问题:

I am drawing polygon by capturing mouse clicks on canvas and then passing these points to fabric.Polygon. So, in this manner I'm drawing multiple polygons.

What I need know is, I want to get the mouse co-ordinates (pixel points on canvas) for the polygon which is selected now?

I have tried with:

canvas.getActiveObject().get('points');

But this is giving some negative and some positive values.

So, can u please tell me a way to find out the polygon points?

回答1:

Polygon points are relative to its center so you can get their "absolute" position like so:

var polygon = canvas.getActiveObject();

var polygonCenter = polygon.getCenterPoint();

var translatedPoints = polygon.get('points').map(function(p) {
  return { 
    x: polygonCenter.x + p.x, 
    y: polygonCenter.y + p.y
  };
});

Let's check how this looks:

translatedPoints.forEach(function(p) {
  canvas.getContext().strokeRect(p.x-5, p.y-5, 10, 10);
});

I think this will only work if polygon's angle is at 0 (otherwise need to "rotate" points coordinates as well).

回答2:

It looks like that from version 2.0 they changed the coordinates of the polygon. Before 2.0 points relative to the center of the polygon; after 2.0 they are absolute to the canvas;

Check out my response to the similar questions https://stackoverflow.com/a/53710375/4681279