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• Our latest improvement to QA : Screenshot Testing

  2 octobre 2013, par benaka — Development

  Introduction to QA in Piwik

  Like any piece of good software, Piwik comes with a comprehensive QA suite that includes unit and integration tests. The unit tests make sure core components of Piwik work properly. The integration tests make sure Piwik’s tracking and report aggregation and APIs work properly.

  To complete our QA suite, we’ve recently added a new type of tests : Screenshot tests, that we use to make sure Piwik’s controller and JavaScript code works properly.

  This blog post will explain how they work and describe our experiences setting them up ; we hope to show you an example of innovative QA practices in an active open source project.

  Screenshot Tests

  As the name implies, our screenshot tests (1) first capture a screenshot of a URL, then (2) compare the result with an expected image. This lets us test the code in Piwik’s controllers and Piwik’s JavaScript simply by specifying a URL.

  Contrast this with conventional UI tests that test for page content changes. Such tests require writing large amounts of test code that, at most, check for changes in HTML. Our tests, on the otherhand, will be able to show regressions in CSS and JavaScript rendering logic with a bare minimum of testing code.

  Capturing Screenshots

  Screenshots are captured using a 3rd party tool. We tried several tools before settling on PhantomJS. PhantomJS executes a JavaScript file with an environment that allows it to create WebKit powered web views. When capturing a screenshot, we supply PhantomJS with a script that :

  • opens a web page view,
  • loads a URL,
  • waits for all AJAX requests to be completed,
  • waits for all images to be loaded
  • waits for all JavaScript to be run.

  Then it renders the completed page to an PNG file.

  • To see how we use PhantomJS see capture.js.
  • To see how we wait for AJAX requests to complete and images to load see override.js.

  Comparing Screenshots

  Once a screenshot is generated we test for UI regressions by comparing it with an expected image. There is no sort of fuzzy matching involved. We just check that the images consist of the same bytes.

  If a screenshot test fails we use ImageMagick’s compare command line tool to generate an image diff :

  

  In this example above, there was a change that caused the Search box to be hidden in the datatable. This resulted in the whole Data table report being shifted up a few pixels. The differences are visible in red color which gives rapid feedback to the developers what has changed in the last commit.

  Screenshot Tests on Travis

  We experienced trouble generating identical screenshots on different machines, so our tests were not initially automated by Travis. Once we surpassed this hurdle, we created a new github repo to store our UI tests and screenshots and then enabled the travis build for it. We also made sure that every time a commit is pushed to the Piwik repo, our travis build will push a commit to the UI test repo to run the UI tests.

  We decided to create a new repository so the main repository wouldn’t be burdened with the large screenshot files (which git would not handle very well). We also made sure the travis build would upload all the generated screenshots to a server so debugging failures would be easier.

  Problems we experienced

  Getting generated screenshots to render identically on separate machines was quite a challenge. It took months to figure out how to get it right. Here’s what we learned :

  Fonts will render identically on different machines, but different machines can pick the wrong fonts. When we first tried getting these tests to run on Travis, we noticed small differences in the way fonts were rendered on different machines. We thought this was an insurmountable problem that would occur due to the libraries installed on these machines. It turns out, the machines were just picking the wrong fonts. After installing certain fonts during our Travis build, everything started working.

  Different versions of GD can generate slightly different images. GD is used in Piwik to, among other things, generate sparkline images. Different versions of GD will result in slightly different images. They look the same to the naked eye, but some pixels will have slightly different colors. This is, unfortunately, a problem we couldn’t solve. We couldn’t make sure that everyone who runs the tests uses the same version of GD, so instead we disabled sparklines for UI testing.

  What we learned about existing screenshot capturing tools

  We tried several screenshot capturing tools before finding one that would work adequately. Here’s what we learned about them :

  • CutyCapt This is the first screenshot capturing tool we tried. CutyCapt is a C++ program that uses QtWebKit to load and take a screenshot of a page. It can’t be used to capture multiple screenshots in one run and it can’t be used to wait for all AJAX/Images/JavaScript to complete/load (at least not currently).

  • PhantomJS This is the solution we eventually chose. PhantomJS is a headless scriptable browser that currently uses WebKit as its rendering engine.

    For the most part, PhantomJS is the best solution we found. It reliably renders screenshots, allows JavaScript to be injected into pages it loads, and since it essentially just runs JavaScript code that you provide, it can be made to do whatever you want.

  • SlimerJS SlimerJS is a clone of PhantomJS that uses Gecko as the rendering engine. It is meant to function similarly to PhantomJS. Unfortunately, due to some limitations hard-coded in Mozilla’s software, we couldn’t use it.

    For one, SlimerJS is not headless. There is, apparently, no way to do that when embedding Mozilla. You can, however, run it through xvfb, however the fact that it has to create a window means some odd things can happen. When using SlimerJS, we would sometimes end up with images where tooltips would display as if the mouse was hovering over an element. This inconsistency meant we couldn’t use it for our tests.

  One tool we didn’t try was Selenium Webdriver. Although Selenium is traditionally used to create tests that check for HTML content, it can be used to generate screenshots. (Note : PhantomJS supports using a remote WebDriver.)

  Our Future Plans for Screenshot Testing

  At the moment we render a couple dozen screenshots. We test how our PHP code, JavaScript code and CSS makes Piwik’s UI look, but we don’t test how it behaves. This is our next step.

