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#version 330 core
in vec3 f_p;
out vec4 f_color;
// -----------------------------------------------------------------------------
// Uniforms
// -----------------------------------------------------------------------------
uniform vec3 u_cam;
uniform float u_time;
uniform int u_mode;
uniform float u_sigma;
uniform int u_steps;
// -----------------------------------------------------------------------------
// Ray-AABB intersection
// ref: https://tavianator.com/2022/ray_box_boundary.html
// -----------------------------------------------------------------------------
bool intersect(vec3 ro, vec3 rd, vec3 bbmin, vec3 bbmax, out float t0, out float t1)
{
vec3 inv = 1.0f / rd;
vec3 ta = (bbmin - ro) * inv;
vec3 tb = (bbmax - ro) * inv;
vec3 tmin = min(ta, tb);
vec3 tmax = max(ta, tb);
t0 = max(max(tmin.x, tmin.y), tmin.z);
t1 = min(min(tmax.x, tmax.y), tmax.z);
return t1 >= max(t0, 0.0f);
}
// -----------------------------------------------------------------------------
// Mode 0: Basic volumetric fog
// -----------------------------------------------------------------------------
float sample_basic(vec3 pos)
{
return 1.0;
}
// -----------------------------------------------------------------------------
// Mode 1: Fade along y axis
// -----------------------------------------------------------------------------
float sample_vfade(vec3 pos)
{
return 1.0f - pos.y;
}
// -----------------------------------------------------------------------------
// Mode 2: Some waves
// -----------------------------------------------------------------------------
float sample_waves(vec3 pos)
{
// Fractal sum
// ref: https://thebookofshaders.com/13/
float h = 0.75f; // baseline
// Big waves
h += 0.08f * sin(pos.x * 2.0f + u_time);
h += 0.08f * cos(pos.z * 2.0f + u_time * -1.1f);
// Medium waves
h += 0.05f * sin(pos.x * 5.0f + u_time * 1.7f);
h += 0.05f * cos(pos.z * 4.0f + u_time * -2.0f);
// Small but choppy waves
h += 0.03f * sin(pos.x * 12.0f + u_time * 3.3f);
h += 0.03f * cos(pos.z * 11.0f + u_time * -2.5f);
return float(pos.y < h);
}
void main()
{
vec3 bbmin = vec3(0.0f, 0.0f, 0.0f);
vec3 bbmax = vec3(1.0f, 1.0f, 1.0f);
vec3 ro = u_cam;
vec3 rd = normalize(f_p - ro);
float t0 = 0.0f;
float t1 = 0.0f;
if (!intersect(ro, rd, bbmin, bbmax, t0, t1)) {
f_color = vec4(0.0f, 0.0f, 0.0f, 0.0f);
return;
}
// Clamp entry point in front of camera
float t_enter = max(t0, 0.0f);
float t_exit = t1;
float t_len = max(0.0f, t_exit - t_enter);
float density = 0;
for (int i = 0; i < u_steps; ++i) {
float t = mix(t_enter, t_exit, (float(i) + 0.5f) / float(u_steps));
vec3 p = ro + rd * t;
switch (u_mode) {
case 0: { density += sample_basic(p); } break;
case 1: { density += sample_vfade(p); } break;
case 2: { density += sample_waves(p); } break;
}
}
density /= float(u_steps);
// Beer-Lambert attenuation
// ref: https://en.wikipedia.org/wiki/Beer%E2%80%93Lambert_law
float a = 1.0f - exp(-u_sigma * t_len * density);
f_color = vec4(vec3(1.0f), a);
}
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