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var img;
var masked;

var mask;

function setup() {
    createCanvas(400, 400);

		// creates an invisible graphic buffer
		// that contains a rectangle
    img = createGraphics(200, 200);
    img.rect(0, 0, 100, 100);

		// creates an invisible graphic buffer
		// that contains a circle
    mask = createGraphics(200, 200);
    mask.ellipse(100, 100, 100, 100);

		// we apply the circle in 'mask' as a clipping mask
		// to the rectangle image contained in 'img'
		// we save the result in a third buffer: 'masked'
    ( masked = img.get() ).mask( mask.get() );
}

function draw() {
    background(130,0,10);

		// print the buffer after the clipping
    image(masked, 0, 0);
}

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