uniform float N_SLOPE_VARIANCE;

uniform sampler2D spectrum_1_2_Sampler;

uniform sampler2D spectrum_3_4_Sampler;

uniform int FFT_SIZE;

uniform vec4 GRID_SIZES;

uniform float slopeVarianceDelta;

uniform float c;

varying vec2 uv;

#ifdef _VERTEX_

void main() {

uv = gl_Vertex.zw;

gl_Position = vec4(gl_Vertex.xy, 0.0, 1.0);

}

#define M_PI 3.14159265

vec2 getSlopeVariances(vec2 k, float A, float B, float C, vec2 spectrumSample) {

float w = 1.0 - exp(A * k.x * k.x + B * k.x * k.y + C * k.y * k.y);

vec2 kw = k * w;

return kw * kw * dot(spectrumSample, spectrumSample) * 2.0;

}

void main() {

const float SCALE = 10.0;

float a = floor(uv.x * N_SLOPE_VARIANCE);

float b = floor(uv.y * N_SLOPE_VARIANCE);

float A = pow(a / (N_SLOPE_VARIANCE - 1.0), 4.0) * SCALE;

float C = pow(c / (N_SLOPE_VARIANCE - 1.0), 4.0) * SCALE;

float B = (2.0 * b / (N_SLOPE_VARIANCE - 1.0) - 1.0) * sqrt(A * C);

A = -0.5 * A;

B = - B;

C = -0.5 * C;

vec2 slopeVariances = vec2(slopeVarianceDelta);

for (int y = 0; y < FFT_SIZE; ++y) {

for (int x = 0; x < FFT_SIZE; ++x) {

int i = x >= FFT_SIZE / 2 ? x - FFT_SIZE : x;

int j = y >= FFT_SIZE / 2 ? y - FFT_SIZE : y;

vec2 k = 2.0 * M_PI * vec2(i, j);

vec4 spectrum12 = texture2D(spectrum_1_2_Sampler, vec2(float(x) + 0.5, float(y) + 0.5) / float(FFT_SIZE));

vec4 spectrum34 = texture2D(spectrum_3_4_Sampler, vec2(float(x) + 0.5, float(y) + 0.5) / float(FFT_SIZE));

slopeVariances += getSlopeVariances(k / GRID_SIZES.x, A, B, C, spectrum12.xy);

slopeVariances += getSlopeVariances(k / GRID_SIZES.y, A, B, C, spectrum12.zw);

slopeVariances += getSlopeVariances(k / GRID_SIZES.z, A, B, C, spectrum34.xy);

slopeVariances += getSlopeVariances(k / GRID_SIZES.w, A, B, C, spectrum34.zw);

}

}

gl_FragColor = slopeVariances.xxxy;

}