creating a canvas clip mask using compositing operations

canvas-clipmask-compositing.js

          ctx.save();
          ctx.beginPath();
          
          ctx.fillStyle = 'white';
          path.context(ctx)(landClipTopo);
          ctx.fill();
          
          ctx.globalCompositeOperation = 'source-in';
          ctx.fillStyle = 'red';
          ctx.fillRect(w/2-200, h/2-200, 200, 200);
          
          ctx.closePath();
          ctx.restore();