  We want to create Screenshot Unit Tests for each UI control Piwik uses (for example, the Data Table View or the Site Selector). These tests would use the Widgetize plugin to load a control by itself, then execute JavaScript that simulates events and user behavior, and finally take a screenshot. This way we can test how our code handles clicks and hovers and all sorts of other behavior.

  Screenshots Tests will make Piwik more stable and keep us agile and able to release early and often. Thank you for your support & Spreading the word about Piwik !

• A systematic approach to making Web Applications accessible

  22 février 2012, par silvia

  With the latest developments in HTML5 and the still fairly new ARIA (Accessible Rich Interface Applications) attributes introduced by the W3C WAI (Web Accessibility Initiative), browsers have now implemented many features that allow you to make your JavaScript-heavy Web applications accessible.

  Since I began working on making a complex web application accessible just over a year ago, I discovered that there was no step-by-step guide to approaching the changes necessary for creating an accessible Web application. Therefore, many people believe that it is still hard, if not impossible, to make Web applications accessible. In fact, it can be approached systematically, as this article will describe.

  This post is based on a talk that Alice Boxhall and I gave at the recent Linux.conf.au titled “Developing accessible Web apps – how hard can it be ?” (slides, video), which in turn was based on a Google Developer Day talk by Rachel Shearer (slides).

  These talks, and this article, introduce a process that you can follow to make your Web applications accessible : each step will take you closer to having an application that can be accessed using a keyboard alone, and by users of screenreaders and other accessibility technology (AT).

  The recommendations here only roughly conform to the requirements of WCAG (Web Content Accessibility Guidelines), which is the basis of legal accessibility requirements in many jurisdictions. The steps in this article may or may not be sufficient to meet a legal requirement. It is focused on the practical outcome of ensuring users with disabilities can use your Web application.

  Step-by-step Approach

  The steps to follow to make your Web apps accessible are as follows :

  1. Use native HTML tags wherever possible
  2. Make interactive elements keyboard accessible
  3. Provide extra markup for AT (accessibility technology)

  If you are a total newcomer to accessibility, I highly recommend installing a screenreader and just trying to read/navigate some Web pages. On Windows you can install the free NVDA screenreader, on Mac you can activate the pre-installed VoiceOver screenreader, on Linux you can use Orca, and if you just want a browser plugin for Chrome try installing ChromeVox.

  1. Use native HTML tags

  As you implement your Web application with interactive controls, try to use as many native HTML tags as possible.

  HTML5 provides a rich set of elements which can be used to both add functionality and provide semantic context to your page. HTML4 already included many useful interactive controls, like <a>, <button>, <input> and <select>, and semantic landmark elements like <h1>. HTML5 adds richer <input> controls, and a more sophisticated set of semantic markup elements like such as <time>, <progress>, <meter>, <nav>, <header>, <article> and <aside>. (Note : check browser support for browser support of the new tags).

  Using as much of the rich HTML5 markup as possible means that you get all of the accessibility features which have been implemented in the browser for those elements, such as keyboard support, short-cut keys and accessibility metadata, for free. For generic tags you have to implement them completely from scratch.

  What exactly do you miss out on when you use a generic tag such as <div> over a specific semantic one such as <button> ?

  1. Generic tags are not focusable. That means you cannot reach them through using the [tab] on the keyboard.
  2. You cannot activate them with the space bar or enter key or perform any other keyboard interaction that would be regarded as typical with such a control.
  3. Since the role that the control represents is not specified in code but is only exposed through your custom visual styling, screenreaders cannot express to their users what type of control it is, e.g. button or link.
  4. Neither can screenreaders add the control to the list of controls on the page that are of a certain type, e.g. to navigate to all headers of a certain level on the page.
  5. And finally you need to manually style the element in order for it to look distinctive compared to other elements on the page ; using a default control will allow the browser to provide the default style for the platform, which you can still override using CSS if you want.

  Example :

  Compare these two buttons. The first one is implemented using a <div> tag, the second one using a <button> tag. Try using a screenreader to experience the difference.

  Send	<style> .custombutton cursor : pointer ; border : 1px solid #000 ; background-color : #F6F6F6 ; display : inline-block ; padding : 2px 5px ; </style> <div class="custombutton" onclick="alert(’sent !’)"> Send </div>
  Send	<button onclick="alert(’sent !’)"> Send </button>

  2. Make interactive elements keyboard accessible

  Many sophisticated web applications have some interactive controls that just have no appropriate HTML tag equivalent. In this case, you will have had to build an interactive element with JavaScript and <div> and/or <span> tags and lots of custom styling. The good news is, it’s possible to make even these custom controls accessible, and as a side benefit you will also make your application smoother to use for power users.

  The first thing you can do to test usability of your control, or your Web app, is to unplug the mouse and try to use only the [TAB] and [ENTER] keys to interact with your application.

  

  Try the following :

  • Can you reach all interactive elements with [TAB] ?
  • Can you activate interactive elements with [ENTER] (or [SPACE]) ?
  • Are the elements in the right tab order ?
  • After interaction : is the right element in focus ?
  • Is there a keyboard shortcut that activates the element (accesskey) ?

  No ? Let’s fix it.

  2.1. Reaching interactive elements

  If you have an element on your page that cannot be reached with [TAB], put a @tabindex attribute on it.

  Example :

  Here we have a <span> tag that works as a link (don’t do this – it’s just a simple example). The first one cannot be reached using [TAB] but the second one has a tabindex and is thus part of the tab order of the HTML page.

  (Note : since we experiment lots with the tabindex in this article, to avoid confusion, click on some text in this paragraph and then hit the [TAB] key to see where it goes next. The click will set your keyboard focus in the DOM.)

