canvas.clearRect

cavas.clearRect(x1, y1, x2, y2)

Make transparent a rectangle defined by the bounding area of x1,y2 to x2,y2.

Parameters:
    x1         The x position of the first corner
    y1         The y position of the first corner
    x2         The x position of the second corner
    y2         The y position of the second corner

Example:
// Fill a canvas with red and poke a transparent hole in the middle of it
function makeTransparentHole(name) {    
    var canvas = sb.getCanvas(name);    
    var holeWidth = canvas.width / 3;    
    var holeHeight = canvas.height / 3;    
    canvas.fill(0xff0000);    
    canvas.clearRect(holeWidth, holeHeight, 2*holeWidth, 2*holeHeight);
}