如何增加Fabric对象的选择区域？

Question

Consider the following code:

var lineObj = new fabric.Line([120,200,320,200],{
  stroke: '#000000',
  strokeWidth: 1
});
canvas.add(lineObj);

This 1 pixel line is very hard to select owing to its very small width. What I wish to do here is to increase its selection area. Like this:

Is there any way in which I can accomplish this?

Answer 1

Yes, we have padding value for that.

var lineObj = new fabric.Line([120,200,320,200], {
  stroke: '#000000',
  strokeWidth: 1,
  padding: 20
});
canvas.add(lineObj);

Did I mention Fabric is flexible and powerful? :)

@markE Nice solution! Glad you're finding it easy to walk through source code.

Answer 2

Bad news--no solution in API / Good news--you can code your solution

The API doesn't provide a way to expand the boundingbox of a line, so there's no API way to get a bigger selection area for lines.

FabricJS is open-source and well-organized and the source code itself has useful comments. Here's what I found...

"Line" objects extend the "Object" object. The "Object" has helper methods and the most pertinent is in the file object_geometry.mixin.js. In there, I found that the boundingbox for any object is being generated using the method getBoundingRect().

You did ask "Is there ANY way...", so here's that way:

So to solve your issue, you must over-ride the getBoundingRect() for lines and make it slightly wider. This will automatically make the selection area of your lines wider and more easily clicked. @Kangax, feel free to indicate any simpler solution!

To get you started, here is the source for getBoundingRect() from the source file object_geometry.mixin.js:

getBoundingRect: function() {
  this.oCoords || this.setCoords();

  var xCoords = [this.oCoords.tl.x, this.oCoords.tr.x, this.oCoords.br.x, this.oCoords.bl.x];
  var minX = fabric.util.array.min(xCoords);
  var maxX = fabric.util.array.max(xCoords);
  var width = Math.abs(minX - maxX);

  var yCoords = [this.oCoords.tl.y, this.oCoords.tr.y, this.oCoords.br.y, this.oCoords.bl.y];
  var minY = fabric.util.array.min(yCoords);
  var maxY = fabric.util.array.max(yCoords);
  var height = Math.abs(minY - maxY);

  return {
    left: minX,
    top: minY,
    width: width,
    height: height
  };
},

/**
 * Returns width of an object
 * @method getWidth
 * @return {Number} width value
 */
getWidth: function() {
  return this.width * this.scaleX;
},

/**
 * Returns height of an object
 * @method getHeight
 * @return {Number} height value
 */
getHeight: function() {
  return this.height * this.scaleY;
},

/**
 * Makes sure the scale is valid and modifies it if necessary
 * @private
 * @method _constrainScale
 * @param {Number} value
 * @return {Number}
 */
_constrainScale: function(value) {
  if (Math.abs(value) < this.minScaleLimit) {
    if (value < 0)
      return -this.minScaleLimit;
    else
      return this.minScaleLimit;
  }

  return value;
}