manipulating_pixels.md

Manipulating Pixels

We can manipulate each pixel in an image. To do this, we first initialize an ImageBuffer with the image width and height.

let mut buf = ImageBuffer::new(width, height);

Then we can access the pixel located at the position (i, j) by the array-like index.

buf[(i, j)]

Through the index, we can read and write the value of the corresponding pixel. Note that (0, 0) indicates the top-left corner of the image.

Here is the complete code that draws a gradient manually:

use image::{ImageBuffer, Rgb};

fn main() {
    let width = 1024;
    let height = 768;
    let mut buf = ImageBuffer::new(width, height);

    for i in 0..width {
        for j in 0..height {
            let value = (i + j) as f32 / (width + height) as f32;
            let value = (value * 255.) as u8;
            buf[(i, j)] = Rgb::from([value, value, value]);
        }
    }

    buf.save("image_buffer.jpg").unwrap();
}

image_buffer.jpg:

We can also load an image, draw it on ImageBuffer and manipulate its pixels.

use image::{imageops, DynamicImage, ImageBuffer, Rgba};

fn main() {
    let img = image::open("my_image.jpg").unwrap();

    let width = 1024;
    let height = 768;
    let mut buf = ImageBuffer::new(width, height);

    imageops::overlay(&mut buf, &img, 0, 0);
    
    for i in 0..width {
        for j in 0..height {
            if (i + j) % 2 == 0 {
                let value = (i + j) as f32 / (width + height) as f32;
                let value = (value * 255.) as u8;
                buf[(i, j)] = Rgba::from([value, value, value, 255]);
            }
        }
    }

    let buf = DynamicImage::from(buf);
    let buf = buf.into_rgb8();
    buf.save("image_buffer_overlay.jpg").unwrap();
}

image_buffer_overlay.jpg:

Note that, to call overlay on the images, we need the two images to have the same color type. So we use Rgba instead of Rgb for pixels, to match the color type of my_image.jpg.

In addition, to save the image as a JPEG file without any errors, we need to transform its color type appropriately.

let buf = DynamicImage::from(buf);
let buf = buf.into_rgb8();

We first transform the ImageBuffer to DynamicImage, and then transform it again to a DynamicImage with color type rgb8.

➡️ Next: Controlling JPEG Quality

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