1#version 450
 2
 3#include "glu_head.glsl"
 4
 5// based on Abramowitz and Stegun formula 7.1.26 or similar Hastings' approximation
 6// ref: https://www.johndcook.com/blog/python_erf/
 7const float p_erf  = 0.3275911f;
 8const float a1_erf = 0.254829592f;
 9const float a2_erf = -0.284496736f;
10const float a3_erf = 1.421413741f;
11const float a4_erf = -1.453152027f;
12const float a5_erf = 1.061405429f;
13
14const float SQRT_2_INV = 0.70710678118654752440084436210484f;
15
16float op(float a, float b) {
17    const float a_div_sqr2 = a * SQRT_2_INV;
18    const float sign_x = sign(a_div_sqr2);
19    const float x = abs(a_div_sqr2);
20    const float t = 1.0f / (1.0f + p_erf * x);
21    const float y = 1.0f - (((((a5_erf * t + a4_erf) * t) + a3_erf) * t + a2_erf) * t + a1_erf) * t * exp(-x * x);
22    const float erf_approx = sign_x * y;
23
24    return 0.5f * a * (1.0f + erf_approx) * b;
25}
26
27#include "glu_main.glsl"