Implement the IFS camera

enkou-shaders/Cargo.toml

@@ -12,5 +12,6 @@ workspace = true
 [dependencies]
 bytemuck.workspace = true
 glam.workspace = true
+libm.workspace = true
 rand.workspace = true
 spirv-std.workspace = true

enkou-shaders/src/camera.rs

@@ -0,0 +1,156 @@
+use bytemuck::{Pod, Zeroable};
+use glam::{Affine2, IVec2, UVec2, Vec2, vec2};
+use libm::powf;
+
+#[derive(Copy, Clone, Pod, Zeroable)]
+#[repr(C)]
+pub struct Camera {
+    transform: Affine2,
+}
+
+impl Camera {
+    /// Construct a new camera that maps IFS coordinates to pixel coordinates.
+    ///
+    /// The camera object is itself an affine transformation, but it's helpful to express
+    /// the parameters in individual steps, and compose them internally.
+    ///
+    /// # Arguments
+    ///
+    /// * `dimensions` - Width and height of the output image (in pixels).
+    /// * `center` - Location of the origin in IFS coordinates. Positive `x` shifts the image
+    ///     left, and positive `y` position shifts the image up.
+    /// * `rotate` - Rotation angle (in radians) of IFS coordinates. Rotation is applied after the
+    ///     `center` translation, so it is about the new origin.
+    /// * `zoom` - Zoom factor applied to IFS coordinates. IFS coordinates are scaled by
+    ///     `pow(2, zoom)`, so a zoom factor of 0 is the identity.
+    /// * `scale` - Pixels per unit of IFS coordinates. By default, this parameter is chosen such
+    ///     that the largest dimension will cover the range `[-2, 2]`, but values higher or lower
+    ///     can be used as a secondary zoom.
+    pub fn new(dimensions: UVec2, center: Vec2, rotate: f32, zoom: Vec2, scale: Vec2) -> Camera {
+        let ifs_center_transform = Affine2::from_translation(-center);
+        let zoom_transform = Affine2::from_scale(vec2(powf(2.0, zoom.x), powf(2.0, zoom.y)));
+        let scale_transform = Affine2::from_scale(scale);
+        let rotate_transform = Affine2::from_angle(rotate);
+        let image_center_transform = Affine2::from_translation((dimensions / 2).as_vec2());
+
+        let transform = image_center_transform
+            * rotate_transform
+            * scale_transform
+            * zoom_transform
+            * ifs_center_transform;
+
+        Camera { transform }
+    }
+
+    /// Map a point from IFS coordinates to pixel coordinates.
+    ///
+    /// ```
+    /// # use glam::{vec2, ivec2, uvec2, Vec2};
+    /// # use crate::enkou_shaders::camera::Camera;
+    /// // Output image is 600x600 pixels, centered at the origin, no rotation, no zoom,
+    /// // and scaled such that it covers the range [-2, 2].
+    /// // Use the origin as the IFS coordinate, so the pixel coordinate is the center of the image
+    /// let camera = Camera::new(
+    ///     uvec2(600, 600),
+    ///     Vec2::ZERO,
+    ///     0.0,
+    ///     Vec2::ZERO,
+    ///     vec2(150.0, 150.0)
+    /// );
+    /// assert_eq!(camera.transform_point(vec2(0.0, 0.0)), ivec2(300, 300));
+    /// ```
+    pub fn transform_point(&self, point: Vec2) -> IVec2 {
+        self.transform.transform_point2(point).as_ivec2()
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use crate::camera::Camera;
+    use glam::{Affine2, Vec2, ivec2, uvec2, vec2};
+    use libm::powf;
+
+    #[test]
+    pub fn manual_camera() {
+        let starting_point = vec2(1.0, 1.0);
+
+        // Move the origin; points move right and up by one unit, giving us (2.0, 2.0)
+        let center = vec2(-1.0, -1.0);
+        let point = starting_point - center;
+
+        // Rotate about the new origin; points move counter-clockwise, giving us (-2.0, 2.0)
+        let rotate = 90.0f32.to_radians();
+        let point = Affine2::from_angle(rotate).transform_point2(point);
+
+        // Zoom in by a factor of 1; points will be twice as far from the origin,
+        // giving us (-4.0, 4.0)
+        let zoom = vec2(1.0, 1.0);
+        let point = point * vec2(powf(2.0, zoom.x), powf(2.0, zoom.y));
+
+        // Apply scaling; scale 100 in a 1000 x 1000 image is an effective range
+        // of [-5, 5] in IFS coordinates.
+        // After scaling, the point is (-400.0, 400.0)
+        let scale = vec2(100.0, 100.0);
+        let point = point * scale;
+
+        // Move the origin from (0, 0) to image center,
+        // giving us (100.0, 900.0)
+        let dimensions = uvec2(1000, 1000);
+        let point = point.as_ivec2() + dimensions.as_ivec2() / 2;
+
+        // Check that the camera implementation ends up at the same point
+        let camera = Camera::new(dimensions, center, rotate, zoom, scale);
+
+        // The camera is implemented by composing affine transforms,
+        // which ends up with a slightly different result because of rounding.
+        let error = camera.transform_point(starting_point) - point;
+        assert!(error.x.abs() <= 1);
+        assert!(error.y.abs() <= 1);
+    }
+
+    #[test]
+    pub fn point_outside_camera() {
+        // Scale 250 for an image 1000 x 1000 gives an effective range of [-2, 2]
+        let camera = Camera::new(
+            uvec2(1000, 1000),
+            Vec2::ZERO,
+            0.0,
+            Vec2::ZERO,
+            vec2(250.0, 250.0),
+        );
+
+        // Converting a point outside the effective range is legal, but outside the image bounds
+        assert_eq!(camera.transform_point(vec2(3.0, 3.0)), ivec2(1250, 1250));
+    }
+
+    #[test]
+    pub fn point_outside_camera_negative() {
+        // Scale 250 for an image 1000 x 1000 gives an effective range of [-2, 2]
+        let camera = Camera::new(
+            uvec2(1000, 1000),
+            Vec2::ZERO,
+            0.0,
+            Vec2::ZERO,
+            vec2(250.0, 250.0),
+        );
+
+        // Converting a point outside the effective range is legal, but outside the image bounds
+        assert_eq!(camera.transform_point(vec2(-3.0, -3.0)), ivec2(-250, -250));
+    }
+
+    #[test]
+    pub fn aspect_ratio() {
+        // Scale 100 for an image 1600 x 900 gives an effective X range of [-8, 8],
+        // and effective Y range of [-4.5, 4.5]
+        let camera = Camera::new(
+            uvec2(1600, 900),
+            Vec2::ZERO,
+            0.0,
+            Vec2::ZERO,
+            vec2(100.0, 100.0),
+        );
+
+        // This point is inside the image width, but outside its height
+        assert_eq!(camera.transform_point(vec2(6.0, 6.0)), ivec2(1400, 1050));
+    }
+}

