS 4
#define GHOST_DISPERSAL (0.7)
#define HALO_WIDTH 0.4
#define CHROMATIC_DISTORTION 3.0
#define ENABLE_CHROMATIC_DISTORTION 1
#define ENABLE_HALO 1
#define HALO_OPACITY 0.5

layout(location = 0) in vec2 in_texCoord;

layout(ANKI_TEX_BINDING(0, 0)) uniform sampler2D u_rt;
layout(ANKI_TEX_BINDING(0, 1)) uniform sampler2D u_lensDirtTex;

layout(location = 0) out vec3 out_color;

vec3 textureDistorted(in sampler2D tex,
	in vec2 texcoord,
	in vec2 direction, // direction of distortion
	in vec3 distortion) // per-channel distortion factor
{
#if ENABLE_CHROMATIC_DISTORTION
	return vec3(texture(tex, texcoord + direction * distortion.r).r,
		texture(tex, texcoord + direction * distortion.g).g,
		texture(tex, texcoord + direction * distortion.b).b);
#else
	return texture(tex, texcoord).rgb;
#endif
}

void main()
{
	vec2 texcoord = vec2(1.0) - in_texCoord;

	vec2 ghostVec = (vec2(0.5) - texcoord) * GHOST_DISPERSAL;

	const vec2 texelSize = 1.0 / vec2(TEX_DIMENSIONS);

	const vec3 distortion = vec3(-texelSize.x * CHROMATIC_DISTORTION, 0.0, texelSize.x * CHROMATIC_DISTORTION);

	const float lenOfHalf = length(vec2(0.5));

	vec2 direction = normalize(ghostVec);

	vec3 result = vec3(0.0);

	// sample ghosts:
	for(int i = 0; i < MAX_GHOSTS; ++i)
	{
		vec2 offset = fract(texcoord + ghostVec * float(i));

		float weight = length(vec2(0.5) - offset) / lenOfHalf;
		weight = pow(1.0 - weight, 10.0);

		result += textureDistorted(u_rt, offset, direction, distortion) * weight;
	}

// sample halo
#if ENABLE_HALO
	vec2 haloVec = normalize(ghostVec) * HALO_WIDTH;
	float weight = length(vec2(0.5) - fract(texcoord + haloVec)) / lenOfHalf;
	weight = pow(1.0 - weight, 20.0);
	result += textureDistorted(u_rt, texcoord + haloVec, direction, distortion) * (weight * HALO_OPACITY);
#endif

	// lens dirt
	result *= texture(u_lensDirtTex, in_texCoord).rgb;

	// Write
	out_color = result;
}