Branchless sky shader

examples/example-runner/src/main.rs

@@ -728,7 +728,7 @@ fn main() {
             })
             .collect();
 
-        let index_buffer_data = [0u32, 1, 2];
+        let index_buffer_data = [0u32, 1, 2, 1, 2, 3];
         let index_buffer_info = vk::BufferCreateInfo::builder()
             .size(std::mem::size_of_val(&index_buffer_data) as u64)
             .usage(vk::BufferUsageFlags::INDEX_BUFFER)
@@ -772,8 +772,27 @@ fn main() {
             .bind_buffer_memory(index_buffer, index_buffer_memory, 0)
             .unwrap();
 
+        let vertices = [
+            Vertex {
+                pos: [-1.0, 1.0, 0.0, 1.0],
+                color: [0.0, 1.0, 0.0, 1.0],
+            },
+            Vertex {
+                pos: [1.0, 1.0, 0.0, 1.0],
+                color: [0.0, 0.0, 1.0, 1.0],
+            },
+            Vertex {
+                pos: [-1.0, -1.0, 0.0, 1.0],
+                color: [1.0, 0.0, 0.0, 1.0],
+            },
+            Vertex {
+                pos: [1.0, -1.0, 0.0, 1.0],
+                color: [1.0, 1.0, 1.0, 1.0],
+            },
+        ];
+
         let vertex_input_buffer_info = vk::BufferCreateInfo {
-            size: 3 * std::mem::size_of::<Vertex>() as u64,
+            size: std::mem::size_of_val(&vertices) as u64 as u64,
             usage: vk::BufferUsageFlags::VERTEX_BUFFER,
             sharing_mode: vk::SharingMode::EXCLUSIVE,
             ..Default::default()
@@ -806,21 +825,6 @@ fn main() {
             .allocate_memory(&vertex_buffer_allocate_info, None)
             .unwrap();
 
-        let vertices = [
-            Vertex {
-                pos: [-1.0, 1.0, 0.0, 1.0],
-                color: [0.0, 1.0, 0.0, 1.0],
-            },
-            Vertex {
-                pos: [1.0, 1.0, 0.0, 1.0],
-                color: [0.0, 0.0, 1.0, 1.0],
-            },
-            Vertex {
-                pos: [0.0, -1.0, 0.0, 1.0],
-                color: [1.0, 0.0, 0.0, 1.0],
-            },
-        ];
-
         let vert_ptr = base
             .device
             .map_memory(
@@ -1061,7 +1065,7 @@ fn main() {
                         1,
                         0,
                         0,
-                        1,
+                        0,
                     );
                     // Or draw without the index buffer
                     // device.cmd_draw(draw_command_buffer, 3, 1, 0, 0);

examples/example-shader/src/lib.rs

@@ -1,17 +1,183 @@
+//! Ported to Rust from https://github.com/Tw1ddle/Sky-Shader/blob/master/src/shaders/glsl/sky.fragment
+
 #![no_std]
 #![feature(register_attr)]
 #![register_attr(spirv)]
 