  Click	<style> .customlink text-decoration : underline ; cursor : pointer ; </style> <span class="customlink" onclick="alert(’activated !’)"> Click </span>
  Click	<style> .customlink text-decoration : underline ; cursor : pointer ; </style> <span class="customlink" onclick="alert(’activated !’)" tabindex="0"> Click </span>

  You set @tabindex=0 to add an element into the native tab order of the page, which is the DOM order.

  2.2. Activating interactive elements

  Next, you typically want to be able to use the [ENTER] and [SPACE] keys to activate your custom control. To do so, you will need to implement an onkeydown event handler. Note that the keyCode for [ENTER] is 13 and for [SPACE] is 32.

  Example :

  Let’s add this functionality to the <span> tag from before. Try tabbing to it and hit the [ENTER] or [SPACE] key.

  Click	<span class="customlink" onclick="alert(’activated !’)" tabindex="0"> Click </span>
  &lt;script&gt;<br /> function handlekey(event) {<br /> var target = event.target || event.srcElement;<br /> if (event.keyCode == 13 || event.keyCode == 32) { target.onclick(); }<br /> }<br /> &lt;/script&gt; Click	<span class="customlink" onclick="alert(’activated !’)" tabindex="0" onkeydown="handlekey(event) ;"> Click </span> <script> function handlekey(event) var target = event.target || event.srcElement ; if (event.keyCode == 13 || event.keyCode == 32) target.onclick() ; </script>

  Note that there are some controls that might need support for keys other than [tab] or [enter] to be able to use them from the keyboard alone, for example a custom list box, menu or slider should respond to arrow keys.

  2.3. Elements in the right tab order

  Have you tried tabbing to all the elements on your page that you care about ? If so, check if the order of tab stops seems right. The default order is given by the order in which interactive elements appear in the DOM. For example, if your page’s code has a right column that is coded before the main article, then the links in the right column will receive tab focus first before the links in the main article.

  You could change this by re-ordering your DOM, but oftentimes this is not possible. So, instead give the elements that should be the first ones to receive tab focus a positive @tabindex. The tab access will start at the smallest non-zero @tabindex value. If multiple elements share the same @tabindex value, these controls receive tab focus in DOM order. After that, interactive elements and those with @tabindex=0 will receive tab focus in DOM order.

  Example :

  The one thing that always annoys me the most is if the tab order in forms that I am supposed to fill in is illogical. Here is an example where the first and last name are separated by the address because they are in a table. We could fix it by moving to a <div> based layout, but let’s use @tabindex to demonstrate the change.

  Firstname : Address : Lastname : City :	<table class="customtabs"> <tr> <td>Firstname : <input type="text" id="firstname"> </td> <td>Address : <input type="text" id="address"> </td> </tr> <tr> <td>Lastname : <input type="text" id="lastname"> </td> <td>City : <input type="text" id="city"> </td> </tr> </table>
  Click here to test this form, then [TAB] : Firstname : Address : Lastname : City :	<table class="customtabs"> <tr> <td>Firstname : <input type="text" id="firstname" tabindex="10"> </td> <td>Address : <input type="text" id="address" tabindex="30"> </td> </tr> <tr> <td>Lastname : <input type="text" id="lastname" tabindex="20"> </td> <td>City : <input type="text" id="city" tabindex="40"> </td> </tr> </table>

  Be very careful with using non-zero tabindex values. Since they change the tab order on the page, you may get side effects that you might not have intended, such as having to give other elements on the page a non-zero tabindex value to avoid skipping too many other elements as I would need to do here.

  2.4. Focus on the right element

  Some of the controls that you create may be rather complex and open elements on the page that were previously hidden. This is particularly the case for drop-downs, pop-ups, and menus in general. Oftentimes the hidden element is not defined in the DOM right after the interactive control, such that a [TAB] will not put your keyboard focus on the next element that you are interacting with.

  The solution is to manage your keyboard focus from JavaScript using the .focus() method.

  Example :

  Here is a menu that is declared ahead of the menu button. If you tab onto the button and hit enter, the menu is revealed. But your tab focus is still on the menu button, so your next [TAB] will take you somewhere else. We fix it by setting the focus on the first menu item after opening the menu.

  Menu item1 Menu item2 Menu &lt;script&gt;<br /> function displayMenu(value) {<br /> document.getElementById(&quot;custommenu&quot;).style.display=value;<br /> }<br /> &lt;/script&gt;	<div id="custommenu" style="display:none ;"> <button id="item1" onclick="displayMenu(’none’) ;">Menu item1</button> <button id="item2" onclick="displayMenu(’none’) ;">Menu item2</button> </div> <button onclick="displayMenu(’block’) ;">Menu</button> <script> function displayMenu(value) document.getElementById("custommenu").style.display=value ; </script>
  Menu item1 Menu item2 Menu &lt;script&gt;<br /> function displayMenu2(value) {<br /> document.getElementById(&quot;custommenu2&quot;).style.display=value;<br /> document.getElementById(&quot;item1&quot;).focus();<br /> }<br /> &lt;/script&gt;	<div id="custommenu" style="display:none ;"> <button id="item1" onclick="displayMenu(’none’) ;">Menu item1</button> <button id="item2" onclick="displayMenu(’none’) ;">Menu item2</button> </div> <button onclick="displayMenu(’block’) ;">Menu</button> <script> function displayMenu(value) document.getElementById("custommenu").style.display=value ; document.getElementById("item1").focus() ; </script>

  You will notice that there are still some things you can improve on here. For example, after you close the menu again with one of the menu items, the focus does not move back onto the menu button.

