From 8a077ca7c421dcea92c9485aa284c6e636f2ddad Mon Sep 17 00:00:00 2001
From: Hunter Kvalevog <hunter@kvog.sh>
Date: Sat, 15 Nov 2025 08:48:09 -0600
Subject: gl-vfog: Add more modes

---
 gl-vfog/fs.glsl | 73 ++++++++++++++++++++++++++++++++++++++++++++++++---------
 1 file changed, 62 insertions(+), 11 deletions(-)

(limited to 'gl-vfog/fs.glsl')

diff --git a/gl-vfog/fs.glsl b/gl-vfog/fs.glsl
index 00ec9b1..12f32b9 100644
--- a/gl-vfog/fs.glsl
+++ b/gl-vfog/fs.glsl
@@ -4,13 +4,19 @@ 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;
@@ -23,8 +29,49 @@ bool intersect(vec3 ro, vec3 rd, vec3 bbmin, vec3 bbmax, out float t0, out float
   return t1 >= max(t0, 0.0f);
 }
 
-// Mode 0: Basic
-void main_0()
+// -----------------------------------------------------------------------------
+// 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);
@@ -44,17 +91,21 @@ void main_0()
   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);
+  float a = 1.0f - exp(-u_sigma * t_len * density);
 
   f_color = vec4(vec3(1.0f), a);
 }
-
-void main()
-{
-  switch (u_mode) {
-  case 0: { main_0(); } break;
-  default: { f_color = vec4(0.0f, 1.0f, 0.0f, 1.0f); } break;
-  }
-}
-- 
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