// draw the phone case on the canvas
context.drawImage(phonecase,0,0);

// set compositing to source-in
// future drawings will only be visible where existing pixels are opaque
context.globalCompositeOperation='source-in';

// draw the artwork over the phonecase
// compositing will display the artwork only inside the opaque phonecase pixels
context.drawImage(artCanvas,offsetX,offsetY);

// always clean up! Return compositing to its default mode
context.globalCompositeOperation='source-out';