  Also, after opening the menu, you may prefer not to move the focus onto the first menu item but rather just onto the menu <div>. You can do so by giving that div a @tabindex and then calling .focus() on it. If you do not want to make the div part of the normal tabbing order, just give it a @tabindex=-1 value. This will allow your div to receive focus from script, but be exempt from accidental tabbing onto (though usually you just want to use @tabindex=0).

  Bonus : If you want to help keyboard users even more, you can also put outlines on the element that is currently in focus using CSS”s outline property. If you want to avoid the outlines for mouse users, you can dynamically add a class that removes the outline in mouseover events but leaves it for :focus.

  2.5. Provide sensible keyboard shortcuts

  At this stage your application is actually keyboard accessible. Congratulations !

  However, it’s still not very efficient : like power-users, screenreader users love keyboard shortcuts : can you imagine if you were forced to tab through an entire page, or navigate back to a menu tree at the top of the page, to reach each control you were interested in ? And, obviously, anything which makes navigating the app via the keyboard more efficient for screenreader users will benefit all power users as well, like the ubiquitous keyboard shortcuts for cut, copy and paste.

  HTML4 introduced so-called accesskeys for this. In HTML5 @accesskey is now allowed on all elements.

  The @accesskey attribute takes the value of a keyboard key (e.g. @accesskey="x") and is activated through platform- and browser-specific activation keys. For example, on the Mac it’s generally the [Ctrl] key, in IE it’ the [Alt] key, in Firefox on Windows [Shift]-[Alt], and in Opera on Windows [Shift]-[ESC]. You press the activation key and the accesskey together which either activates or focuses the element with the @accesskey attribute.

  Example :

  Send &lt;script&gt;<br /> var button = document.getElementById('accessbutton');<br /> if (button.accessKeyLabel) {<br />  button.innerHTML += ' (' + button.accessKeyLabel + ')';<br /> }<br /> &lt;/script&gt;

  <button id="accessbutton" onclick="alert(’sent !’)" accesskey="e"> Send </button> <script> var button = document.getElementById(’accessbutton’) ; if (button.accessKeyLabel) button.innerHTML += ’ (’ + button.accessKeyLabel + ’)’ ; </script>

  Now, the idea behind this is clever, but the execution is pretty poor. Firstly, the different activation keys between different platforms and browsers make it really hard for people to get used to the accesskeys. Secondly, the key combinations can conflict with browser and screenreader shortcut keys, the first of which will render browser shortcuts unusable and the second will effectively remove the accesskeys.

  In the end it is up to the Web application developer whether to use the accesskey attribute or whether to implement explicit shortcut keys for the application through key event handlers on the window object. In either case, make sure to provide a help list for your shortcut keys.

  Also note that a page with a really good hierarchical heading layout and use of ARIA landmarks can help to eliminate the need for accesskeys to jump around the page, since there are typically default navigations available in screen readers to jump directly to headings, hyperlinks, and ARIA landmarks.

  3. Provide markup for AT

  Having made the application keyboard accessible also has advantages for screenreaders, since they can now reach the controls individually and activate them. So, next we will use a screenreader and close our eyes to find out where we only provide visual cues to understand the necessary interaction.

  Here are some of the issues to consider :

  • Role may need to get identified
  • States may need to be kept track of
  • Properties may need to be made explicit
  • Labels may need to be provided for elements

  This is where the W3C’s ARIA (Accessible Rich Internet Applications) standard comes in. ARIA attributes provide semantic information to screen readers and other AT that is otherwise conveyed only visually.

  Note that using ARIA does not automatically implement the standard widget behavior – you’ll still need to add focus management, keyboard navigation, and change aria attribute values in script.

  3.1. ARIA roles

  After implementing a custom interactive widget, you need to add a @role attribute to indicate what type of controls it is, e.g. that it is playing the role of a standard tag such as a button.

  Example :

  This menu button is implemented as a <div>, but with a role of “button” it is announced as a button by a screenreader.

  Menu	<div tabindex="0" role="button">Menu</div>

  ARIA roles also describe composite controls that do not have a native HTML equivalent.

  Example :

  This menu with menu items is implemented as a set of <div> tags, but with a role of “menu” and “menuitem” items.

  Cut Copy Paste

  <div role="menu"> <div tabindex="0" role="menuitem">Cut</div> <div tabindex="0" role="menuitem">Copy</div> <div tabindex="0" role="menuitem">Paste</div> </div>

  3.2. ARIA states

  Some interactive controls represent different states, e.g. a checkbox can be checked or unchecked, or a menu can be expanded or collapsed.

  Example :

  The following menu has states on the menu items, which are here not just used to give an aural indication through the screenreader, but also a visual one through CSS.

  Left Center Right

  <style> .custombutton[aria-checked=true]:before content : "\2713 " ; </style> <div role="menu"> <div tabindex="0" role="menuitem" aria-checked="true">Left</div> <div tabindex="0" role="menuitem" aria-checked="false">Center</div> <div tabindex="0" role="menuitem" aria-checked="false">Right</div> </div>

  3.3. ARIA properties

  Some of the functionality of interactive controls cannot be captured by the role attribute alone. We have ARIA properties to add features that the screenreader needs to announce, such as aria-label, aria-haspopup, aria-activedescendant, or aria-live.

