/** 
 * @file class1/deferred/genbrdflut.glsl
 *
 * $LicenseInfo:firstyear=2022&license=viewerlgpl$
 * Second Life Viewer Source Code
 * Copyright (C) 2022, Linden Research, Inc.
 * 
 * This library is free software; you can redistribute it and/or
 * modify it under the terms of the GNU Lesser General Public
 * License as published by the Free Software Foundation;
 * version 2.1 of the License only.
 * 
 * This library is distributed in the hope that it will be useful,
 * but WITHOUT ANY WARRANTY; without even the implied warranty of
 * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
 * Lesser General Public License for more details.
 * 
 * You should have received a copy of the GNU Lesser General Public
 * License along with this library; if not, write to the Free Software
 * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA  02110-1301  USA
 * 
 * Linden Research, Inc., 945 Battery Street, San Francisco, CA  94111  USA
 * $/LicenseInfo$
 */

/*  Taken from Sascha Willem's Vulkan GLTF refernce implementation
MIT License

Copyright (c) 2018 Sascha Willems

Permission is hereby granted, free of charge, to any person obtaining a copy
of this software and associated documentation files (the "Software"), to deal
in the Software without restriction, including without limitation the rights
to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
copies of the Software, and to permit persons to whom the Software is
furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all
copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
SOFTWARE.
*/

/*[EXTRA_CODE_HERE]*/

VARYING vec2 vary_uv;

out vec4 outColor;

#define NUM_SAMPLES 1024

const float PI = 3.1415926536;

// Based omn http://byteblacksmith.com/improvements-to-the-canonical-one-liner-glsl-rand-for-opengl-es-2-0/
float random(vec2 co)
{
	float a = 12.9898;
	float b = 78.233;
	float c = 43758.5453;
	float dt= dot(co.xy ,vec2(a,b));
	float sn= mod(dt,3.14);
	return fract(sin(sn) * c);
}

vec2 hammersley2d(uint i, uint N) 
{
	// Radical inverse based on http://holger.dammertz.org/stuff/notes_HammersleyOnHemisphere.html
	uint bits = (i << 16u) | (i >> 16u);
	bits = ((bits & 0x55555555u) << 1u) | ((bits & 0xAAAAAAAAu) >> 1u);
	bits = ((bits & 0x33333333u) << 2u) | ((bits & 0xCCCCCCCCu) >> 2u);
	bits = ((bits & 0x0F0F0F0Fu) << 4u) | ((bits & 0xF0F0F0F0u) >> 4u);
	bits = ((bits & 0x00FF00FFu) << 8u) | ((bits & 0xFF00FF00u) >> 8u);
	float rdi = float(bits) * 2.3283064365386963e-10;
	return vec2(float(i) /float(N), rdi);
}

// Based on http://blog.selfshadow.com/publications/s2013-shading-course/karis/s2013_pbs_epic_slides.pdf
vec3 importanceSample_GGX(vec2 Xi, float roughness, vec3 normal) 
{
	// Maps a 2D point to a hemisphere with spread based on roughness
	float alpha = roughness * roughness;
	float phi = 2.0 * PI * Xi.x + random(normal.xz) * 0.1;
	float cosTheta = sqrt((1.0 - Xi.y) / (1.0 + (alpha*alpha - 1.0) * Xi.y));
	float sinTheta = sqrt(1.0 - cosTheta * cosTheta);
	vec3 H = vec3(sinTheta * cos(phi), sinTheta * sin(phi), cosTheta);

	// Tangent space
	vec3 up = abs(normal.z) < 0.999 ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
	vec3 tangentX = normalize(cross(up, normal));
	vec3 tangentY = normalize(cross(normal, tangentX));

	// Convert to world Space
	return normalize(tangentX * H.x + tangentY * H.y + normal * H.z);
}

// Geometric Shadowing function
float G_SchlicksmithGGX(float dotNL, float dotNV, float roughness)
{
	float k = (roughness * roughness) / 2.0;
	float GL = dotNL / (dotNL * (1.0 - k) + k);
	float GV = dotNV / (dotNV * (1.0 - k) + k);
	return GL * GV;
}

vec2 BRDF(float NoV, float roughness)
{
	// Normal always points along z-axis for the 2D lookup 
	const vec3 N = vec3(0.0, 0.0, 1.0);
	vec3 V = vec3(sqrt(1.0 - NoV*NoV), 0.0, NoV);

	vec2 LUT = vec2(0.0);
	for(uint i = 0u; i < NUM_SAMPLES; i++) {
		vec2 Xi = hammersley2d(i, NUM_SAMPLES);
		vec3 H = importanceSample_GGX(Xi, roughness, N);
		vec3 L = 2.0 * dot(V, H) * H - V;

		float dotNL = max(dot(N, L), 0.0);
		float dotNV = max(dot(N, V), 0.0);
		float dotVH = max(dot(V, H), 0.0); 
		float dotNH = max(dot(H, N), 0.0);

		if (dotNL > 0.0) {
			float G = G_SchlicksmithGGX(dotNL, dotNV, roughness);
			float G_Vis = (G * dotVH) / (dotNH * dotNV);
			float Fc = pow(1.0 - dotVH, 5.0);
			LUT += vec2((1.0 - Fc) * G_Vis, Fc * G_Vis);
		}
	}
	return LUT / float(NUM_SAMPLES);
}

void main() 
{
	outColor = vec4(BRDF(vary_uv.s, 1.0-vary_uv.t), 0.0, 1.0);
}