<canvas width="600" height="400"></canvas>
<script>
var tela = document.querySelector('canvas');
var pincel = tela.getContext('2d');
pincel.fillStyle = 'grey';
pincel.fillRect(0, 0, 600, 400);
var boll = 10
function desenhaCirculo(evento) {
var x = evento.pageX - tela.offsetLeft;
var y = evento.pageY - tela.offsetTop;
var shift = evento.shiftKey;
console.log(x + ',' + y,shift);
if(evento.shiftKey){
boll = boll + 10
}
pincel.fillStyle = "blue"
pincel.beginPath();
pincel.arc(x, y, boll, 0, 2 * 3.14);
pincel.fill();
}
</script>
