[vertex]


layout(location=0) in highp vec2 vertex;

layout(location=1) in highp vec2 uv;

out highp vec2 uv_interp;

void main() {

	uv_interp=uv;
	gl_Position=vec4(vertex,0,1);
}

[fragment]


uniform samplerCube source_cube; //texunit:1
uniform int face_id;
uniform float roughness;
in highp vec2 uv_interp;


layout(location = 0) vec4 frag_color;


vec3 texelCoordToVec(vec2 uv, int faceID)
{
    mat3 faceUvVectors[6];

    // -x
    faceUvVectors[1][0] = vec3(0.0, 0.0, 1.0);  // u -> +z
    faceUvVectors[1][1] = vec3(0.0, -1.0, 0.0); // v -> -y
    faceUvVectors[1][2] = vec3(-1.0, 0.0, 0.0); // -x face

    // +x
    faceUvVectors[0][0] = vec3(0.0, 0.0, -1.0); // u -> -z
    faceUvVectors[0][1] = vec3(0.0, -1.0, 0.0); // v -> -y
    faceUvVectors[0][2] = vec3(1.0, 0.0, 0.0);  // +x face

    // -y
    faceUvVectors[3][0] = vec3(1.0, 0.0, 0.0);  // u -> +x
    faceUvVectors[3][1] = vec3(0.0, 0.0, -1.0); // v -> -z
    faceUvVectors[3][2] = vec3(0.0, -1.0, 0.0); // -y face

    // +y
    faceUvVectors[2][0] = vec3(1.0, 0.0, 0.0);  // u -> +x
    faceUvVectors[2][1] = vec3(0.0, 0.0, 1.0);  // v -> +z
    faceUvVectors[2][2] = vec3(0.0, 1.0, 0.0);  // +y face

    // -z
    faceUvVectors[5][0] = vec3(-1.0, 0.0, 0.0); // u -> -x
    faceUvVectors[5][1] = vec3(0.0, -1.0, 0.0); // v -> -y
    faceUvVectors[5][2] = vec3(0.0, 0.0, -1.0); // -z face

    // +z
    faceUvVectors[4][0] = vec3(1.0, 0.0, 0.0);  // u -> +x
    faceUvVectors[4][1] = vec3(0.0, -1.0, 0.0); // v -> -y
    faceUvVectors[4][2] = vec3(0.0, 0.0, 1.0);  // +z face

    // out = u * s_faceUv[0] + v * s_faceUv[1] + s_faceUv[2].
    vec3 result = (faceUvVectors[faceID][0] * uv.x) + (faceUvVectors[faceID][1] * uv.y) + faceUvVectors[faceID][2];
    return normalize(result);
}

vec3 ImportanceSampleGGX(vec2 Xi, float Roughness, vec3 N)
{
	float a = Roughness * Roughness; // DISNEY'S ROUGHNESS [see Burley'12 siggraph]

	// Compute distribution direction
	float Phi = 2.0 * M_PI * Xi.x;
	float CosTheta = sqrt((1.0 - Xi.y) / (1.0 + (a*a - 1.0) * Xi.y));
	float SinTheta = sqrt(1.0 - CosTheta * CosTheta);

	// Convert to spherical direction
	vec3 H;
	H.x = SinTheta * cos(Phi);
	H.y = SinTheta * sin(Phi);
	H.z = CosTheta;

	vec3 UpVector = abs(N.z) < 0.999 ? vec3(0.0, 0.0, 1.0) : vec3(1.0, 0.0, 0.0);
	vec3 TangentX = normalize(cross(UpVector, N));
	vec3 TangentY = cross(N, TangentX);

	// Tangent to world space
	return TangentX * H.x + TangentY * H.y + N * H.z;
}

// http://graphicrants.blogspot.com.au/2013/08/specular-brdf-reference.html
float GGX(float NdotV, float a)
{
	float k = a / 2.0;
	return NdotV / (NdotV * (1.0 - k) + k);
}

// http://graphicrants.blogspot.com.au/2013/08/specular-brdf-reference.html
float G_Smith(float a, float nDotV, float nDotL)
{
	return GGX(nDotL, a * a) * GGX(nDotV, a * a);
}

float radicalInverse_VdC(uint bits) {
    bits = (bits << 16u) | (bits >> 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);
    return float(bits) * 2.3283064365386963e-10; // / 0x100000000
}

vec2 Hammersley(uint i, uint N) {
     return vec2(float(i)/float(N), radicalInverse_VdC(i));
}

#define SAMPLE_COUNT 1024

void main() {

	vec2 uv         = (uv_interp * 2.0) - 1.0;
	vec3 N          = texelCoordToVec(uv, face_id);

	//vec4 color = color_interp;
	vec4 sum = vec4(0.0, 0.0, 0.0, 0.0);

	for(int sampleNum = 0; sampleNum < SAMPLE_COUNT; sampleNum++) {
		vec2 xi = Hammersley(sampleNum, SAMPLE_COUNT);
		vec2 xi = texture2DLod(Texture0, vec2(float(sampleNum) / float(SAMPLE_COUNT), 0.5), 0.0).xy;

		vec3 H  = ImportanceSampleGGX( xi, roughness, N );
		vec3 V  = N;
		vec3 L  = normalize(2.0 * dot( V, H ) * H - V);

		float ndotl = max(0.0, dot(N, L));
		vec3 s = textureCubeLod(u_skyCube, H, 0.0).rgb * ndotl;

		sum += vec4(s, 1.0);
	}
	sum /= sum.w;

	frag_color = vec4(sum.rgb, 1.0);
}