+use core::f32::consts::PI;
 use core::panic::PanicInfo;
-use spirv_std::{f32x4, Input, Output};
+use spirv_std::{
+    builtin::*, f32x4, Input, Mat4, Output, Vec3, Vec4,
+};
+
+const DEPOLARIZATION_FACTOR: f32 = 0.035;
+const LUMINANCE: f32 = 1.0;
+const MIE_COEFFICIENT: f32 = 0.005;
+const MIE_DIRECTIONAL_G: f32 = 0.8;
+const MIE_K_COEFFICIENT: Vec3 = Vec3::new(0.686, 0.678, 0.666);
+const MIE_V: f32 = 4.0;
+const MIE_ZENITH_LENGTH: f32 = 1.25e3;
+const NUM_MOLECULES: f32 = 2.542e25f32;
+const PRIMARIES: Vec3 = Vec3::new(6.8e-7f32, 5.5e-7f32, 4.5e-7f32);
+const RAYLEIGH: f32 = 1.0;
+const RAYLEIGH_ZENITH_LENGTH: f32 = 8.4e3;
+const REFRACTIVE_INDEX: f32 = 1.0003;
+const SUN_ANGULAR_DIAMETER_DEGREES: f32 = 0.0093333;
+const SUN_INTENSITY_FACTOR: f32 = 1000.0;
+const SUN_INTENSITY_FALLOFF_STEEPNESS: f32 = 1.5;
+const TONEMAP_WEIGHTING: Vec3 = Vec3::splat(9.50);
+const TURBIDITY: f32 = 2.0;
+
+/// Based on: https://seblagarde.wordpress.com/2014/12/01/inverse-trigonometric-functions-gpu-optimization-for-amd-gcn-architecture/
+fn acos_approx(v: f32) -> f32 {
+    let x = absf32(v);
+    let mut res = -0.155972 * x + 1.56467; // p(x)
+    res *= sqrtf32(1.0f32 - x);
+
+    let mask = (v >= 0.0) as u32 as f32;
+
+    // can't use if-statement so do oldskool shader masking instead to avoid conditional
+    (res * mask) + ((1.0f32 - mask) * (PI - res))
+}
+
+fn clamp(a: f32, b: f32, c: f32) -> f32 {
+    a.max(b).min(c)
+}
+
+fn smoothstep(edge0: f32, edge1: f32, x: f32) -> f32 {
+    // Scale, bias and saturate x to 0..1 range
+    let x = clamp((x - edge0) / (edge1 - edge0), 0.0, 1.0);
+    // Evaluate polynomial
+    return x * x * (3.0 - 2.0 * x);
+}
+
+fn total_rayleigh(lambda: Vec3) -> Vec3 {
+    (8.0 * powf32(PI, 3.0)
+        * powf32(powf32(REFRACTIVE_INDEX, 2.0) - 1.0, 2.0)
+        * (6.0 + 3.0 * DEPOLARIZATION_FACTOR))
+        / (3.0 * NUM_MOLECULES * lambda.pow(4.0) * (6.0 - 7.0 * DEPOLARIZATION_FACTOR))
+}
+
+fn total_mie(lambda: Vec3, k: Vec3, t: f32) -> Vec3 {
+    let c = 0.2 * t * 10e-18;
+    0.434 * c * PI * ((2.0 * PI) / lambda).pow(MIE_V - 2.0) * k
+}
+
+fn rayleigh_phase(cos_theta: f32) -> f32 {
+    (3.0 / (16.0 * PI)) * (1.0 + powf32(cos_theta, 2.0))
+}
+
+fn henyey_greenstein_phase(cos_theta: f32, g: f32) -> f32 {
+    (1.0 / (4.0 * PI))
+        * ((1.0 - powf32(g, 2.0))
+            / powf32(1.0 - 2.0 * g * cos_theta + powf32(g, 2.0), 1.5))
+}
+
+fn sun_intensity(zenith_angle_cos: f32) -> f32 {
+    let cutoff_angle = PI / 1.95; // Earth shadow hack
+    SUN_INTENSITY_FACTOR
+        * 0.0f32.max(
+            1.0 - expf32(
+                -((cutoff_angle - acos_approx(zenith_angle_cos)) / SUN_INTENSITY_FALLOFF_STEEPNESS),
+            ),
+        )
+}
+
+fn uncharted2_tonemap(w: Vec3) -> Vec3 {
+    let a = Vec3::splat(0.15); // Shoulder strength
+    let b = Vec3::splat(0.50); // Linear strength
+    let c = Vec3::splat(0.10); // Linear angle
+    let d = Vec3::splat(0.20); // Toe strength
+    let e = Vec3::splat(0.02); // Toe numerator
+    let f = Vec3::splat(0.30); // Toe denominator
+
+    ((w * (a * w + c * b) + d * e) / (w * (a * w + b) + d * f)) - e / f
+}
+
+fn sky(dir: Vec3, sun_position: Vec3) -> Vec3 {
+    let up = Vec3::new(0.0, 1.0, 0.0);
+    let sunfade = 1.0 - clamp(1.0 - expf32(sun_position.1 / 450000.0), 0.0, 1.0);
+    let rayleigh_coefficient = RAYLEIGH - (1.0 * (1.0 - sunfade));
+    let beta_r = total_rayleigh(PRIMARIES) * rayleigh_coefficient;
+
+    // Mie coefficient
+    let beta_m = total_mie(PRIMARIES, MIE_K_COEFFICIENT, TURBIDITY) * MIE_COEFFICIENT;
+
+    // Optical length, cutoff angle at 90 to avoid singularity
+    let zenith_angle = acos_approx(up.dot(dir).max(0.0));
+    let denom = cosf32(zenith_angle)
+        + 0.15 * powf32(93.885 - ((zenith_angle * 180.0) / PI), -1.253);
+    let s_r = RAYLEIGH_ZENITH_LENGTH / denom;
+    let s_m = MIE_ZENITH_LENGTH / denom;
+
+    // Combined extinction factor
+    let fex = (-(beta_r * s_r + beta_m * s_m)).exp();
+
+    // In-scattering
+    let sun_direction = sun_position.normalize();
+    let cos_theta = dir.dot(sun_direction);
+    let beta_r_theta = beta_r * rayleigh_phase(cos_theta * 0.5 + 0.5);
+
+    let beta_m_theta = beta_m * henyey_greenstein_phase(cos_theta, MIE_DIRECTIONAL_G);
+    let sun_e = sun_intensity(sun_direction.dot(up));
+    let mut lin =
+        (sun_e * ((beta_r_theta + beta_m_theta) / (beta_r + beta_m)) * (Vec3::splat(1.0) - fex))
+            .pow(1.5);
+    lin *= Vec3::splat(1.0).lerp(
+        (sun_e * ((beta_r_theta + beta_m_theta) / (beta_r + beta_m)) * fex).pow(0.5),
+        clamp(powf32(1.0 - up.dot(sun_direction), 5.0), 0.0, 1.0),
+    );
+
+    // Composition + solar disc
+    let sun_angular_diameter_cos = cosf32(SUN_ANGULAR_DIAMETER_DEGREES);
+    let sundisk = smoothstep(
+        sun_angular_diameter_cos,
+        sun_angular_diameter_cos + 0.00002,
+        cos_theta,
+    );
+    let mut l0 = 0.1 * fex;
+    l0 += sun_e * 19000.0 * fex * sundisk;
+    let mut tex_color = lin + l0;
+    tex_color *= Vec3::splat(0.04);
+    tex_color += Vec3::new(0.0, 0.001, 0.0025) * 0.3;
+
+    // Tonemapping
+    let white_scale = 1.0 / uncharted2_tonemap(TONEMAP_WEIGHTING);
+    let curr =
+        uncharted2_tonemap((log2f32(2.0 / powf32(LUMINANCE, 4.0))) * tex_color);
+    let color = curr * white_scale;
+
+    color.pow(1.0 / (1.2 + (1.2 * sunfade)))
+}
 
