fn emphasize(p: &Pixel) -> Pixel {

    let emphasize_component = |c| {

        // emphasize

        let mut c = c as u32 * 4;

        // make sure it's visible

        if c > 0 {

            c += 128;

        }

        c.min(255) as u8

    };

    p.map(emphasize_component)

}

pub fn compare_surfaces(

    surf_a: &SharedImageSurface,

    surf_b: &SharedImageSurface,

) -> Result<BufferDiff, cairo::Error> {

    let a_width = surf_a.width();

    let a_height = surf_a.height();

    let b_width = surf_b.width();

    let b_height = surf_b.height();

    if a_width != b_width || a_height != b_height {

    }

    let mut surf_diff = cairo::ImageSurface::create(cairo::Format::ARgb32, a_width, a_height)?;

    let diff_stride = surf_diff.stride() as usize;

    let mut num_pixels_changed = 0;

    let mut max_diff = 0;

    let black = Pixel::default().with_alpha(255);

    {

        let mut diff_data = surf_diff.data().unwrap();

        for ((xa, ya, pixel_a), (_, _, pixel_b)) in Pixels::new(surf_a).zip(Pixels::new(surf_b)) {

            let dest = if pixel_a != pixel_b {

                num_pixels_changed += 1;

                let pixel_diff = pixel_a.diff(&pixel_b);

                max_diff = pixel_diff.iter().fold(max_diff, |acc, c| acc.max(c));

                let pixel_diff = emphasize(&pixel_diff);

                if pixel_diff.rgb() == RGB::default() {

                    let a = pixel_diff.a;

                    pixel_diff.map_colors(|_| a)

                    pixel_diff.with_alpha(255)

                black

            diff_data.set_pixel(diff_stride, dest, xa, ya);

    let surface = SharedImageSurface::wrap(surf_diff, SurfaceType::SRgb)?;

    Ok(BufferDiff::Diff(Diff {

        num_pixels_changed,

        max_diff,

        surface,

    }))

}