  Example :

  The following drop-down menu uses aria-haspopup to tell the screenreader that there is a popup hidden behind the menu button together with an ARIA state of aria-expanded to track whether it’s open or closed.

  Justify Left Center Right &lt;script&gt;<br /> var button = document.getElementById(&quot;button&quot;);<br /> var menu = document.getElementById(&quot;menu&quot;);<br /> var items = document.getElementsByClassName(&quot;menuitem&quot;);<br /> var focused = 0;<br /> function showMenu(evt) {<br />    evt.stopPropagation();<br />    menu.style.visibility = 'visible';<br />    button.setAttribute('aria-expanded','true');<br />    focused = getSelected();<br />    items[focused].focus();<br />  }<br />  function hideMenu(evt) {<br />    evt.stopPropagation();<br />    menu.style.visibility = 'hidden';<br />    button.setAttribute('aria-expanded','false');<br />    button.focus();<br />  }<br />  function getSelected() {<br />    for (var i=0; i &lt; items.length; i++) {<br />      if (items[i].getAttribute('aria-checked') == 'true') {<br />        return i;<br />      }<br />    }<br />  }<br />  function setSelected(elem) {<br />    var curSelected = getSelected();<br />    items[curSelected].setAttribute('aria-checked', 'false');<br />    elem.setAttribute('aria-checked', 'true');<br />  }<br />  function selectItem(evt) {<br />    setSelected(evt.target);<br />    hideMenu(evt);<br />  }<br /> function getPrevItem(index) {<br />    var prev = index - 1;<br />    if (prev &lt; 0) {<br />      prev = items.length - 1;<br />    }<br />    return prev;<br />  }<br />  function getNextItem(index) {<br />    var next = index + 1;<br />    if (next == items.length) {<br />      next = 0;<br />    }<br />    return next;<br />  }<br /> function handleButtonKeys(evt) {<br />    evt.stopPropagation();<br />    var key = evt.keyCode;<br />    switch(key) {<br />      case (13): /* ENTER */<br />      case (32): /* SPACE */<br />        showMenu(evt);<br />      default:<br />    }<br />  }<br />  function handleMenuKeys(evt) {<br />    evt.stopPropagation();<br />    var key = evt.keyCode;<br />    switch(key) {<br />      case (38): /* UP */<br />        focused = getPrevItem(focused);<br />        items[focused].focus();<br />        break;<br />      case (40): /* DOWN */<br />        focused = getNextItem(focused);<br />        items[focused].focus();<br />        break;<br />      case (13): /* ENTER */<br />      case (32): /* SPACE */<br />        setSelected(evt.target);<br />          hideMenu(evt);<br />          break;<br />      case (27): /* ESC */<br />        hideMenu(evt);<br />         break;<br />      default:<br />    }<br />  }<br />  button.addEventListener('click', showMenu, false);<br />  button.addEventListener('keydown', handleButtonKeys, false);<br />  for (var i = 0;  i &lt; items.length; i++) {<br />    items[i].addEventListener('click', selectItem, false);<br />    items[i].addEventListener('keydown', handleMenuKeys, false);<br />  }<br /> &lt;/script&gt;

  <div class="custombutton" id="button" tabindex="0" role="button" aria-expanded="false" aria-haspopup="true"> <span>Justify</span> </div> <div role="menu" class="menu" id="menu" style="display : none ;"> <div tabindex="0" role="menuitem" class="menuitem" aria-checked="true"> Left </div> <div tabindex="0" role="menuitem" class="menuitem" aria-checked="false"> Center </div> <div tabindex="0" role="menuitem" class="menuitem" aria-checked="false"> Right </div> </div> [CSS and JavaScript for example omitted]

  3.4. Labelling

  The main issue that people know about accessibility seems to be that they have to put alt text onto images. This is only one means to provide labels to screenreaders for page content. Labels are short informative pieces of text that provide a name to a control.

  There are actually several ways of providing labels for controls :

  • on img elements use @alt
  • on input elements use the label element
  • use @aria-labelledby if there is another element that contains the label
  • use @title if you also want a label to be used as a tooltip
  • otherwise use @aria-label

  I’ll provide examples for the first two use cases - the other use cases are simple to deduce.

  Example :

  The following two images show the rough concept for providing alt text for images : images that provide information should be transcribed, images that are just decorative should receive an empty @alt attribute.

  Image by Noah Sussman

  <img src="texture.jpg" alt=""> <img src="lolcat.jpg" alt="shocked lolcat titled ’HTML cannot do that !"> <img src="texture.jpg" alt="">

  When marking up decorative images with an empty @alt attribute, the image is actually completely removed from the accessibility tree and does not confuse the blind user. This is a desired effect, so do remember to mark up all your images with @alt attributes, even those that don’t contain anything of interest to AT.

  Example :

  In the example form above in Section 2.3, when tabbing directly on the input elements, the screen reader will only say "edit text" without announcing what meaning that text has. That’s not very useful. So let’s introduce a label element for the input elements. We’ll also add checkboxes with a label.

  Doctor title : Firstname : Lastname : Address : City : Remember me :

  <label>Doctor title :</label> <input type="checkbox" id="doctor"/> <label>Firstname :</label> <input type="text" id="firstname2"/> <label for="lastname2">Lastname :</label> <input type="text" id="lastname2"/> <label>Address : <input type="text" id="address2"> </label> <label for="city2">City : <input type="text" id="city2"> </label> <label for="remember">Remember me :</label> <input type="checkbox" id="remember">

  In this example we use several different approaches to show what a different it makes to use the <label> element to mark up input boxes.

