Processing Mode refers to py5’s ability to serve as a bridge from Java to Python, allowing Processing Sketches to call Python code using a new callPython() method. This is a solid feature that in time will add a significant amount of value to the Processing community.

Processing Mode is documented on py5's website. The first two examples are code samples from the online documentation.

Here is a quick example of a Processing Sketch that uses py5 in Processing Mode:

package test;

import processing.core.PImage;
import py5.core.SketchBase;

public class Example1Sketch extends SketchBase {

  public void settings() {
    size(400, 400, P2D);
  }

  public void setup() {
    String msg = "Hello from Java!";
    PImage img = createImage(200, 200, RGB);

    // call Python function `alter_image(msg, img)` and get back a PImage object
    PImage imgResponse = (PImage) callPython("test_transfer", msg, img);
    image(imgResponse, 100, 100);

    // call numpy `random.randint()` function
    long randomNumber = (long) callPython("np.random.randint", 0, 100);
    py5Println("JAVA: Random number from numpy: " + randomNumber);
  }

  public void draw() {
    rect(mouseX, mouseY, 20, 20);
  }

}

The callPython() method in this Sketch is added to Processing with py5's py5.core.SketchBase class. It can pass parameters and return objects back to Java.

The Python code used to execute the above example Sketch is the following:

import numpy as np

import py5_tools
import py5


def alter_image(msg: str, img: py5.Py5Image):
    py5.println("PYTHON:", msg)
    py5.println("PYTHON:", img)

    img.load_np_pixels()
    img.np_pixels[::2, ::2] = [255, 255, 0, 0]
    img.update_np_pixels()

    return img


# register processing mode keys so the Java `callPython()` method can find them
py5_tools.register_processing_mode_key('test_transfer', alter_image)
py5_tools.register_processing_mode_key('np', np)

# run the sketch in processing mode, specifying the Java class to instantiate
py5.run_sketch(jclassname='test.Example1Sketch')

The main idea is for py5 to provide Processing users with efficient access to the Python Ecosystem. Calls to callPython() in Java are linked to Python through registered "keys" that map to Python functions and modules. Refer to the online Processing Mode documentation to learn more.