使用Fabric JS canvas中心对象后的位置错误

Question

I have an image on a canvas using FabricJS, after I load the image I call canvas.centerObject(img). I have the same image scaled 2X, I am trying to imaplement a Magnify Glass functionality, everything works fine without centering the image, but as soon as I call canvas.centerObject(img) the bigger image doesn't show in the rigth position.

    fabric.Image.fromURL('http://fabricjs.com/assets/pug_small.jpg', function (img) {
        img1 = img;
    img.set({
        width: originalWidth,
        height: originalHeight,
        evented: true,
        selectable: false
    });

    lens = fabric.util.object.clone(img);
    lens.set({
        width: scale * originalWidth,
        height: scale * originalHeight,
        clipTo: function (ctx) {
            ctx.arc(-this.left + x - this.width / 2, -this.top + y - this.height / 2, radius, 0, Math.PI * 2, true);
        }
    });

    canvas.add(img, lens);
    canvas.centerObject(img);//*******issue here****
});

canvas.on('mouse:move', function (e) {
    x = e.e.layerX;
    y = e.e.layerY;
    lens.set('left', -(scale - 1) * x);
    lens.set('top', -(scale - 1) * y);
    canvas.renderAll();
});

Please check on this fiddle

Thanks.

Answer 1

I re-aligned the bigger image so it shows correctly when you call canvas.centerObject(img) . However, you might like to rethink your design a little to perhaps incorporate object scaling, scale() - depending on your implementation needs, of course.

Here's an updated JSFiddle and the tweaked code:

https://jsfiddle.net/rekrah/rfdxff5h/

  fabric.Image.fromURL('http://fabricjs.com/assets/pug_small.jpg', function(img) {
    img1 = img;
    img.set({
      width: originalWidth,
      height: originalHeight,
      evented: true,
      selectable: false
    });

    lens = fabric.util.object.clone(img);
    lens.set({
      top: -225,
      left: 150,
      width: scale * originalWidth,
      height: scale * originalHeight,
      clipTo: function(ctx) {
        ctx.arc(-this.left + x - this.width / 2, -this.top + y - this.height / 2, radius, 0, Math.PI * 2, true);
      }
    });

    canvas.add(img, lens);
    canvas.centerObject(img);//******* issue no longer here****
  });

  canvas.on('mouse:move', function(e) {
    x = e.e.layerX;
    y = e.e.layerY;
    if (x > 150 && x < 750) {
      lens.set('left', -(scale - 1) * x + 300);
      lens.set('top', -(scale - 1) * y);
    }
    canvas.renderAll();
  });