HTML5 Canvas Balloon

Today we’re going to draw a balloon on the HTML5 canvas element. Why not, right? Here’s the final result:

HTML5 Canvas Balloon

The balloon shape is basically a circle stretched out in various places. However, it’s not that straightforward to simply draw a circle and then stretch it with the canvas API. Instead, we need to recreate a circle using four cubic Bézier curves:

Circle Bezier Curve Count

After beginning this process, I discovered that it is actually impossible to create a perfect circle using cubic Bézier curves. Instead, you can only get a very close approximation using a constant called “kappa”. So to calculate the length of the “handles” (or the distance from a control point to a corresponding point on the curve), we use the following formula:

var KAPPA = (4 * (Math.sqrt(2) - 1))/3;
var handleLength = KAPPA * radius;

Now we can create a near-perfect circle using four, separate cubic Bézier curves, created with the bezierCurveTo method:

// Begin balloon path


// Top Left Curve
var topLeftCurveStartX = centerX - radius;
var topLeftCurveStartY = centerY;
var topLeftCurveEndX = centerX;
var topLeftCurveEndY = centerY - radius;
gfxContext.moveTo(topLeftCurveStartX, topLeftCurveStartY);
gfxContext.bezierCurveTo(topLeftCurveStartX, topLeftCurveStartY - handleLength,
			topLeftCurveEndX - handleLength, topLeftCurveEndY,
			topLeftCurveEndX, topLeftCurveEndY);
// Top Right Curve
var topRightCurveStartX = centerX;
var topRightCurveStartY = centerY - radius;
var topRightCurveEndX = centerX + radius;
var topRightCurveEndY = centerY;
gfxContext.bezierCurveTo(topRightCurveStartX + handleLength, topRightCurveStartY,
			topRightCurveEndX, topRightCurveEndY - handleLength,
			topRightCurveEndX, topRightCurveEndY);
// Bottom Right Curve
var bottomRightCurveStartX = centerX + radius;
var bottomRightCurveStartY = centerY;
var bottomRightCurveEndX = centerX;
var bottomRightCurveEndY = centerY + radius;
gfxContext.bezierCurveTo(bottomRightCurveStartX, bottomRightCurveStartY + handleLength,
			bottomRightCurveEndX + handleLength, bottomRightCurveEndY,
			bottomRightCurveEndX, bottomRightCurveEndY);							
// Bottom Left Curve
var bottomLeftCurveStartX = centerX;
var bottomLeftCurveStartY = centerY + radius;
var bottomLeftCurveEndX = centerX - radius;
var bottomLeftCurveEndY = centerY;
gfxContext.bezierCurveTo(bottomLeftCurveStartX - handleLength, bottomLeftCurveStartY,
			bottomLeftCurveEndX, bottomLeftCurveEndY + handleLength,
			bottomLeftCurveEndX, bottomLeftCurveEndY);

Once we have our circle in place, we can stretch out the curves to create the balloon shape. I just fiddled around and came out with arbitrary values for the factors used to stretch the shape. For example, I stretched out the bottom using a heightDiff value, calculated as follows:

var heightDiff = (radius * 0.4);

We can now modify the above circle equation to use these values:

var balloonBottomY = centerY + radius + heightDiff

// Updated Bottom Right Curve Value
var bottomRightCurveEndY = balloonBottomY;

// Updated Bottom Left Curve Value
var bottomLeftCurveStartY = baloonBottomY;

To add color to the balloon, I used a radial gradient with, again, some arbitrary values thrown in there:

var gradientOffset = (radius/3);
var balloonGradient =
	gfxContext.createRadialGradient(centerX + gradientOffset,
					centerY - gradientOffset, 3,
					centerX, centerY,
					radius + heightDiff);

	balloonGradient.addColorStop(0, this.lightColor.rgbString());
	balloonGradient.addColorStop(0.7, this.darkColor.rgbString());
	gfxContext.fillStyle = balloonGradient;

To calculate the gradient colors, I found this fantastic color manipulation library, that makes working with colors a snap:

this.baseColor = new Color(color);
this.darkColor = (new Color(color)).darken(0.3);
this.lightColor = (new Color(color)).lighten(0.3);

Check out the full script!