html5 upload image, pick color, create image

gistfile1.js

// http://www.solidbg.com/beta/app.js

var file = document.getElementById('file'),
    canvas = document.getElementById('canvas'),
    preview = document.getElementById('preview'),
    ctx = canvas.getContext('2d'),
    ua = navigator.userAgent.toLowerCase(),
    isMobileSafari = (ua.indexOf('mobile') > -1 && ua.indexOf('safari') > -1);

file.onchange = readFile;

canvas.onmousemove = getPixel;
canvas.ontouchstart = getPixel;
canvas.ontouchmove = getPixel;

preview.onclick = goToColor;
preview.ontouchend = goToColor;

function goToColor() {
    var color = preview.style.backgroundColor,
        colorCanvas = document.createElement('canvas'),
        colorCtx = colorCanvas.getContext('2d');

    colorCanvas.width = screen.width;
    colorCanvas.height = screen.height;

    colorCtx.fillStyle = color;
    colorCtx.beginPath();
    colorCtx.rect (0, 0, colorCanvas.width, colorCanvas.height);
    colorCtx.fill();

    window.location = colorCanvas.toDataURL('image/png');
}

function readFile() {
    var reader = new FileReader();
    reader.onload = loadImage;
    reader.readAsDataURL(this.files[0]);
    preview.style.display = '';
}

function loadImage(e) {
    var img = new Image();
    img.onload = drawImage;
    img.src = e.target.result;
}

function drawImage() {
    var imgw = this.width,
        imgh = this.height,
        isWide = imgw > imgh,
        r = Math.min(imgw, imgh) / Math.max(imgw, imgh),
        cw = Math.min(imgw, 300),
        ch = isWide ? cw * r : cw * (1 - (r - 1));

    canvas.width = cw;
    canvas.height = ch;
    ctx.drawImage(this, 0, 0, imgw, imgh, 0, 0, cw, isMobileSafari ? ch * 4 : ch);
}

function getPixel(e) {
    var x = e.offsetX || e.touches[0].clientX - this.offsetLeft,
        y = e.offsetY || e.touches[0].clientY - this.offsetTop,
        pixel = ctx.getImageData(x, y, 1, 1).data;

    preview.style.backgroundColor = 'rgb(' + pixel[0] + ',' + pixel[1] + ',' + pixel[2] + ')';

    e.preventDefault();
    e.stopPropagation();
}