Pixel manipulation in P5

lisasketch.js

var img;

function preload(){
  img = loadImage("lisa.png");
}
 
function setup(){
  createCanvas(500,382);
  image(img,0,0);
  loadPixels();
  for (var i = 0; i < (width*height); i++) {
    var index = i*4;
    pixels[index] = 255; // red
    pixels[index+1] = 255; // green
    pixels[index+2] = 255; // blue
    pixels[index+3] = 100; //alpha

    // Note: you don't have to change ALL of the RGBA values. 
    // You could only change alpha to make the image more transparent
    //    or change only the blue to make it more or less blue
    // Try commenting out some of the 4 lines for setting pixels and see what results you get.
  }
  updatePixels();
}