javascript - Drawing circle/ellipse on HTML5 canvas using mouse events

Question

I want something like ellipse option in paint for drawing on my canvas. I have achieved this partially. The problem is I am not able to get radius of circle; currently I have hard coded it to 15. Also I want to draw an ellipse (like in paint) not exact circle.

This is my code for drawing circle on canvas using mouse events. Please help me with code to achieve my above mentioned requirements.

function tool_circle() {
    var tool = this;
    this.started = false;

    this.mousedown = function (ev) {
        tool.started = true;
        tool.x0 = ev._x;
        tool.y0 = ev._y;
    };

    this.mousemove = function (ev) {
        if (!tool.started) {
            return;
        }

        context.fillStyle = 'red';

        var distance = Math.sqrt(Math.pow(tool.x0 - ev._x, 2) + Math.pow(tool.y0 - ev._y));
        context.beginPath();
    
        context.arc(tool.x0, tool.y0,15, 0, Math.PI * 2, false);
        context.stroke();
        context.fill();
    };

    this.mouseup = function (ev) {
        if (tool.started) {
            tool.mousemove(ev);
            tool.started = false;
            img_update();
        }
    };
}

Answer 1

I would something similar as with markE's answer however, using Bezier curve will draw ellipses but it won't give you the exact radius that you probably would need.

For that a function to draw a manual ellipse is needed, and it's rather simple -

This function will take a corner start point and and end point and draw an ellipse exactly within that boundary:

Live demo

function drawEllipse(x1, y1, x2, y2) {

    var radiusX = (x2 - x1) * 0.5,   /// radius for x based on input
        radiusY = (y2 - y1) * 0.5,   /// radius for y based on input
        centerX = x1 + radiusX,      /// calc center
        centerY = y1 + radiusY,
        step = 0.01,                 /// resolution of ellipse
        a = step,                    /// counter
        pi2 = Math.PI * 2 - step;    /// end angle
    
    /// start a new path
    ctx.beginPath();

    /// set start point at angle 0
    ctx.moveTo(centerX + radiusX * Math.cos(0),
               centerY + radiusY * Math.sin(0));

    /// create the ellipse    
    for(; a < pi2; a += step) {
        ctx.lineTo(centerX + radiusX * Math.cos(a),
                   centerY + radiusY * Math.sin(a));
    }
    
    /// close it and stroke it for demo
    ctx.closePath();
    ctx.strokeStyle = '#000';
    ctx.stroke();
}

The demo marks the rectangle area too to show that the ellipse is exactly within it.

Draw

To handle mouse operation that will let you draw the ellipse you can do:

var canvas = document.getElementById('myCanvas'),
    ctx = canvas.getContext('2d'),
    w = canvas.width,
    h = canvas.height,
    x1,                 /// start points
    y1,
    isDown = false;     /// if mouse button is down

/// handle mouse down    
canvas.onmousedown = function(e) {

    /// get corrected mouse position and store as first point
    var rect = canvas.getBoundingClientRect();
    x1 = e.clientX - rect.left;
    y1 = e.clientY - rect.top;
    isDown = true;
}

/// clear isDown flag to stop drawing
canvas.onmouseup = function() {
    isDown = false;
}

/// draw ellipse from start point
canvas.onmousemove = function(e) {

    if (!isDown) return;
    
    var rect = canvas.getBoundingClientRect(),
        x2 = e.clientX - rect.left,
        y2 = e.clientY - rect.top;
    
    /// clear canvas
    ctx.clearRect(0, 0, w, h);

    /// draw ellipse
    drawEllipse(x1, y1, x2, y2);
}

A tip can be to create a top canvas on top of your main canvas and do the drawing itself there. When mouse button is released then transfer the drawing to your main canvas. This way you don't have to redraw everything when drawing a new shape.

Hope this helps!