Thursday, January 15, 2009

ellipse and circle for canvas 2d context

These days I am working inside an ExtJS panel to create some sophisticated drawing stuff, and I discovered (too late ...) that there is almost nothing in canvas 2d context to draw circle or ellipse.

That's why I have extended Google excanvas library and/or native canvas 2d context prototype to let us create circles and ellipse in the same simple way we can create rectangles.

Here the extended proto:

// Andrea Giammarchi - Mit Style License
var extend = {
// Circle methods
circle:function(aX, aY, aDiameter){
this.ellipse(aX, aY, aDiameter, aDiameter);
fillCircle:function(aX, aY, aDiameter){
this.beginPath();, aY, aDiameter);
strokeCircle:function(aX, aY, aDiameter){
this.beginPath();, aY, aDiameter);
// Ellipse methods
ellipse:function(aX, aY, aWidth, aHeight){
var hB = (aWidth / 2) * .5522848,
vB = (aHeight / 2) * .5522848,
eX = aX + aWidth,
eY = aY + aHeight,
mX = aX + aWidth / 2,
mY = aY + aHeight / 2;
this.moveTo(aX, mY);
this.bezierCurveTo(aX, mY - vB, mX - hB, aY, mX, aY);
this.bezierCurveTo(mX + hB, aY, eX, mY - vB, eX, mY);
this.bezierCurveTo(eX, mY + vB, mX + hB, eY, mX, eY);
this.bezierCurveTo(mX - hB, eY, aX, mY + vB, aX, mY);
fillEllipse:function(aX, aY, aWidth, aHeight){
this.ellipse(aX, aY, aWidth, aHeight);
strokeEllipse:function(aX, aY, aWidth, aHeight){
this.ellipse(aX, aY, aWidth, aHeight);
for(var key in extend)
CanvasRenderingContext2D.prototype[key] = extend[key];
G_vmlCanvasManager = {init:function(){}, initElement:function(el){return el}};

The last part is to obtain the same behavior with any browser and runtime create canvas:

var myCanvas = G_vmlCanvasManager.initElement(document.createElement("canvas"));

SImple and efficient? I hope so, since I spent more than a couple of minutes to be able to create a perfect circle via 1/4 of a rectangle (square ... but how could I remember the bloody constant to do that?) :P

P.S. I remind you that the simplest function to create a circle via a center is canvas.arc(x, y, radius, 0, Math.PI*2); and that this proto create a circle inside a specified square but ithout the same arc performances ( anyway, runtime rendering in IE and my personal version of excanvas even faster than FireFox 3 ... anybody interested? :-) )

No comments:

Post a Comment