  The first two fields just have a <label> element next to a <input> element. When using a screenreader you will not notice a difference between this and not using the <label> element because there is no connection between the <label> and the <input> element.

  In the third field we use the @for attribute to create that link. Now the input field isn’t just announced as "edit text", but rather as "Lastname edit text", which is much more useful. Also, the screenreader can now skip the labels and get straight on the input element.

  In the fourth and fifth field we actually encapsulate the <input> element inside the <label> element, thus avoiding the need for a @for attribute, though it doesn’t hurt to explicity add it.

  Finally we look at the checkbox. By including a referenced <label> element with the checkbox, we change the screenreaders announcement from just "checkbox not checked" to "Remember me checkbox not checked". Also notice that the click target now includes the label, making the checkbox not only more usable to screenreaders, but also for mouse users.

  4. Conclusions

  This article introduced a process that you can follow to make your Web applications accessible. As you do that, you will noticed that there are other things that you may need to do in order to give the best experience to a power user on a keyboard, a blind user using a screenreader, or a vision-impaired user using a screen magnifier. But once you’ve made a start, you will notice that it’s not all black magic and a lot can be achieved with just a little markup.

  You will find more markup in the WAI ARIA specification and many more resources at Mozilla’s ARIA portal. Now go and change the world !

  Many thanks to Alice Boxhall and Dominic Mazzoni for their proof-reading and suggested changes that really helped improve the article !

• I am using ffmpeg java library to convert captured screenshots to video. Video output is blurry

  2 octobre 2020, par dark prince

  I am using ffmpeg java library to convert captured screenshots to video. Video which is generated as output is blurry.

  &#xA;

  I am using bit rate as 9000, frames per sec as 25 and video size as that of desktop screen size.

  &#xA;

  Any suggestions on how to solve this issue.

  &#xA;

  P.S. I cannot use ffmpeg.exe and command line due to certain restrictions and hence I am opting for ffmpeg java library.

  &#xA;

  Any suggestions on the issue or suggestions on any better approach will be helpful.

  &#xA;

