jhdevuk jhdevuk - 4 years ago 224
Javascript Question

Fabric.js - Free draw a rectangle

I have the following which doesn't work correctly:

var canvas = new fabric.Canvas('canvas');


canvas.observe('mouse:down', function(e) { mousedown(e); });
canvas.observe('mouse:move', function(e) { mousemove(e); });
canvas.observe('mouse:up', function(e) { mouseup(e); });


var started = false;


var x = 0;
var y = 0;


/* Mousedown */
function mousedown(e) {

var mouse = canvas.getPointer(e.memo.e);

started = true;

x = mouse.x;
y = mouse.y;

var square = new fabric.Rect({

width: 1,
height: 1,
left: mouse.x,
top: mouse.y,
fill: '#000'

});


canvas.add(square);
canvas.renderAll();
canvas.setActiveObject(square);

}


/* Mousemove */
function mousemove(e) {

if(!started) {

return false;

}

var mouse = canvas.getPointer(e.memo.e);

var x = Math.min(mouse.x, x),
y = Math.min(mouse.y, y),
w = Math.abs(mouse.x - x),
h = Math.abs(mouse.y - y);

if (!w || !h) {

return false;

}

var square = canvas.getActiveObject();

square.set('top', y).set('left', x).set('width', w).set('height', h);

canvas.renderAll();

}


/* Mouseup */
function mouseup(e) {

if(started) {

started = false;

}

}


The above logic is from a simple rectangle drawing system I used without fabric.js so I know it works, just not with fabric.js.

It seems the maths is off or I'm setting the incorrect params with the width/height/x/y values, as when you draw the rectangle does not follow the cursor correctly.

Any help is much appreciated, thanks in advance :)

Answer Source

Looks like Fabric.js calculates everything from the origin. So, 'Top' and 'Left' are a bit misleading. Check the following link: Canvas Coordinates Have Offset. Also, I've changed a bit of your code:

var canvas = new fabric.Canvas('canvas');
canvas.observe('mouse:down', function(e) { mousedown(e); });
canvas.observe('mouse:move', function(e) { mousemove(e); });
canvas.observe('mouse:up', function(e) { mouseup(e); });

var started = false;
var x = 0;
var y = 0;

/* Mousedown */
function mousedown(e) {
    var mouse = canvas.getPointer(e.memo.e);
    started = true;
    x = mouse.x;
    y = mouse.y;

    var square = new fabric.Rect({ 
        width: 0, 
        height: 0, 
        left: x, 
        top: y, 
        fill: '#000'
    });

    canvas.add(square); 
    canvas.renderAll();
    canvas.setActiveObject(square); 

}


/* Mousemove */
function mousemove(e) {
    if(!started) {
        return false;
    }

    var mouse = canvas.getPointer(e.memo.e);

    var w = Math.abs(mouse.x - x),
    h = Math.abs(mouse.y - y);

    if (!w || !h) {
        return false;
    }

    var square = canvas.getActiveObject(); 
    square.set('width', w).set('height', h);
    canvas.renderAll(); 
}

/* Mouseup */
function mouseup(e) {
    if(started) {
        started = false;
    }

    var square = canvas.getActiveObject();

    canvas.add(square); 
    canvas.renderAll();
 } 
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