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/** * @license * Copyright 2017 Google LLC * SPDX-License-Identifier: Apache-2.0 */ /** * @fileoverview Checks to see if the images used on the page are larger than * their display sizes. The audit will list all images that are larger than * their display size with DPR (a 1000px wide image displayed as a * 500px high-res image on a Retina display is 100% used); * However, the audit will only fail pages that use images that have waste * beyond a particular byte threshold. */ import {ByteEfficiencyAudit} from './byte-efficiency-audit.js'; import {NetworkRequest} from '../../lib/network-request.js'; import {ImageRecords} from '../../computed/image-records.js'; import UrlUtils from '../../lib/url-utils.js'; import * as i18n from '../../lib/i18n/i18n.js'; const UIStrings = { /** Imperative title of a Lighthouse audit that tells the user to resize images to match the display dimensions. This is displayed in a list of audit titles that Lighthouse generates. */ title: 'Properly size images', /** Description of a Lighthouse audit that tells the user *why* they need to serve appropriately sized images. This is displayed after a user expands the section to see more. No character length limits. The last sentence starting with 'Learn' becomes link text to additional documentation. */ description: 'Serve images that are appropriately-sized to save cellular data ' + 'and improve load time. ' + '[Learn how to size images](https://developer.chrome.com/docs/lighthouse/performance/uses-responsive-images/).', }; const str_ = i18n.createIcuMessageFn(import.meta.url, UIStrings); const IGNORE_THRESHOLD_IN_BYTES = 4096; // Ignore up to 12KB of waste if an effort was made with breakpoints. const IGNORE_THRESHOLD_IN_BYTES_BREAKPOINTS_PRESENT = 12288; class UsesResponsiveImages extends ByteEfficiencyAudit { /** * @return {LH.Audit.Meta} */ static get meta() { return { id: 'uses-responsive-images', title: str_(UIStrings.title), description: str_(UIStrings.description), scoreDisplayMode: ByteEfficiencyAudit.SCORING_MODES.METRIC_SAVINGS, guidanceLevel: 2, requiredArtifacts: ['ImageElements', 'ViewportDimensions', 'GatherContext', 'devtoolsLogs', 'traces', 'URL'], }; } /** * @param {LH.Artifacts.ImageElement & {naturalWidth: number, naturalHeight: number}} image * @param {LH.Artifacts.ViewportDimensions} ViewportDimensions * @return {{width: number, height: number}}; */ static getDisplayedDimensions(image, ViewportDimensions) { if (image.displayedWidth && image.displayedHeight) { return { width: image.displayedWidth * ViewportDimensions.devicePixelRatio, height: image.displayedHeight * ViewportDimensions.devicePixelRatio, }; } // If the image has 0 dimensions, it's probably hidden/offscreen, so we'll be as forgiving as possible // and assume it's the size of two viewports. See https://github.com/GoogleChrome/lighthouse/issues/7236 const viewportWidth = ViewportDimensions.innerWidth; const viewportHeight = ViewportDimensions.innerHeight * 2; const imageAspectRatio = image.naturalWidth / image.naturalHeight; const viewportAspectRatio = viewportWidth / viewportHeight; let usedViewportWidth = viewportWidth; let usedViewportHeight = viewportHeight; if (imageAspectRatio > viewportAspectRatio) { usedViewportHeight = viewportWidth / imageAspectRatio; } else { usedViewportWidth = viewportHeight * imageAspectRatio; } return { width: usedViewportWidth * ViewportDimensions.devicePixelRatio, height: usedViewportHeight * ViewportDimensions.devicePixelRatio, }; } /** * @param {LH.Artifacts.ImageElement & {naturalWidth: number, naturalHeight: number}} image * @param {LH.Artifacts.ViewportDimensions} ViewportDimensions * @param {Array} networkRecords * @return {null|LH.Audit.ByteEfficiencyItem}; */ static computeWaste(image, ViewportDimensions, networkRecords) { const networkRecord = networkRecords.find(record => record.url === image.src); // Nothing can be done without network info, ignore images without resource size information. if (!networkRecord) { return null; } const displayed = this.getDisplayedDimensions(image, ViewportDimensions); const usedPixels = displayed.width * displayed.height; const url = UrlUtils.elideDataURI(image.src); const actualPixels = image.naturalWidth * image.naturalHeight; const wastedRatio = 1 - (usedPixels / actualPixels); const totalBytes = NetworkRequest.getResourceSizeOnNetwork(networkRecord); const wastedBytes = Math.round(totalBytes * wastedRatio); return { node: ByteEfficiencyAudit.makeNodeItem(image.node), url, totalBytes, wastedBytes, wastedPercent: 100 * wastedRatio, }; } /** * @param {LH.Artifacts.ImageElement} image * @return {number}; */ static determineAllowableWaste(image) { if (image.srcset || image.isPicture) { return IGNORE_THRESHOLD_IN_BYTES_BREAKPOINTS_PRESENT; } return IGNORE_THRESHOLD_IN_BYTES; } /** * @param {LH.Artifacts} artifacts * @param {Array} networkRecords * @param {LH.Audit.Context} context * @return {Promise} */ static async audit_(artifacts, networkRecords, context) { const images = await ImageRecords.request({ ImageElements: artifacts.ImageElements, networkRecords, }, context); const ViewportDimensions = artifacts.ViewportDimensions; /** @type {Map} */ const resultsMap = new Map(); /** @type {Array} */ const passedImageList = []; for (const image of images) { // Give SVG a free pass because creating a "responsive" SVG is of questionable value. // Ignore CSS images because it's difficult to determine what is a spritesheet, // and the reward-to-effort ratio for responsive CSS images is quite low https://css-tricks.com/responsive-images-css/. if (image.mimeType === 'image/svg+xml' || image.isCss) { continue; } // Skip if we couldn't collect natural image size information. if (!image.naturalDimensions) continue; const naturalHeight = image.naturalDimensions.height; const naturalWidth = image.naturalDimensions.width; // If naturalHeight or naturalWidth are falsy, information is not valid, skip. if (!naturalWidth || !naturalHeight) continue; const processed = UsesResponsiveImages.computeWaste( {...image, naturalHeight, naturalWidth}, ViewportDimensions, networkRecords ); if (!processed) continue; // Verify the image wastes more than the minimum. const exceedsAllowableWaste = processed.wastedBytes > this.determineAllowableWaste(image); const existing = resultsMap.get(processed.url); // Don't warn about an image that was later used appropriately, or wastes a trivial amount of data. if (exceedsAllowableWaste && !passedImageList.includes(processed.url)) { if ((!existing || existing.wastedBytes > processed.wastedBytes)) { resultsMap.set(processed.url, processed); } } else { // Ensure this url passes for future tests. resultsMap.delete(processed.url); passedImageList.push(processed.url); } } const items = Array.from(resultsMap.values()); /** @type {LH.Audit.Details.Opportunity['headings']} */ const headings = [ {key: 'node', valueType: 'node', label: ''}, {key: 'url', valueType: 'url', label: str_(i18n.UIStrings.columnURL)}, {key: 'totalBytes', valueType: 'bytes', label: str_(i18n.UIStrings.columnResourceSize)}, {key: 'wastedBytes', valueType: 'bytes', label: str_(i18n.UIStrings.columnWastedBytes)}, ]; return { items, headings, }; } } export default UsesResponsiveImages; export {UIStrings, str_};

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