How to draw a rounded Rectangle on HTML Canvas?

Question

I found that there are only can fill rectangle, but no rounded corner one, how can I do that?

Answer 1

I needed to do the same thing and created a method to do it.

// Now you can just call
var ctx = document.getElementById("rounded-rect").getContext("2d");
// Draw using default border radius, 
// stroke it but no fill (function's default values)
roundRect(ctx, 5, 5, 50, 50);
// To change the color on the rectangle, just manipulate the context
ctx.strokeStyle = "rgb(255, 0, 0)";
ctx.fillStyle = "rgba(255, 255, 0, .5)";
roundRect(ctx, 100, 5, 100, 100, 20, true);
// Manipulate it again
ctx.strokeStyle = "#0f0";
ctx.fillStyle = "#ffffd";
// Different radii for each corner, others default to 0
roundRect(ctx, 300, 5, 200, 100, {
  tl: 50,
  br: 25
}, true);

/**
 * Draws a rounded rectangle using the current state of the canvas.
 * If you omit the last three params, it will draw a rectangle
 * outline with a 5 pixel border radius
 * @param {CanvasRenderingContext2D} ctx
 * @param {Number} x The top left x coordinate
 * @param {Number} y The top left y coordinate
 * @param {Number} width The width of the rectangle
 * @param {Number} height The height of the rectangle
 * @param {Number} [radius = 5] The corner radius; It can also be an object 
 *                 to specify different radii for corners
 * @param {Number} [radius.tl = 0] Top left
 * @param {Number} [radius.tr = 0] Top right
 * @param {Number} [radius.br = 0] Bottom right
 * @param {Number} [radius.bl = 0] Bottom left
 * @param {Boolean} [fill = false] Whether to fill the rectangle.
 * @param {Boolean} [stroke = true] Whether to stroke the rectangle.
 */
function roundRect(ctx, x, y, width, height, radius, fill, stroke) {
  if (typeof stroke == 'undefined') {
    stroke = true;
  }
  if (typeof radius === 'undefined') {
    radius = 5;
  }
  if (typeof radius === 'number') {
    radius = {tl: radius, tr: radius, br: radius, bl: radius};
  } else {
    var defaultRadius = {tl: 0, tr: 0, br: 0, bl: 0};
    for (var side in defaultRadius) {
      radius[side] = radius[side] || defaultRadius[side];
    }
  }
  ctx.beginPath();
  ctx.moveTo(x + radius.tl, y);
  ctx.lineTo(x + width - radius.tr, y);
  ctx.quadraticCurveTo(x + width, y, x + width, y + radius.tr);
  ctx.lineTo(x + width, y + height - radius.br);
  ctx.quadraticCurveTo(x + width, y + height, x + width - radius.br, y + height);
  ctx.lineTo(x + radius.bl, y + height);
  ctx.quadraticCurveTo(x, y + height, x, y + height - radius.bl);
  ctx.lineTo(x, y + radius.tl);
  ctx.quadraticCurveTo(x, y, x + radius.tl, y);
  ctx.closePath();
  if (fill) {
    ctx.fill();
  }
  if (stroke) {
    ctx.stroke();
  }

}

<canvas id="rounded-rect" width="500" height="200">
  <!-- Insert fallback content here -->
</canvas>

• Different radii per corner provided by Corgalore
• See http://js-bits.blogspot.com/2010/07/canvas-rounded-corner-rectangles.html for further explanation

Answer 2

The HTML5 canvas doesn't provide a method to draw a rectangle with rounded corners.

How about using the lineTo() and arc() methods?

You can also use the quadraticCurveTo() method instead of the arc() method.

Answer 3

Here's one I wrote... uses arcs instead of quadratic curves for better control over radius. Also, it leaves the stroking and filling up to you

    /* Canvas 2d context - roundRect
 *
 * Accepts 5 parameters, the start_x and start_y points, the end_x and end_y points, and the radius of the corners
 * 
 * No return value
 */

CanvasRenderingContext2D.prototype.roundRect = function(sx,sy,ex,ey,r) {
    var r2d = Math.PI/180;
    if( ( ex - sx ) - ( 2 * r ) < 0 ) { r = ( ( ex - sx ) / 2 ); } //ensure that the radius isn't too large for x
    if( ( ey - sy ) - ( 2 * r ) < 0 ) { r = ( ( ey - sy ) / 2 ); } //ensure that the radius isn't too large for y
    this.beginPath();
    this.moveTo(sx+r,sy);
    this.lineTo(ex-r,sy);
    this.arc(ex-r,sy+r,r,r2d*270,r2d*360,false);
    this.lineTo(ex,ey-r);
    this.arc(ex-r,ey-r,r,r2d*0,r2d*90,false);
    this.lineTo(sx+r,ey);
    this.arc(sx+r,ey-r,r,r2d*90,r2d*180,false);
    this.lineTo(sx,sy+r);
    this.arc(sx+r,sy+r,r,r2d*180,r2d*270,false);
    this.closePath();
}