enkou-shaders/src/lib.rs

@@ -2,11 +2,13 @@
 #![no_std]
 #![warn(missing_docs)]
 
+pub mod camera;
+
 use bytemuck::{Pod, Zeroable};
 use core::f32::consts::PI;
-use glam::{Affine2, Vec3, Vec4, vec2, vec3, Vec2};
-use rand::{Rng, RngExt};
+use glam::{Affine2, Vec2, Vec3, Vec4, vec2, vec3};
 use rand::distr::StandardUniform;
+use rand::{Rng, RngExt};
 #[cfg(target_arch = "spirv")]
 use spirv_std::num_traits::Float;
 use spirv_std::spirv;
@@ -97,7 +99,12 @@ impl Transform {
 /// # Arguments
 ///
 /// * `weights` - Weights are assumed to be normalized; adding all elements together should return the value 1
-pub fn step_chaos_game<R: Rng>(rng: &mut R, point: Vec2, weights: &[f32], transforms: &[Transform]) -> (Vec2, u32) {
+pub fn step_chaos_game<R: Rng>(
+    rng: &mut R,
+    point: Vec2,
+    weights: &[f32],
+    transforms: &[Transform],
+) -> (Vec2, u32) {
     let mut choice_weight = rng.sample::<f32, _>(StandardUniform);
     let mut transform_index: u32 = 0;
 
@@ -110,7 +117,10 @@ pub fn step_chaos_game<R: Rng>(rng: &mut R, point: Vec2, weights: &[f32], transf
         transform_index += 1;
     }
 
-    (transforms[transform_index as usize].transform_point(point), transform_index)
+    (
+        transforms[transform_index as usize].transform_point(point),
+        transform_index,
+    )
 }
 
 #[derive(Copy, Clone, Pod, Zeroable)]