      import java.awt.AWTException;&#xA;    import java.awt.Dimension;&#xA;    import java.awt.FlowLayout;&#xA;    import java.awt.Rectangle;&#xA;    import java.awt.Robot;&#xA;    import java.awt.Toolkit;&#xA;    import java.awt.event.ActionEvent;&#xA;    import java.awt.event.ActionListener;&#xA;    import java.awt.image.BufferedImage;&#xA;    import java.io.File;&#xA;    import java.io.IOException;&#xA;    import java.util.Date;&#xA;    &#xA;    import javax.imageio.ImageIO;&#xA;    import javax.swing.JButton;&#xA;    import javax.swing.JFrame;&#xA;    import javax.swing.JLabel;&#xA;    import javax.swing.JOptionPane;&#xA;    &#xA;    import org.bytedeco.javacpp.avcodec;&#xA;    import org.bytedeco.javacv.FFmpegFrameRecorder;&#xA;    import org.bytedeco.javacv.OpenCVFrameConverter;&#xA;    &#xA;    public class ScreenRecorder{&#xA;    &#xA;        public static boolean videoComplete=false;&#xA;        public static String inputImageDir="inputImgFolder"&#x2B;File.separator;&#xA;        public static String inputImgExt="png";&#xA;        public static String outputVideo="recording.mp4"; &#xA;        public static int counter=0;&#xA;        public static int imgProcessed=0;&#xA;        public static FFmpegFrameRecorder recorder=null;&#xA;        public static int videoWidth=1920;&#xA;        public static int videoHeight=1080;&#xA;        public static int videoFrameRate=3;&#xA;        public static int videoQuality=0; // 0 is the max quality&#xA;        public static int videoBitRate=9000;&#xA;        public static String videoFormat="mp4";&#xA;        public static int videoCodec=avcodec.AV_CODEC_ID_MPEG4;&#xA;        public static Thread t1=null;&#xA;        public static Thread t2=null;&#xA;        public static JFrame frame=null;&#xA;        public static boolean isRegionSelected=false;&#xA;        public static int c1=0;&#xA;        public static int c2=0;&#xA;        public static int c3=0;&#xA;        public static int c4=0;&#xA;        &#xA;        &#xA;        public static void main(String[] args) {&#xA;            &#xA;            try {&#xA;                if(getRecorder()==null)&#xA;                {&#xA;                    System.out.println("Cannot make recorder object, Exiting program");&#xA;                    System.exit(0);&#xA;                }&#xA;                if(getRobot()==null)&#xA;                {&#xA;                    System.out.println("Cannot make robot object, Exiting program");&#xA;                    System.exit(0);&#xA;                }&#xA;                File scanFolder=new File(inputImageDir);&#xA;                scanFolder.delete();&#xA;                scanFolder.mkdirs();&#xA;                &#xA;                createGUI();&#xA;            } catch (Exception e) {&#xA;                System.out.println("Exception in program "&#x2B;e.getMessage());&#xA;            }&#xA;        }&#xA;        &#xA;        public static void createGUI()&#xA;        {&#xA;            frame=new JFrame("Screen Recorder");&#xA;            JButton b1=new JButton("Select Region for Recording");&#xA;            JButton b2=new JButton("Start Recording");&#xA;            JButton b3=new JButton("Stop Recording");&#xA;            JLabel l1=new JLabel("<br />If you dont select a region then full screen recording <br /> will be made when you click on Start Recording");&#xA;            l1.setFont (l1.getFont ().deriveFont (20.0f));&#xA;            b1.addActionListener(new ActionListener() {&#xA;                @Override&#xA;                public void actionPerformed(ActionEvent e) {&#xA;                    try {&#xA;                        JOptionPane.showMessageDialog(frame, "A new window will open. Use your mouse to select the region you like to record");&#xA;                        new CropRegion().getImage();&#xA;                    } catch (Exception e1) {&#xA;                        // TODO Auto-generated catch block&#xA;                        System.out.println("Issue while trying to call the module to crop region");&#xA;                        e1.printStackTrace();&#xA;                    } &#xA;                }&#xA;            });&#xA;            b2.addActionListener(new ActionListener() {&#xA;                @Override&#xA;                public void actionPerformed(ActionEvent e) {&#xA;                    counter=0;&#xA;                    startRecording();&#xA;                }&#xA;            });&#xA;            b3.addActionListener(new ActionListener() {&#xA;                @Override&#xA;                public void actionPerformed(ActionEvent e) {&#xA;                    stopRecording();&#xA;                    System.out.print("Exiting...");&#xA;                    System.exit(0);&#xA;                }&#xA;            });&#xA;            &#xA;            frame.add(b1);&#xA;            frame.add(b2);&#xA;            frame.add(b3);&#xA;            frame.add(l1);&#xA;            frame.setLayout(new FlowLayout(0));&#xA;            frame.setVisible(true);&#xA;            frame.setSize(1000, 170);&#xA;            frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);&#xA;        }&#xA;        &#xA;        public static void startRecording()&#xA;        {&#xA;            t1=new Thread()&#xA;            {&#xA;                public void run() {&#xA;                    try {&#xA;                        takeScreenshot(getRobot());&#xA;                    } catch (Exception e) {&#xA;                        JOptionPane.showMessageDialog(frame, "Cannot make robot object, Exiting program "&#x2B;e.getMessage());&#xA;                        System.out.println("Cannot make robot object, Exiting program "&#x2B;e.getMessage());&#xA;                        System.exit(0);&#xA;                    }&#xA;                }&#xA;            };&#xA;            &#xA;            t2=new Thread()&#xA;            {&#xA;                public void run() {&#xA;                    prepareVideo();&#xA;                }&#xA;            };&#xA;            &#xA;            t1.start();&#xA;            t2.start();&#xA;            System.out.println("Started recording at "&#x2B;new Date());&#xA;        }&#xA;        &#xA;        public static Robot getRobot() throws Exception&#xA;        {&#xA;            Robot r=null;&#xA;            try {&#xA;                r = new Robot();&#xA;                return r;&#xA;            } catch (AWTException e) {&#xA;                JOptionPane.showMessageDialog(frame, "Issue while initiating Robot object "&#x2B;e.getMessage());&#xA;                System.out.println("Issue while initiating Robot object "&#x2B;e.getMessage());&#xA;                throw new Exception("Issue while initiating Robot object");&#xA;            }&#xA;        }&#xA;        &#xA;        public static void takeScreenshot(Robot r)&#xA;        {&#xA;            Dimension size = Toolkit.getDefaultToolkit().getScreenSize();&#xA;            Rectangle rec=new Rectangle(size);&#xA;            if(isRegionSelected)&#xA;            {&#xA;                rec=new Rectangle(c1, c2, c3-c1, c4-c2);&#xA;            }&#xA;            while(!videoComplete)&#xA;            {&#xA;            counter&#x2B;&#x2B;;&#xA;            BufferedImage img = r.createScreenCapture(rec);&#xA;            try {&#xA;                ImageIO.write(img, inputImgExt, new File(inputImageDir&#x2B;counter&#x2B;"."&#x2B;inputImgExt));&#xA;            } catch (IOException e) {&#xA;                JOptionPane.showMessageDialog(frame, "Got an issue while writing the screenshot to disk "&#x2B;e.getMessage());&#xA;                System.out.println("Got an issue while writing the screenshot to disk "&#x2B;e.getMessage());&#xA;                counter--;&#xA;            }&#xA;            }&#xA;        }&#xA;        &#xA;        public static void prepareVideo()&#xA;        {&#xA;            File scanFolder=new File(inputImageDir);&#xA;            while(!videoComplete)&#xA;            {&#xA;                File[] inputFiles=scanFolder.listFiles();&#xA;                try {&#xA;                    getRobot().delay(500);&#xA;                } catch (Exception e) {&#xA;                }&#xA;                //for(int i=0;i/imgProcessed&#x2B;&#x2B;;&#xA;                    addImageToVideo(inputFiles[i].getAbsolutePath());&#xA;                    //String imgToAdd=scanFolder.getAbsolutePath()&#x2B;File.separator&#x2B;imgProcessed&#x2B;"."&#x2B;inputImgExt;&#xA;                    //addImageToVideo(imgToAdd);&#xA;                    //new File(imgToAdd).delete();&#xA;                    inputFiles[i].delete();&#xA;                }&#xA;            }&#xA;            &#xA;            File[] inputFiles=scanFolder.listFiles();&#xA;            for(int i=0;i/ maximum quality&#xA;             recorder.start();&#xA;             }&#xA;             catch(Exception e)&#xA;             {&#xA;                 JOptionPane.showMessageDialog(frame, "Exception while starting the recorder object "&#x2B;e.getMessage());&#xA;                 System.out.println("Exception while starting the recorder object "&#x2B;e.getMessage());&#xA;                 throw new Exception("Unable to start recorder");&#xA;             }&#xA;             return recorder;&#xA;        }&#xA;        &#xA;        public static OpenCVFrameConverter.ToIplImage getFrameConverter()&#xA;        {&#xA;            OpenCVFrameConverter.ToIplImage grabberConverter = new OpenCVFrameConverter.ToIplImage();&#xA;            return grabberConverter;&#xA;        }&#xA;        &#xA;        public static void addImageToVideo(String imgPath)&#xA;        {&#xA;            try {&#xA;                getRecorder().record(getFrameConverter().convert(cvLoadImage(imgPath)));&#xA;            } catch (Exception e) {&#xA;                JOptionPane.showMessageDialog(frame, "Exception while adding image to video "&#x2B;e.getMessage());&#xA;                System.out.println("Exception while adding image to video "&#x2B;e.getMessage());&#xA;            }&#xA;        }&#xA;        &#xA;        public static void stopRecording()&#xA;        {&#xA;            try {&#xA;                videoComplete=true;&#xA;                System.out.println("Stopping recording at "&#x2B;new Date());&#xA;                t1.join();&#xA;                System.out.println("Screenshot thread complete");&#xA;                t2.join();&#xA;                System.out.println("Video maker thread complete");&#xA;                getRecorder().stop();&#xA;                System.out.println("Recording has been saved successfully at "&#x2B;new File(outputVideo).getAbsolutePath());&#xA;                JOptionPane.showMessageDialog(frame, "Recording has been saved successfully at "&#x2B;new File(outputVideo).getAbsolutePath());&#xA;            } catch (Exception e) {&#xA;                System.out.println("Exception while stopping the recorder "&#x2B;e.getMessage());&#xA;            }&#xA;        }&#xA;    }&#xA;

