From 49cda13e509c6b15f69f72df70e608e3d2b9a75a Mon Sep 17 00:00:00 2001
From: mars <mars@tebibyte.media>
Date: Sat, 7 May 2022 18:37:49 -0600
Subject: [PATCH] Add initial oct_encoding.wgsl

---
 shaders/oct_encoding.wgsl | 52 +++++++++++++++++++++++++++++++++++++++
 1 file changed, 52 insertions(+)
 create mode 100644 shaders/oct_encoding.wgsl

diff --git a/shaders/oct_encoding.wgsl b/shaders/oct_encoding.wgsl
new file mode 100644
index 0000000..201cd09
--- /dev/null
+++ b/shaders/oct_encoding.wgsl
@@ -0,0 +1,52 @@
+struct TangentFrame {
+  normal: vec3<f32>;
+  tangent: vec3<f32>;
+};
+
+// https://knarkowicz.wordpress.com/2014/04/16/octahedron-normal-vector-encoding/?like_comment=12
+fn oct_norm_wrap(v: vec2<f32>) -> vec2<f32> {
+  return (1.0 - abs(v.yx)) * select(vec2<f32>(-1.0), vec2<f32>(1.0), v.xy >= vec2<f32>(0.0));
+}
+
+fn oct_norm_encode(n: vec3<f32>) -> vec2<f32> {
+  let ml = (abs(n.x) + abs(n.y) + abs(n.z)); // ml = manhattan length
+  let n = n / ml;
+  let v = select(oct_norm_wrap(n.xy), n.xy, n.z >= 0.0);
+  return fma(v, vec2<f32>(0.5), vec2<f32>(0.5));
+}
+
+fn oct_norm_decode(f: vec2<f32>) -> vec3<f32> {
+  let f = fma(f, vec2<f32>(2.0), vec2<f32>(-1.0));
+  let n = vec3<f32>(f.xy, 1.0 - abs(f.x) - abs(f.y));
+  let t = vec2<f32>(clamp(-n.z, 0.0, 1.0));
+  let nxy = n.xy + select(t, -t, n.xy >= vec2<f32>(0.0));
+  return normalize(vec3<f32>(nxy, n.z));
+}
+
+fn tan_frame_encode(tf: TangentFrame) -> u32 {
+  return u32(0); // TODO
+}
+
+fn tan_frame_decode(i: u32) -> TangentFrame {
+  let n = i & u32(0xffff);
+  let nx = n >> u32(8);
+  let ny = n & u32(0xff);
+  let n = vec2<f32>(f32(nx), f32(ny)) / 255.0;
+
+  let t = i >> u32(16);
+  let tx = t >> u32(8);
+  let ty = t & u32(0xff);
+  let t = vec2<f32>(f32(tx), f32(ty)) / 255.0;
+
+  var tf: TangentFrame;
+  tf.normal = oct_norm_decode(n);
+  tf.tangent = oct_norm_decode(n);
+  return tf;
+}
+
+fn tan_frame_transform(i: u32, transform: mat4x4<f32>) -> u32 {
+  var tf = tan_frame_decode(i);
+  tf.normal = (transform * vec4<f32>(tf.normal, 0.0)).xyz;
+  tf.tangent = (transform * vec4<f32>(tf.tangent, 0.0)).xyz;
+  return tan_frame_encode(tf);
+}