All shapes drawn to the screen have a position that is specified as a coordinate. All coordinates are measured as the distance from the origin in units of pixels. The origin [0, 0] is the coordinate is in the upper left of the window and the coordinate in the lower right is [width-1, height-1].
Original Processing.org Example: Coordinates
// All Examples Written by Casey Reas and Ben Fry // unless otherwise stated. // Sets the screen to be 200, 200, so the width of the window is 200 pixels // and the height of the window is 200 pixels size(200, 200); background(0); noFill(); stroke(255); // The two parameters of the point() method each specify coordinates. // This call to point() draws at the position [100, 100] point(width/2, height/2); // Draws to the position [100, 50] point(width/2, height/4); // It is also possible to specify a point with any parameter, // but only coordinates on the screen are visible point(60, 30); point(60, 134); point(160, 50); point(280, -800); point(201, 100); // Coordinates are used for drawing all shapes, not just points. // Parameters for different methods are used for different purposes. // For example, the first two parameters to line() specify the coordinates of the // first point and the second two parameters specify the second point stroke(204); line(0, 73, width, 73); // The first two parameters to rect() are coordinates // and the second two are the width and height rect(110, 55, 40, 36);