Here is an example:

var _e = document.getElementById('#my_canvas');
var _cxt = _e.getContext("2d");
_cxt.roundRect(35,10,260,120,20);
_cxt.strokeStyle = "#000";
_cxt.stroke();

Answer 4

To make the function more consistent with the normal means of using a canvas context, the canvas context class can be extended to include a 'fillRoundedRect' method -- that can be called in the same way fillRect is called:

var canv = document.createElement("canvas");
var cctx = canv.getContext("2d");

// If thie canvasContext class doesn't have  a fillRoundedRect, extend it now
if (!cctx.constructor.prototype.fillRoundedRect) {
  // Extend the canvaseContext class with a fillRoundedRect method
  cctx.constructor.prototype.fillRoundedRect = 
    function (xx,yy, ww,hh, rad, fill, stroke) {
      if (typeof(rad) == "undefined") rad = 5;
      this.beginPath();
      this.moveTo(xx+rad, yy);
      this.arcTo(xx+ww, yy,    xx+ww, yy+hh, rad);
      this.arcTo(xx+ww, yy+hh, xx,    yy+hh, rad);
      this.arcTo(xx,    yy+hh, xx,    yy,    rad);
      this.arcTo(xx,    yy,    xx+ww, yy,    rad);
      if (stroke) this.stroke();  // Default to no stroke
      if (fill || typeof(fill)=="undefined") this.fill();  // Default to fill
  }; // end of fillRoundedRect method
} 

The code checks to see if the prototype for the constructor for the canvas context object contains a 'fillRoundedRect' property and adds one -- the first time around. It is invoked in the same manner as the fillRect method:

  ctx.fillStyle = "#eef";  ctx.strokeStyle = "#ddf";
  // ctx.fillRect(10,10, 200,100);
  ctx.fillRoundedRect(10,10, 200,100, 5);

The method uses the arcTo method as Grumdring did. In the method, this is a reference to the ctx object. The stroke argument defaults to false if undefined. The fill argument defaults to fill the rectangle if undefined.

(Tested on Firefox, I don't know if all implementations permit extension in this manner.)

Answer 5

I started with @jhoff's solution, but rewrote it to use width/height parameters, and using arcTo makes it quite a bit more terse:

CanvasRenderingContext2D.prototype.roundRect = function (x, y, w, h, r) {
  if (w < 2 * r) r = w / 2;
  if (h < 2 * r) r = h / 2;
  this.beginPath();
  this.moveTo(x+r, y);
  this.arcTo(x+w, y,   x+w, y+h, r);
  this.arcTo(x+w, y+h, x,   y+h, r);
  this.arcTo(x,   y+h, x,   y,   r);
  this.arcTo(x,   y,   x+w, y,   r);
  this.closePath();
  return this;
}

Also returning the context so you can chain a little. E.g.:

ctx.roundRect(35, 10, 225, 110, 20).stroke(); //or .fill() for a filled rect

Answer 6

The drawPolygon function below can be used to draw any polygon with rounded corners.

See it running here.

function drawPolygon(ctx, pts, radius) {
  if (radius > 0) {
    pts = getRoundedPoints(pts, radius);
  }
  var i, pt, len = pts.length;
  ctx.beginPath();
  for (i = 0; i < len; i++) {
    pt = pts[i];
    if (i == 0) {          
      ctx.moveTo(pt[0], pt[1]);
    } else {
      ctx.lineTo(pt[0], pt[1]);
    }
    if (radius > 0) {
      ctx.quadraticCurveTo(pt[2], pt[3], pt[4], pt[5]);
    }
  }
  ctx.closePath();
}

function getRoundedPoints(pts, radius) {
  var i1, i2, i3, p1, p2, p3, prevPt, nextPt,
      len = pts.length,
      res = new Array(len);
  for (i2 = 0; i2 < len; i2++) {
    i1 = i2-1;
    i3 = i2+1;
    if (i1 < 0) {
      i1 = len - 1;
    }
    if (i3 == len) {
      i3 = 0;
    }
    p1 = pts[i1];
    p2 = pts[i2];
    p3 = pts[i3];
    prevPt = getRoundedPoint(p1[0], p1[1], p2[0], p2[1], radius, false);
    nextPt = getRoundedPoint(p2[0], p2[1], p3[0], p3[1], radius, true);
    res[i2] = [prevPt[0], prevPt[1], p2[0], p2[1], nextPt[0], nextPt[1]];
  }
  return res;
};

function getRoundedPoint(x1, y1, x2, y2, radius, first) {
  var total = Math.sqrt(Math.pow(x2 - x1, 2) + Math.pow(y2 - y1, 2)),
      idx = first ? radius / total : (total - radius) / total;
  return [x1 + (idx * (x2 - x1)), y1 + (idx * (y2 - y1))];
};

The function receives an array with the polygon points, like this:

var canvas = document.getElementById("cv");
var ctx = canvas.getContext("2d");
ctx.strokeStyle = "#000000";
ctx.lineWidth = 5;

drawPolygon(ctx, [[20,   20],
                  [120,  20],
                  [120, 120],
                  [ 20, 120]], 10);
ctx.stroke();

This is a port and a more generic version of a solution posted here.