pub fn gray_bench_image(width: u32, height: u32) -> GrayImage {
    let mut image = GrayImage::new(width, height);
    for y in 0..image.height() {
        for x in 0..image.width() {
            let intensity = (x % 7 + y % 6) as u8;
            image.put_pixel(x, y, Luma([intensity]));
        }
    }
    black_box(image)
}
Gray image to use in benchmarks. This is neither noise nor
similar to natural images - it's just a convenience method
to produce an image that's not constant.
src/utils.rs