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