Understanding the Contrast Ratio

HTML

            
              <div class="nes-container">
  <main class="main-content">
    <h1> Understanding Contrast</h1>

    <div class="nes-field is-inline">
      <label for="color1">Color 1</label>
      <input type="text" maxlength="7" placeholder="Hex Color 1" id="hex1" class="nes-input hexcolor">
      <div class="nes-container is-rounded color" id="color1"></div>
    </div>

    <div class="nes-field is-inline">
      <label for="color1">Color 2</label>
      <input type="text" placeholder="Hex Color 2" id="hex2" class="nes-input hexcolor">
      <div class="nes-container color is-rounded " id="color2"></div>
    </div>

    <table class="nes-table is-bordered is-centered results">
      <tr>
        <td class="reportgrade">
          <p class="textcenter">Contrast Ratio</p>
          <h3 id="ratioResult" class="rr">###</h3><br/>
          <div class="resultHearts">
            <div class="icon-list">
              <i class="nes-icon is-medium heart is-empty"></i>
              <i class="nes-icon is-medium heart is-empty"></i>
              <i class="nes-icon is-medium heart is-empty"></i><br/>
              <i class="nes-icon is-medium star is-empty"></i>
            </div>
          </div>

        </td>
        <td>
          <p class="textcenter">WCAG Grade</p>
<!--           <h3 id="wcagResult" class="wr">###</h3><br/> -->
          <div class="lists">
            <ul id="wcagChart" class="nes-list is-disc">
              <li id="ratio-Fail">Fail: Less than 3.0</li>

              <li id="ratio-AA">&nbsp;AA: 3.0 for text 18pt+</li>

              <li id="ratio-AAL">&nbsp;AA: 4.5 for smaller text</li>

              <li id="ratio-AAA">AAA: 7.0 gold star</li>
          </div>

        </td>
      </tr>
    </table>

    <section class="message-list">
      <div class="message">
        <span class="avatar contrastPrincess right"></span>
        <!-- Balloon -->
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          <p>How do we calculate the contrast ratio?</p>
        </div>
      </div>
      <div class="message">
        <span class="avatar contrastPrincess left"></span>
        <!-- Balloon -->
        <div class="nes-balloon from-left -left right">
          <p>First, We need to determine the <br/>
            <em>relative luminance</em> for each color.</p>
        </div>
      </div>
    </section>
    <h4 class="nes-text is-primary">1. Convert the HEX color to RGB.</h4>

    <div class="nes-table-responsive">
      <table class="nes-table is-bordered is-centered results ">
        <tr>
          <td id="rgb1">RGB 1</td>
          <td id="rgb2">RGB 2</td>
        </tr>
      </table>
    </div>
    <h4 class="nes-text is-primary">2. Divide each color by 255 to get a linear value between 0 and 1. </h4>

    <div class="nes-table-responsive results">
      <table class="nes-table is-bordered is-centered results">
        <tr>
          <td>
            r: <span id="c1-d-r"></span><br/> g: <span id="c1-d-g"></span><br/> b:
            <span id="c1-d-b"></span><br/></td>
          <td>
            r: <span id="c2-d-r"></span><br/> g: <span id="c2-d-g"></span><br/> b:
            <span id="c2-d-b"></span><br/></td>
        </tr>
      </table>
    </div>
    <h4 class="nes-text is-primary">3. Apply Gamma Correction</h4>

    <p>If value
      < than .03928, divide by 12.92.</p>
        <p> Otherwise, add .055 and divide total by 1.055 and take result to the power of 2.4</p>
        <div class="nes-table-responsive">
          <table class="nes-table is-bordered is-centered results">
            <tr>
              <td>
                r: <span id="c1-gc-r"></span><br/> g:
                <span id="c1-gc-g"></span><br/> b:
                <span id="c1-gc-b"></span><br/></td>
              <td>r: <span id="c2-gc-r"></span><br/> g:
                <span id="c2-gc-g"></span><br/> b:
                <span id="c2-gc-b"></span><br/></td>
            </tr>
          </table>

          <div class="message-list">
            <div class="message">
              <div class="nes-balloon from-right left">
                <p>This translates what the computer <em>"sees"</em> into the human perception of brightness.</p>
              </div>
              <div class="avatar contrastPrincess right"></div>
            </div>
            <div class="message">
              <div class="nes-balloon from-left right">

                <p>Computers record light directly. Twice the photons equals twice the brightness.</p>
              </div>
              <div class="avatar contrastPrincess left"></div>
            </div>
            <div class="message">
              <div class="nes-balloon from-right left">
                <p>Our eyes percieve more levels of light in dim conditions, and less in bright conditions. </p>
              </div>
              <div class="avatar contrastPrincess right"></div>
            </div>
          </div><br/>
          <h4 class="nes-text is-primary">4. Calculate luminance</h3>
            <h5>(r X .2126) + (g X .7152) + (b X 0.0722)</h5>
            <table class="nes-table is-bordered is-centered results">
              <tr>
                <td id="L1">Luminance 1</td>
                <td id="L2">Luminance 2</td>
              </tr>
            </table>
            <div class="message-list">
              <div class="message">
                <div class="nes-balloon from-right left">
                  <p>We multiply the values of each color by a number that signifies how bright that color appears to us. You'll notice green gets the highest number.<sup><a href="#fn1" id="ref1">1</a></sup> </p>
                </div>
                <div class="avatar contrastPrincess right"></div>
              </div>
            </div>
        </div> <br/>
        <h4 class="nes-text is-primary">5. Determine contrast between colors. </h4>
        <h5>(L1 + 0.05) / (L2 + 0.05)</h5><br/>
        <div class="nes-container cr is-rounded">Contrast Result</div>

