pub fn rotate(settings: &Settings, degree: f32) -> RgbaImage {
    // angle in radians
    let angle = degree * PI / 180.0;

    // calculate the size of the image
    let (new_width, new_height) = {
        let (width, height) = (image.width() as f32, image.height() as f32);
        (
            (width * angle.cos().abs() + height * angle.sin().abs()) as u32,
            (height * angle.cos().abs() + width * angle.sin().abs()) as u32,
        )
    };

    let mut new_image = DynamicImage::new_rgba8(new_width, new_height).to_rgba();

    // we need to write our original image to the center of the new image. after this, we can rotate the new one around its center.
    let (offset_x, offset_y) = (new_width - image.width(), new_height - image.height());
    for x in 0..image.width() {
        for y in 0..image.height() {
            let pixel = image.get_pixel(x, y);
            if pixel.data[3] == 0 {
                continue;
            }
            let (target_x, target_y) = (x + offset_x / 2, y + offset_y / 2);
            new_image.put_pixel(target_x, target_y, *pixel);
        }
    }

    imageproc::affine::rotate_with_default(&new_image,
                                           (new_width as f32 / 2.0, new_height as f32 / 2.0),
                                           angle,
                                           Rgba([0, 0, 0, 0u8]),
                                           Interpolation::Nearest)
}