Heatmap based on mouse movement

gistfile1.js

/* 
Copyright (c) 2010, Patrick Wied. All rights reserved. 
Code licensed under the BSD License: 
http://patrick-wied.at/static/license.txt 
*/  
var heatmapApp = (function(){  
    // var definition  
    // canvas: the canvas element  
    // ctx: the canvas 2d context  
    // width: the heatmap width for border calculations  
    // height: the heatmap height for border calculations  
    // invoke: the app doesn't react on the mouse events unless the invoke var is set to true  
    var canvas,  
        ctx,  
        width,  
        height,  
        radius1 = 20,  
        radius2 = 40,  
        invoke = false,  
    // function for coloring the heatmap  
    colorize = function(x,y,x2){  
        // initial check if x and y is outside the app  
        // -> resetting values  
        if(x+x2>width)  
            x=width-x2;  
        if(x<0)  
            x=0;  
        if(y<0)  
            y=0;  
        if(y+x2>height)  
            y=height-x2;  
        // get the image data for the mouse movement area  
        var image = ctx.getImageData(x,y,x2,x2),  
        // some performance tweaks  
            imageData = image.data,  
            length = imageData.length;  
        // loop thru the area  
        for(var i=3; i < length; i+=4){  
              
            var r = 0,  
                g = 0,  
                b = 0,  
                tmp = 0,  
                // [0] -> r, [1] -> g, [2] -> b, [3] -> alpha  
                alpha = imageData[i];  
                  
            // coloring depending on the current alpha value  
            if(alpha<=255 && alpha >= 235){  
                tmp=255-alpha;  
                r=255-tmp;  
                g=tmp*12;  
            }else if(alpha<=234 && alpha >= 200){  
                tmp=234-alpha;  
                r=255-(tmp*8);  
                g=255;  
            }else if(alpha<= 199 && alpha >= 150){  
                tmp=199-alpha;  
                g=255;  
                b=tmp*5;  
            }else if(alpha<= 149 && alpha >= 100){  
                tmp=149-alpha;  
                g=255-(tmp*5);  
                b=255;  
            }else  
                b=255;  
            // we ve started with i=3  
            // set the new r, g and b values  
            imageData[i-3]=r;  
            imageData[i-2]=g;  
            imageData[i-1]=b;  
        }  
        // the rgb data manipulation didn't affect the ImageData object(defined on the top)  
        // after the manipulation process we have to set the manipulated data to the ImageData object  
        image.data = imageData;  
        ctx.putImageData(image,x,y);      
    },  
    // this handler is listening to the mouse movement of the user  
    mouseMoveHandler = function(ev){  
        // if the invoke variable is set to true -> do the alphamap manipulation  
        if(invoke){  
            // at first we have to get the x and y values of the user's mouse position  
            var x, y;  
            if (ev.layerX) { // Firefox  
                x = ev.layerX;  
                y = ev.layerY;  
            } else if (ev.offsetX) { // Opera  
                x = ev.offsetX;  
                y = ev.offsetY;  
            }  
            if(typeof(x)=='undefined')  
                return;  
              
            // storing the variables because they will be often used  
            var r1 = radius1;  
            var r2 = radius2;  
              
            //console.log("x: "+x+"; y:" +y);  
            // create a radial gradient with the defined parameters. we want to draw an alphamap  
            var rgr = ctx.createRadialGradient(x,y,r1,x,y,r2);  
            // the center of the radial gradient has .1 alpha value  
            rgr.addColorStop(0, 'rgba(0,0,0,0.1)');    
            // and it fades out to 0  
            rgr.addColorStop(1, 'rgba(0,0,0,0)');  
            // drawing the gradient  
            ctx.fillStyle = rgr;    
            ctx.fillRect(x-r2,y-r2,2*r2,2*r2);  
            // negate the invoke variable   
            // next execution of the logic is when the activate method activates the invoke var again  
            invoke=!invoke;  
            // at least colorize the area  
            colorize(x-r2,y-r2,2*r2);  
        }  
    };  
      
    return {  
        // initialization  
        initialize: function(c, wt, ht){  
            canvas = document.getElementById(c);  
            canvas.width = wt;  
            canvas.height = ht;  
            canvas.style.border = "2px solid black";  
            ctx = canvas.getContext("2d");  
            width = wt;  
            height = ht;  
            canvas["onmousemove"] =  function(ev){ mouseMoveHandler(ev); };  
        },  
        // activating the logic  
        activate: function(){  
            invoke = !invoke;  
        }  
    };  
}());  
window.onload=function(){  
    //call the initialization  
    heatmapApp.initialize("c",300,900);  
    // and call the activate function in an interval of 50ms  
    setInterval("heatmapApp.activate();",50);  
}