gistfile1.txt

function mouseGridPosition(e,
  {
    pixel,
    w,
    h
  }) {
  // w and h are the width and height of the canvas
  var { canvas, context: ctx } = getCanvasAndContext();

  var mX = e.offsetX;
  var mY = e.offsetY;

  var gridOffsetX = Math.floor( (mX / canvas.width) * pixel);
  var gridOffsetY = Math.floor( (mY / canvas.height) * pixel);
  var x = (gridOffsetX/pixel) * w;
  var y = (gridOffsetY/pixel) * h;

  var data = {
    left: x,
    top: y,
    right: x + w/pixel,
    bottom: y + h/pixel,
    canvasWidth: w,
    canvasHeight: h,
    pixel,
    canvas,
    context: ctx,
    width: w/pixel,
    height: h/pixel,
    rawMouseX: mX,
    rawMouseY: mY
  };
  data.centerX = data.left + (data.width / 2);
  data.centerY = data.top + (data.height / 2);

  return data;
}