examples/colorwheel.js

var page = require('webpage').create();
page.viewportSize = { width: 400, height : 400 };
page.content = '<html><body><canvas id="surface"></canvas></body></html>';
page.evaluate(function() {
    var el = document.getElementById('surface'),
        context = el.getContext('2d'),
        width = window.innerWidth,
        height = window.innerHeight,
        cx = width / 2,
        cy = height / 2,
        radius = width  / 2.3,
        imageData,
        pixels,
        hue, sat, value,
        i = 0, x, y, rx, ry, d,
        f, g, p, u, v, w, rgb;

    el.width = width;
    el.height = height;
    imageData = context.createImageData(width, height);
    pixels = imageData.data;

    for (y = 0; y < height; y = y + 1) {
        for (x = 0; x < width; x = x + 1, i = i + 4) {
            rx = x - cx;
            ry = y - cy;
            d = rx * rx + ry * ry;
            if (d < radius * radius) {
                hue = 6 * (Math.atan2(ry, rx) + Math.PI) / (2 * Math.PI);
                sat = Math.sqrt(d) / radius;
                g = Math.floor(hue);
                f = hue - g;
                u = 255 * (1 - sat);
                v = 255 * (1 - sat * f);
                w = 255 * (1 - sat * (1 - f));
                pixels[i] = [255, v, u, u, w, 255, 255][g];
                pixels[i + 1] = [w, 255, 255, v, u, u, w][g];
                pixels[i + 2] = [u, u, w, 255, 255, v, u][g];
                pixels[i + 3] = 255;
            }
        }
    }

    context.putImageData(imageData, 0, 0);
    document.body.style.backgroundColor = 'white';
    document.body.style.margin = '0px';
});

page.render('colorwheel.png');

phantom.exit();