  &#xA;

  Imagepanel.java

  &#xA;

  import java.awt.Dimension;
import java.awt.Graphics;
import java.awt.Image;
import javax.swing.ImageIcon;
import javax.swing.JPanel;

class ImagePanel
  extends JPanel
{
  private Image img;
  
  public ImagePanel(String img)
  {
    this(new ImageIcon(img).getImage());
  }
  
  public ImagePanel(Image img)
  {
    this.img = img;
    Dimension size = new Dimension(img.getWidth(null), img.getHeight(null));
    
    setPreferredSize(size);
    setMinimumSize(size);
    setMaximumSize(size);
    setSize(size);
    setLayout(null);
  }
  
  public void paintComponent(Graphics g)
  {
    g.drawImage(this.img, 0, 0, null);
  }
}


  &#xA;

  CropRegion.java

  &#xA;

  import java.awt.AWTException;
import java.awt.Dimension;
import java.awt.FlowLayout;
import java.awt.Graphics;
import java.awt.Rectangle;
import java.awt.Robot;
import java.awt.Toolkit;
import java.awt.event.MouseEvent;
import java.awt.event.MouseListener;
import java.awt.event.MouseMotionListener;
import java.awt.image.BufferedImage;
import java.io.IOException;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;


public class CropRegion implements MouseListener,
        MouseMotionListener {

    int drag_status = 0;
    int c1;
    int c2;
    int c3;
    int c4;
    JFrame frame=null;
    static int counter=0;
    JLabel background=null;

    
    public void getImage() throws AWTException, IOException, InterruptedException {
        Dimension size = Toolkit.getDefaultToolkit().getScreenSize();
        Robot robot = new Robot();
        BufferedImage img = robot.createScreenCapture(new Rectangle(size));
        ImagePanel panel = new ImagePanel(img);
        frame=new JFrame();
        frame.add(panel);
        frame.setLocation(0, 0);
        frame.setSize(size);
        frame.setLayout(new FlowLayout());
        frame.setUndecorated(true);
        frame.setVisible(true);
        frame.addMouseListener(this);
        frame.addMouseMotionListener(this);
        frame.setDefaultCloseOperation(JFrame.DISPOSE_ON_CLOSE);
    }

    public void draggedScreen() throws Exception {
        ScreenRecorder.c1=c1;
        ScreenRecorder.c2=c2;
        ScreenRecorder.c3=c3;
        ScreenRecorder.c4=c4;
        ScreenRecorder.isRegionSelected=true;
        JOptionPane.showMessageDialog(frame, "Region Selected.Please click on Start Recording button to record the selected region.");
        frame.dispose();
    }

    public void mouseClicked(MouseEvent arg0) {
    }

    public void mouseEntered(MouseEvent arg0) {
    }

    public void mouseExited(MouseEvent arg0) {
    }

    public void mousePressed(MouseEvent arg0) {
        paint();
        this.c1 = arg0.getX();
        this.c2 = arg0.getY();
    }

    public void mouseReleased(MouseEvent arg0) {
        paint();
        if (this.drag_status == 1) {
            this.c3 = arg0.getX();
            this.c4 = arg0.getY();
            try {
                draggedScreen();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }
    }

    public void mouseDragged(MouseEvent arg0) {
        paint();
        this.drag_status = 1;
        this.c3 = arg0.getX();
        this.c4 = arg0.getY();
    }

    public void mouseMoved(MouseEvent arg0) {
    }

    public void paint() {
        Graphics g = frame.getGraphics();
        frame.repaint();
        int w = this.c1 - this.c3;
        int h = this.c2 - this.c4;
        w *= -1;
        h *= -1;
        if (w < 0) {
            w *= -1;
        }
        g.drawRect(this.c1, this.c2, w, h);
    }
}


  &#xA;