<!--         <h4 class="nes-text is-primary">6. Apply the WCAG grading equation</h4>
        <h5>21 * (L1-L2)</h5><br/>
        <div class="nes-container wr is-rounded">WCAG Grade</div>
     -->
     <hr/>
        <h5>Credits:</h5>
        <ul>
          <li><a href="https://nostalgic-css.github.io/NES.css">NES.css</a></li>
          <li><a href="https://contrast-ratio.com/">Lee Verou's Contrast Ratio Site</a>(helped me understand the math)</li>
          <li>The Princess Icon by <a href="https://www.freepik.com/" title="Freepik">Freepik</a> from <a href="https://www.flaticon.com/" title="Flaticon">www.flaticon.com</a> is licensed by <a href="http://creativecommons.org/licenses/by/3.0/" title="Creative Commons BY 3.0"
              target="_blank">CC 3.0 BY</a></li>

        </ul>
        <h5>Reference</h5>
        <ul>
          <li><a href="https://www.cambridgeincolour.com/tutorials/gamma-correction.htm">Understanding Gamma Correction</a>
            <li><a href="https://www.sciencedirect.com/topics/computer-science/gamma-correction">Learn more about Gamma Correction</a></li>
            <li><a href="https://en.wikipedia.org/wiki/Relative_luminance">Relative luminance</a></li>
            <li> <a href="https://www.staciearellano.com/accessibility-resources/">Stacie's list of Accessibility Resources</></li>
          <li><span id="fn1">1.</span> <a href="https://hypertextbook.com/facts/2007/SusanZhao.shtml">Wavelength of Maximum Human Visual Sensitivity </a>Susan Zhao 2007</li>

        </ul>
   
    <p class="byline nes-container is-rounded is-dark">'Understanding Contrast' supports the article 'The Bright Stuff', posted at: <a href="https://www.staciearellano.com/the-bright-stuff/">staciearellano.com/the-bright-stuff/</a>, written by Stacie Arellano, 2019. <br /> <a href="https://twitter.com/dreamling"<i class="nes-icon twitter is-small"></i> @dreamling</a></p>
  </main>
</div>
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Handle Hex Short codes
Make small version tool
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CSS

            
              // https://nostalgic-css.github.io/NES.css/

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  // background-color:pink;
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.results{
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  margin:0 auto;
}



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$hair:"#FF6243";
$bangs:"#F47C5A";// not used at the moment. but tagged
$dress:"#B0DBD0;";
$dress-inset:"#2C7B7E";
$crown:"#FF9929";
$skin:"#F6DAB9";

.skin{
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.hair{
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  body { 
    padding: 0 .02rem;
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        font-size: 14px;
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  }
#Capa_1{
  // background-color:pink;
  // width:200px;
  // height:200px;
}

JS

            
              //set hexes via form element.
// Failing Contrast Colors Teal/Gray
// var hex1 = "1ABC9C";
// var hex2 = "888888";

// test colors
// ff0000 fffff ratio 3.99
// 8d8d8d fffff ratio 3.31
// 000000 fffff ratio 21?
var hexbox1 = document.getElementById("hex1");
var hexbox2 = document.getElementById("hex2");
var colorbox1 = document.getElementById("color1");
var colorbox2 = document.getElementById("color2");

// hexbox1.addEventListener("focusout", setHexandGo);
hexbox1.addEventListener("focusout", setHexandGo);
hexbox1.addEventListener("keydown", function(event) {
  if (event.key === "Enter") {
    setHexandGo();
  }
});
hexbox2.addEventListener("focusout", setHexandGo);
hexbox2.addEventListener("keydown", function(event) {
  if (event.key === "Enter") {
    setHexandGo();
  }
});

function setHexandGo() {
  console.log("********\nStarting New Contrast Comparison");
  // get hex
  var hex1 = hexbox1.value
    .split("#")
    .pop()
    .slice(-6);
  var hex2 = hexbox2.value
    .split("#")
    .pop()
    .slice(-6);
  //set some defaults if blank
  if (hex1 == "") {
    hex1 = "000000";
  }
  if (hex2 == "") {
    hex2 = "FFFFFF";
  }
  // display hex
  hexbox1.value = "#" + hex1;
  hexbox2.value = "#" + hex2;
  //display colors in boxes
  colorbox1.style.backgroundColor = hexbox1.value;
  colorbox2.style.backgroundColor = hexbox2.value;
  // todo? change display ie hair or bg here.

