Generate normals from a height map

generate_normals.rs

// https://stackoverflow.com/questions/44120220/calculating-normals-on-terrain-mesh
fn generate_normals(
    height_map: &image::ImageBuffer<image::Luma<u16>, Vec<u16>>, 
    normal_map: &mut image::ImageBuffer<image::Rgb<u8>, Vec<u8>>)
{
    let (cols, rows) = height_map.dimensions();
    let up = cgmath::vec3(0.0, 0.0, 1.0);
    //let mut normals = vec![up; (cols * rows) as usize];
    // the `size_factor` reduces to `width / 1024` with this `cell_size`.
    let cell_size = 1.0 / (cols as f32 / 128.0);
    let size_factor = 1.0 / (8.0 * cell_size);
            
    for row in 0..rows {
        for col in 0..cols {
            if row == 0 || col == 0 || row == rows - 1 || col == cols - 1 {
                let normal = up;
                let r = ((1.0 + normal.x) * 0.5 * 255.0) as u8;
                let g = ((1.0 + normal.y) * 0.5 * 255.0) as u8;
                let b = ((1.0 + normal.z) * 0.5 * 255.0) as u8;
                let pixel = image::Rgb([r, g, b]);
                normal_map.put_pixel(col, row, pixel);
            } else {
                const U16_MAX: f32 = u16::MAX as f32;

                let nw = height_map.get_pixel(col - 1, row - 1)[0] as f32 / U16_MAX;
                let n = height_map.get_pixel(col, row - 1)[0] as f32 / U16_MAX;
                let ne = height_map.get_pixel(col + 1, row - 1)[0] as f32 / U16_MAX;
                let e = height_map.get_pixel(col + 1, row)[0] as f32 / U16_MAX;
                let se = height_map.get_pixel(col + 1, row + 1)[0] as f32 / U16_MAX;
                let s = height_map.get_pixel(col, row + 1)[0] as f32 / U16_MAX;
                let sw = height_map.get_pixel(col - 1, row + 1)[0] as f32 / U16_MAX;
                let w = height_map.get_pixel(col - 1, row)[0] as f32 / U16_MAX;

                let dydx = ((ne + 2.0 * e + se) - (nw + 2.0 * w + sw)) * size_factor;
                let dydz = ((sw + 2.0 * s + se) - (nw + 2.0 * n + ne)) * size_factor;

                let normal = cgmath::vec3(dydx, dydz, 1.0).normalize();
                //normals[(row * cols + col) as usize] = normal;
                let r = ((1.0 + normal.x) * 0.5 * 255.0) as u8;
                let g = ((1.0 + normal.y) * 0.5 * 255.0) as u8;
                let b = ((1.0 + normal.z) * 0.5 * 255.0) as u8;
                let pixel = image::Rgb([r, g, b]);
                normal_map.put_pixel(col, row, pixel);
            }
        }
    }
}