Function window_luminance_stats

tools/bloom-diff/src/main.rs:192–230

Mean and variance of luminance in an N×N window plus the covariance between reference and candidate. Returned as f32s.

(
    reference: &[[f32; 3]],
    candidate: &[[f32; 3]],
    width: usize,
    x0: usize,
    y0: usize,
    size: usize,
)

/// Mean and variance of luminance in an N×N window plus the
/// covariance between reference and candidate. Returned as f32s.
fn window_luminance_stats(
    reference: &[[f32; 3]],
    candidate: &[[f32; 3]],
    width: usize,
    x0: usize,
    y0: usize,
    size: usize,
) -> (f32, f32, f32, f32, f32) {
    let mut sum_r = 0f32;
    let mut sum_c = 0f32;
    for yy in 0..size {
        for xx in 0..size {
            let i = (y0 + yy) * width + (x0 + xx);
            sum_r += luminance(reference[i]);
            sum_c += luminance(candidate[i]);
        }
    }
    let n = (size * size) as f32;
    let mean_r = sum_r / n;
    let mean_c = sum_c / n;

    let mut var_r = 0f32;
    let mut var_c = 0f32;
    let mut cov = 0f32;
    for yy in 0..size {
        for xx in 0..size {
            let i = (y0 + yy) * width + (x0 + xx);
            let dr = luminance(reference[i]) - mean_r;
            let dc = luminance(candidate[i]) - mean_c;
            var_r += dr * dr;
            var_c += dc * dc;
            cov += dr * dc;
        }
    }
    var_r /= n;
    var_c /= n;
    cov /= n;
    (mean_r, mean_c, var_r, var_c, cov)
}

// ============================================================
// Heatmap + composite output