function canvas() {
var c = document.getElementById("canvas");
var ctx = c.getContext("2d");
ctx.fillStyle = "rgb(0,128,0)";
ctx.fillRect(0, 0, 20, 20);
ctx.globalAlpha = 0; // any future drawing operations will be invisible
ctx.drawImage(document.getElementById('red.png'), 0, 0); // draw invisible red square over the green square
}