Add notes on camera coordinate system

notes/camera.md

@@ -0,0 +1,18 @@
+# `Camera` coordinate system
+
+The `Camera` class contains a coordinate system that can be used to generate rays using the thin-lens approximation for depth of field effects. Note: `right`, `up` are just normalized `horizontal`, `vertical` vectors.
+
+![camera-coordinates](img/camera.jpg)
+
+```rust
+// A WGSL shader example of how to generate a ray using `Camera`.
+fn generateCameraRay(camera: Camera, rngState: ptr<function, u32>, u: f32, v: f32) -> Ray {
+    let randomPointInAperture = camera.lensRadius * randomVec3InUnitDisk(rngState);
+    let lensOffset = randomPointInAperture.x * camera.right + randomPointInAperture.y * camera.up;
+
+    let origin = camera.origin + lensOffset;
+    let direction = camera.lowerLeftCorner + u * camera.horizontal + v * camera.vertical - origin;
+
+    return Ray(origin, direction);
+}
+```

src/pt/raytracer.wgsl

@@ -60,7 +60,7 @@ struct FrameData {
 }
 
 struct Camera {
-    eye: vec3f,
+    origin: vec3f,
     lowerLeftCorner: vec3f,
     horizontal: vec3f,
     vertical: vec3f,
@@ -121,7 +121,7 @@ fn rayColor(primaryRay: Ray, rngState: ptr<function, u32>) -> vec3f {
 }
 
 fn generateCameraRay(camera: Camera, rngState: ptr<function, u32>, u: f32, v: f32) -> Ray {
-    let origin = camera.eye;
+    let origin = camera.origin;
     let direction = camera.lowerLeftCorner + u * camera.horizontal + v * camera.vertical - origin;
 
     return Ray(origin, direction);