Merge branch 'xacrimon/sgrbf' into 'master'

Faster sgrb saturation

See merge request veloren/veloren!508

common/src/util/mod.rs

@@ -2,6 +2,39 @@ pub const GIT_HASH: &str = include_str!(concat!(env!("OUT_DIR"), "/githash"));
 
 use vek::{Mat3, Rgb, Rgba, Vec3};
 
+/// This is a fast approximation of powf. This should only be used when minor accuracy loss is acceptable.
+#[inline(always)]
+#[allow(unsafe_code)]
+fn approx_powf(b: f32, e: f32) -> f32 {
+    unsafe {
+        let b = b as f64;
+        let e = e as f64;
+        union Swagger {
+            f: f64,
+            a: [i32; 2],
+        }
+        let mut b = Swagger { f: b };
+        b.a[1] = (e * (b.a[1] as f64 - 1072632447.0) + 1072632447.0) as i32;
+        b.a[0] = 0;
+        b.f as f32
+    }
+}
+
+#[cfg(test)]
+mod approx_powf_tests {
+    fn close_ei(a: f32, b: f32) -> bool {
+        (a - b < 1.0 && a - b > 0.0) || (b - a < 1.0 && b - a > 0.0)
+    }
+
+    #[test]
+    fn accuracy_1() {
+        let test_values: Vec<f32> = vec![3.0, 2.5, 1.5, 2.2];
+        test_values.windows(2).for_each(|a| {
+            assert!(close_ei(a[0].powf(a[1]), super::approx_powf(a[0], a[1])));
+        });
+    }
+}
+
 #[inline(always)]
 pub fn srgb_to_linear(col: Rgb<f32>) -> Rgb<f32> {
     #[inline(always)]
@@ -9,7 +42,7 @@ pub fn srgb_to_linear(col: Rgb<f32>) -> Rgb<f32> {
         if x <= 0.04045 {
             x / 12.92
         } else {
-            ((x + 0.055) / 1.055).powf(2.4)
+            approx_powf((x + 0.055) / 1.055, 2.4)
         }
     }
     col.map(to_linear)
@@ -21,7 +54,7 @@ pub fn linear_to_srgb(col: Rgb<f32>) -> Rgb<f32> {
         if x <= 0.0031308 {
             x * 12.92
         } else {
-            x.powf(1.0 / 2.4) * 1.055 - 0.055
+            approx_powf(x, 1.0 / 2.4) * 1.055 - 0.055
         }
     }
     col.map(to_srgb)
@@ -118,6 +151,7 @@ pub fn xyy_to_rgb(xyy: Vec3<f32>) -> Rgb<f32> {
     )
 }
 
+// TO-DO: speed this up
 #[inline(always)]
 pub fn saturate_srgb(col: Rgb<f32>, value: f32) -> Rgb<f32> {
     let mut hsv = rgb_to_hsv(srgb_to_linear(col));