-#[allow(unused_attributes)]
 #[spirv(entry = "fragment")]
 pub fn main_fs(input: Input<f32x4>, mut output: Output<f32x4>) {
-    output.store(input.load());
+    let color = input.load();
+    let mut dir = Vec3::new(color.0, color.1, 0.0);
+
+    // hard-code information because we can't bind buffers at the moment
+    let eye_pos = Vec3(0.0, 0.0997, 0.2);
+    let sun_pos = Vec3::new(0.0, 75.0, -1000.0);
+    let clip_to_world = Mat4 {
+        x_axis: Vec4(-0.5522849, 0.0, 0.0, 0.0),
+        y_axis: Vec4(0.0, 0.4096309, -0.061444636, 0.0),
+        z_axis: Vec4(0.0, 99.99999, 199.99998, 999.99994),
+        w_axis: Vec4(0.0, -0.14834046, -0.98893654, 0.0),
+    };
+
+    let cs_pos = Vec4(dir.0, -dir.1, 1.0, 1.0);
+    let mut ws_pos = clip_to_world.mul_vec4(cs_pos);
+    let ws_pos = Vec3(
+        ws_pos.0 / ws_pos.3,
+        ws_pos.1 / ws_pos.3,
+        ws_pos.2 / ws_pos.3,
+    );
+    let dir = (ws_pos - eye_pos).normalize();
+    let k = sky(dir, sun_pos);
+
+    output.store(f32x4(k.0, k.1, k.2, 0.0))
 }
 
-#[allow(unused_attributes)]
 #[spirv(entry = "vertex")]
 pub fn main_vs(
     in_pos: Input<f32x4>,
@@ -20,7 +186,7 @@ pub fn main_vs(
     mut out_color: Output<f32x4>,
 ) {
     out_pos.store(in_pos.load());
-    out_color.store(in_color.load());
+    out_color.store(in_pos.load());
 }
 
 #[panic_handler]

spirv-std/src/lib.rs

@@ -1,7 +1,11 @@
 #![no_std]
-#![feature(register_attr, repr_simd)]
+#![feature(register_attr, repr_simd, core_intrinsics)]
 #![register_attr(spirv)]
 
+pub mod math;
+pub use crate::math::builtin::*;
+pub use crate::math::*;
+
 macro_rules! pointer_addrspace_write {
     (false) => {};
     (true) => {