Sketcher.js v1.1

function Sketcher( canvasID, brushImage ) {
    this.renderFunction = (brushImage == null || brushImage == undefined) ? this.updateCanvasByLine : this.updateCanvasByBrush;
    this.brush = brushImage;
    this.touchSupported = Modernizr.touch;
    this.canvasID = canvasID;
    this.canvas = $("#"+canvasID);
    this.canvasOffset = null;
    this.context = this.canvas.get(0).getContext("2d");
    this.context.strokeStyle = "#000000";
    this.context.lineWidth = 3;
    this.lastMousePoint = {x:0, y:0};

    if (this.touchSupported) {
        this.mouseDownEvent = "touchstart";
        this.mouseMoveEvent = "touchmove";
        this.mouseUpEvent = "touchend";
    }
    else {
        this.mouseDownEvent = "mousedown";
        this.mouseMoveEvent = "mousemove";
        this.mouseUpEvent = "mouseup";
    }

    this.canvas.bind( this.mouseDownEvent, this.onCanvasMouseDown() );
}

Sketcher.prototype.onCanvasMouseDown = function () {
    var self = this;

    return function(event) {
        // Save canvas offset on mousedown event rather than calculate it on mousemove
        self.canvasOffset = self.canvas.offset();

        self.mouseMoveHandler = self.onCanvasMouseMove()
        self.mouseUpHandler = self.onCanvasMouseUp()

        $(document).bind( self.mouseMoveEvent, self.mouseMoveHandler );
        $(document).bind( self.mouseUpEvent, self.mouseUpHandler );

        self.updateMousePosition( event );
        self.renderFunction( event );
    }
}

Sketcher.prototype.onCanvasMouseMove = function () {
    var self = this;
    return function(event) {
        self.renderFunction( event );
        event.preventDefault();
        return false;
    }
}

Sketcher.prototype.onCanvasMouseUp = function (event) {
    var self = this;
    return function(event) {
        $(document).unbind( self.mouseMoveEvent, self.mouseMoveHandler );
        $(document).unbind( self.mouseUpEvent, self.mouseUpHandler );

        self.mouseMoveHandler = null;
        self.mouseUpHandler = null;
    }
}

Sketcher.prototype.updateMousePosition = function (event) {
    var target = this.touchSupported ? event.touches[0] : event;

    this.lastMousePoint.x = target.pageX - this.canvasOffset.left;
    this.lastMousePoint.y = target.pageY - this.canvasOffset.top;
}

Sketcher.prototype.updateCanvasByLine = function (event) {
    this.context.beginPath();
    this.context.moveTo( this.lastMousePoint.x, this.lastMousePoint.y );

    // Run the code of updateMousePosition within this function (at the expense of repeating code) for improved execution speed
    var target = this.touchSupported ? event.touches[0] : event;
    this.lastMousePoint.x = target.pageX - this.canvasOffset.left;
    this.lastMousePoint.y = target.pageY - this.canvasOffset.top;
    //this.updateMousePosition( event );

    this.context.lineTo( this.lastMousePoint.x, this.lastMousePoint.y );
    this.context.stroke();
}

Sketcher.prototype.updateCanvasByBrush = function (event) {
    var halfBrushW = this.brush.width/2;
    var halfBrushH = this.brush.height/2;

    var start = { x:this.lastMousePoint.x, y: this.lastMousePoint.y };
    this.updateMousePosition( event );
    var end = { x:this.lastMousePoint.x, y: this.lastMousePoint.y };

    var distance = parseInt( Trig.distanceBetween2Points( start, end ) );
    var angle = Trig.angleBetween2Points( start, end );

    var x,y;

    for ( var z=0; (z<=distance || z==0); z++ )
    {
        x = start.x + (Math.sin(angle) * z) - halfBrushW;
        y = start.y + (Math.cos(angle) * z) - halfBrushH;

        this.context.drawImage(this.brush, x, y);
    }
}

Sketcher.prototype.toData = function () {

    var dataString = this.canvas.get(0).toDataURL("image/png");
    var index = dataString.indexOf( "," )+1;
    dataString = dataString.substring( index );

    return dataString;
}

Sketcher.prototype.toDataURL = function () {

    var dataString = this.canvas.get(0).toDataURL("image/png");
    return dataString;
}

Sketcher.prototype.clear = function () {

    var c = this.canvas[0];
    this.context.clearRect( 0, 0, c.width, c.height );
}