How to use the assetgraph.assets function in assetgraph

.parEach(function (imageInfo, i) {
                var processedAsset = new AssetGraph.assets.Png({
                    rawSrc: this.vars[i]
                });
                processedAsset.url = urlTools.resolveUrl(assetGraph.root, processedAsset.id + processedAsset.defaultExtension);
                assetGraph.addAsset(processedAsset);
                imageInfo.incomingRelations.forEach(function (incomingRelation) {
                    var style = incomingRelation.cssRule.style;
                    style.removeProperty(postProcessPropertyName);
                    if (imageInfo.ie6) {
                        // Designates that the processed image should only be used in IE6
                        // Keep the original relation and use the underscore hack for getting
                        // IE6 to fetch the processed version:
                        if (('_' + incomingRelation.propertyName) in style) {
                            throw new Error("transforms.postProcessCssImages: Underscore hack already in use in Css rule");
                        }
                        style.setProperty('_' + incomingRelation.propertyName, 'url(...)', style.getPropertyPriority(incomingRelation.propertyName));
                        var relation = new AssetGraph.relations.CssImage({

var childProcess = require('child_process'),
    _ = require('underscore'),
    seq = require('seq'),
    urlTools = require('assetgraph/lib/util/urlTools'),
    pipeImageThroughChildProcessAndBuffer = require('../pipeImageThroughChildProcessAndBuffer'),
    passError = require('passerror'),
    AssetGraph = require('assetgraph'),
    postProcessPropertyName = AssetGraph.assets.Css.vendorPrefix + 'image-postprocess';

function applyFilters(buffer, filters, cb) {
    if (filters.length === 0) {
        return process.nextTick(function () {
            cb(null, buffer);
        });
    }

    var nextFilterNum = 0;
    function proceed () {
        if (nextFilterNum < filters.length) {
            var nextFilter = filters[nextFilterNum];
            nextFilterNum += 1;
            nextFilter(buffer, passError(cb, function (resultBuffer) {
                buffer = resultBuffer;
                proceed();