more experiments with canvas

gistfile1.js

// demo: http://jsfiddle.net/7wG3a/14/
var c = document.getElementById('canvas');
var ctx = c.getContext('2d');
var drawing = false, dx, dy;
var x = 100, y = 100, w = 100, h = 100;

c.onmousedown = function (e) {
  dx = e.clientX - x;
  dy = e.clientY - y;
  drawing = true;
}
    
c.onmousemove = function (e) {
  if (drawing) {
    ctx.clearRect(0, 0, 400, 400);
    x = e.clientX - dx;
    y = e.clientY - dy;
    draw(ctx, x, y, 100, 100);
  }
}
    
c.onmouseup = function (e) {
  drawing = false;
}

function draw(ctx, x, y, w, h) {
  ctx.save();
  ctx.fillStyle = "#ff0000";
  ctx.beginPath();
  ctx.translate(x, y);
  ctx.rotate(45 * Math.PI / 180);
  ctx.fillRect(-w/2, -h/2, w, h);
  ctx.restore();
}

ctx.clearRect(0, 0, 400, 400);
draw(ctx, x, y, w, h);