Error in colorspace conversion from RGBA to HSLA

[fgiordana]

Bevy version

commit 5e6400a

Operating system & version

macOS 12.2.1 on MBP 2021, Apple M1 Max

What you did

#[cfg(test)]
mod test {
    use bevy::prelude::*;

    const EPSILON: f32 = 0.0001;
    fn are_equal(a: &[f32; 4], b: &[f32; 4]) -> bool {
        for i in 0..4 {
            if (a[i] - b[i]).abs() > EPSILON {
                return false;
            }
        }
        true
    }

    #[test]
    fn test_hsl() {
        for r in 0..255 {
            for g in 0..255 {
                for b in 0..255 {
                    let c_rgba = Color::rgba(r as f32 / 255.0, g as f32 / 255.0, b as f32 / 255.0, 1.0);
                    let c_hsla = c_rgba.as_hsla();
                    if !are_equal(&(c_rgba.as_rgba_f32()), &(c_hsla.as_rgba_f32())) {
                        panic!("colors are different: {:?}, {:?}\n{:?}, {:?}", c_rgba, c_hsla, c_rgba.as_rgba_f32(), c_hsla.as_rgba_f32());
                    }
                }
            }
        }
    }
}

What you expected to happen

The test should be successful. There is an error in the conversion formula when the R and G components are the same

What actually happened

thread 'scene::test::test_hsl' panicked at 'colors are different: Rgba { red: 0.003921569, green: 0.003921569, blue: 0.0, alpha: 1.0 }, Hsla { hue: 240.0, saturation: 1.0, lightness: 0.0019607844, alpha: 1.0 }
[0.003921569, 0.003921569, 0.0, 1.0], [2.3283064e-10, 2.3283064e-10, 0.0039215684, 1.0]'

Additional information

There is an error in the conversion formula, it should actually be:

/// converts a color in sRGB space to HLS space
#[inline]
pub fn nonlinear_srgb_to_hsl([red, green, blue]: [f32; 3]) -> (f32, f32, f32) {
    // https://en.wikipedia.org/wiki/HSL_and_HSV#From_RGB
    let x_max = red.max(green.max(blue));
    let x_min = red.min(green.min(blue));
    let chroma = x_max - x_min;
    let lightness = (x_max + x_min) / 2.0;
    let hue = if chroma == 0.0 {
        0.0
    } else if red == x_max {
        60.0 * (green - blue) / chroma
    } else if green == x_max {
        60.0 * (2.0 + (blue - red) / chroma)
    } else {
        60.0 * (4.0 + (red - green) / chroma)
    };
    let hue = if hue < 0.0 { 360.0 + hue } else { hue };
    let saturation = if lightness <= 0.0 || lightness >= 1.0 {
        0.0
    } else {
        (x_max - lightness) / lightness.min(1.0 - lightness)
    };

    (hue, saturation, lightness)
}