ff-preview 0.14.1

//! Unsafe-free helpers for the timeline presentation loop.

#![allow(clippy::cast_possible_truncation)]
#![allow(clippy::cast_sign_loss)]

/// Blend two packed-RGBA buffers: `dst[i] = (1 − alpha) · a[i] + alpha · b[i]`.
///
/// If `a` and `b` have different lengths, `dst` is set to a copy of `a`.
/// The alpha channel (byte index 3, 7, 11, …) is blended identically to the
/// colour channels so that transparency transitions work correctly.
pub(super) fn blend_rgba(a: &[u8], b: &[u8], alpha: f32, dst: &mut Vec<u8>) {
    if a.len() != b.len() {
        dst.resize(a.len(), 0);
        dst.copy_from_slice(a);
        return;
    }
    dst.resize(a.len(), 0);
    let inv = 1.0_f32 - alpha;
    for ((d, av), bv) in dst.iter_mut().zip(a.iter()).zip(b.iter()) {
        *d = (f32::from(*av) * inv + f32::from(*bv) * alpha) as u8;
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn blend_rgba_at_zero_alpha_should_return_a() {
        let a = vec![200u8, 100, 50, 255];
        let b = vec![0u8, 0, 0, 255];
        let mut dst = Vec::new();
        blend_rgba(&a, &b, 0.0, &mut dst);
        assert_eq!(dst, a);
    }

    #[test]
    fn blend_rgba_at_full_alpha_should_return_b() {
        let a = vec![0u8, 0, 0, 255];
        let b = vec![200u8, 100, 50, 255];
        let mut dst = Vec::new();
        blend_rgba(&a, &b, 1.0, &mut dst);
        assert_eq!(dst, b);
    }

    #[test]
    fn blend_rgba_at_half_alpha_should_average() {
        let a = vec![100u8, 200, 0, 255];
        let b = vec![200u8, 0, 100, 255];
        let mut dst = Vec::new();
        blend_rgba(&a, &b, 0.5, &mut dst);
        // (100 * 0.5 + 200 * 0.5) as u8 = 150
        assert_eq!(dst[0], 150);
        // (200 * 0.5 + 0 * 0.5) as u8 = 100
        assert_eq!(dst[1], 100);
    }

    #[test]
    fn blend_rgba_mismatched_lengths_should_copy_a() {
        let a = vec![1u8, 2, 3, 4];
        let b = vec![5u8, 6];
        let mut dst = Vec::new();
        blend_rgba(&a, &b, 0.5, &mut dst);
        assert_eq!(dst, a);
    }
}