A Processing sketch to save JPG images with variable compression

JPEGCompression

/*
JPEG COMPRESSION
 Jeff Thompson | 2016 | www.jeffreythompson.org

 For web-based images, controlling the compression of a JPG
 can optimize dimensions and file size. The built-in save()
 command for Processing doesn't give us this control, but we
 can use Java's ImageIO to add this functionality.
 */

import java.awt.image.BufferedImage;
import javax.imageio.plugins.jpeg.*;
import javax.imageio.*;
import javax.imageio.stream.*;


void setup() {
  
  // set the amount of compression
  // 1.0 = no compression, largest image file
  // 0.5 = 50% compression
  // 0.0 = 100% compression, smallest image file
  float compressionLevel = 0.5;
  
  // image to compress and image to output
  String imageFile = sketchPath("") + "test.jpg";              
  String outputFilename = sketchPath("") + "compressed.jpg";

  try {

    // setup JPG output
    JPEGImageWriteParam jpegParams = new JPEGImageWriteParam(null);
    jpegParams.setCompressionMode(ImageWriteParam.MODE_EXPLICIT);
    jpegParams.setCompressionQuality(compressionLevel);
    final ImageWriter writer = ImageIO.getImageWritersByFormatName("jpg").next();
    writer.setOutput(new FileImageOutputStream(new File(outputFilename)));

    // load image to compress
    // alternatively, you could access the sketch's pixel array and save it
    PImage img = loadImage(imageFile);
    img.loadPixels();

    // output to BufferedImage object
    // create the object, then write the pixel data to it
    BufferedImage out = new BufferedImage(img.width, img.height, BufferedImage.TYPE_INT_RGB);
    for (int i=0; i<img.pixels.length; i++) {
      out.setRGB(i%img.width, i/img.width, img.pixels[i]);
    }

    // save it!
    writer.write(null, new IIOImage(out, null, null), jpegParams);
    println("Saved!");
  }
  catch (Exception e) {
    println("Problem saving... :(");
    println(e);
  }

  // all done
  exit();
}

Thanks for sharing this code. I've utilised it in an example that shows how to upload images with Processing to a webserver. See: https://www.kasperkamperman.com/blog/processing-save-image-to-web/

I've modified the loadImage part to get rid of the for-loop, that walks through all the pixels. I suppose the solution below could be faster (although I didn't test it).

Instead of the code from line 41 - 49 I've used:
BufferedImage in = (BufferedImage) img.getNative();
BufferedImage out = new BufferedImage(img.width, img.height, BufferedImage.TYPE_INT_RGB);
out.getGraphics().drawImage(in,0,0,null);

(based on this resource: https://blog.idrsolutions.com/2009/10/converting-java-bufferedimage-between-colorspaces/).