index.js

/*global Canvas, define*/
(function () {
    var Canvas,
        getCanvasImageData = function (imageBuffer, callback) {
            var canvasImage = new Canvas.Image();

            canvasImage.onerror = callback;
            canvasImage.onload = function () {
                var context = new Canvas(canvasImage.width, canvasImage.height).getContext('2d');
                context.drawImage(canvasImage, 0, 0);
                callback(null, context.getImageData(0, 0, canvasImage.width, canvasImage.height).data);
            };
            canvasImage.src = imageBuffer;
        },
        histogram = function (imageBuffer, callback) {
            getCanvasImageData(imageBuffer, function (err, data) {
                if (err) {
                    return callback(err);
                }

                var hist = {
                        red: new Array(256),
                        green: new Array(256),
                        blue: new Array(256),
                        alpha: new Array(256),

                        colors: {
                            rgb: 0,
                            rgba: 0
                        },

                        palettes: {
                            rgb: [],
                            rgba: []
                        },

                        greyscale: true,
                        alphachannel: false
                    },
                    red,
                    green,
                    blue,
                    alpha,
                    hexmap = {},
                    hexamap = {},
                    i;

                for (i = 0; i < 256; i += 1) {
                    hist.red[i] =
                    hist.green[i] =
                    hist.blue[i] =
                    hist.alpha[i] = 0;
                }

                for (i = 0; i < data.length; i += 4) {
                    red   = data[i];
                    green = data[i + 1];
                    blue  = data[i + 2];
                    alpha = data[i + 3];

                    if (alpha < 255) {
                        hist.alphachannel = true;
                    }

                    if (hist.greyscale && red !== green || red !== blue) {
                        hist.greyscale = false;
                    }

                    hist.red[red]     += 1;
                    hist.green[green] += 1;
                    hist.blue[blue]   += 1;
                    hist.alpha[alpha] += 1;

                    var hexaString = (red * 0x1000000 + green * 0x10000 + blue * 0x100 + alpha).toString(16),
                        hexa  = '#' + ('0000000'.substr(0, 8 - hexaString.length)) + hexaString,
                        hex = hexa.substr(0, 7);

                    if (!(hex in hexmap)) {
                        hexmap[hex] = 1;
                        hist.palettes.rgb.push(hex);
                        hist.colors.rgb += 1;
                    } else {
                        hexmap[hex] += 1;
                    }

                    if (!(hexa in hexamap)) {
                        hexamap[hexa] = 1;
                        hist.palettes.rgba.push(hexa);
                        hist.colors.rgba += 1;
                    } else {
                        hexamap[hexa] += 1;
                    }

                }

                callback(null, hist);
            });
        };

    if (typeof window === 'undefined') {
        Canvas = require('canvas');
    } else {
        // Polyfill canvas constructor
        Canvas = function (width, height) {
            var canvas = document.createElement('canvas');
            canvas.width = width;
            canvas.height = height;

            return canvas;
        };
        Canvas.Image = Image;
    }
    if (typeof module !== 'undefined') {
        module.exports = histogram;
    } else if (typeof define === 'function') {
        // AMD module
        define([], function () {
            return histogram;
        });
    } else if (window) {
        // Fall back to installing histogram in window scope
        window.histogram = histogram;
    }
}());