  //convert to rgb
  console.log("1. Convert Hex colors to RGB.");
  var rgb1 = hex2rgb(hex1);
  var rgb2 = hex2rgb(hex2);

  // display rgb
  document.getElementById("rgb1").innerHTML = rgb1;
  document.getElementById("rgb2").innerHTML = rgb2;
  
  // display rgb/255
  displayRgb(rgb1, rgb2);

  // display gamma corrected rgb
  displayGC(rgb1, rgb2);
  
  // get luminance
  var L1 = luminance(rgb1);
  var L2 = luminance(rgb2);
  console.log("returned luminances: "+L1+", "+L2);
    // display luminance
  document.getElementById("L1").innerHTML = formatNumbers(L1);
  document.getElementById("L2").innerHTML = formatNumbers(L2);
  
  // get contrast ratio
  var ratio = getContrastRatio(L1, L2);
  
  // not sure where i got this from...
  // var grade = 21 * Math.abs(L1 - L2);
  // grade = grade.toFixed(3);
  
  // display contrast ratio
  document.getElementById("ratioResult").innerHTML = ratio.toFixed(3) + ":1";
  document.getElementsByClassName("cr")[0].innerHTML = ratio.toFixed(3) + ":1";
// i could do a nicer format for 21 or 1 here...

  // display contrast score
  // document.getElementById("wcagResult").innerHTML = grade;
  // document.getElementsByClassName("wr")[1].innerHTML = grade;

  //display resultHearts
  resultHearts(ratio);

}

function formatNumbers(num) {
  // Display all numbers to 8 decimals, unless 1 or 0.
  if (num != 0 && num != 1) {
    num = num.toFixed(8);
  }

  return num;
}

function hex2rgb(hex) {
  // convert to rgb
  let rgb = [];
  //split rgb values out, and return the value by parsing the hex.
  for (var i = 0; i < 6; i += 2) {
    rgb.push(parseInt(hex.substr(i, 2), 16));
  }

  console.log("Hex: #" + hex + " to rgb: " + rgb);
  return rgb;
}

function luminance(color) {
  // Formula: http://www.w3.org/TR/2008/REC-WCAG20-20081211/#relativeluminancedef
  console.log(
    "DIVIDE EACH COLOR COMPONENT BY 255 TO GET A LINEAR VALUE BETWEEN 0 AND 1."
  );

  let rgba = color;
  var rgb_text = ["R", "G", "B"];
  var linear_rgb = [];
  for (var i = 0; i < 3; i++) {
    var rgb = rgba[i]; //take in the linear rgb value for each, r g and b.
    rgb /= 255; //divide by 255 to get each color's value between 0 and 1
    linear_rgb[i] = rgb;

    console.log("  " + rgb_text[i] + ": " + rgba[i] + " / 255: " + rgb);

    rgb = rgb <= 0.03928 ? rgb / 12.92 : Math.pow((rgb + 0.055) / 1.055, 2.4);
// console.log("rgb:" + rgb);
    rgba[i] = rgb; // reassign relative luminance corrected value.
  }
  console.log(`color/255: ${linear_rgb[0]}, ${linear_rgb[1]}, ${linear_rgb[2]}`);
  console.log(`rel-lumi : ${rgba[0]}, ${rgba[1]}, ${rgba[2]}`);
  console.log("APPLY GAMMA CORRECTION");
  console.log(
    "     If value is < .03928, divide by 12.92.\n     Otherwise, add .055 and divide total by 1.055\n     and take result to the power of 2.4\n"
  );
  
  var calcLumi = 0.2126 * rgba[0] + 0.7152 * rgba[1] + 0.0722 * rgba[2];

  /* the magic numbers here compensate for which colors the human eyes percieve as brighter, ie, green the most, blue the least. */
  console.log("Calculate and return luminance: " + calcLumi);
  
  return calcLumi;
}


function getContrastRatio(L1, L2) {
  // make sure to evaluate which color is largest/smallest? lighter/darker.
  // because luminance is 0 to 1 the larger number is brighter, 
 
   var ratio 
       // (L1 + 0.05) / (L2 + 0.05);
   var lum1 = (L1 + 0.05);
   var lum2 = (L2 + 0.05);
 
  if(lum1 > lum2){
    ratio = lum1/lum2;
  } else {
    ratio = lum2/lum1;
  }
// this was my bug, half the time i wasn't checking for light divided by dark, so the scoring was off. 
  // console.log("L1/L2 = " + (lum1/lum2));
  // console.log("L2/L1 = " + (lum2/lum1));

  console.log("Contrast ratio: " + ratio + ":1");
  return ratio;
}
// 0.2526 / 1.05 = 0.1684
function displayRgb(rgb1, rgb2) {
  // Display results for html viewing. (not part of final calc)
  
  // column/color - divideby255 - colorpart
  // console.error("rgb2:"+ rgb2);
  var color1_red = formatNumbers(rgb1[0] / 255);
  var color1_green = formatNumbers(rgb1[1] / 255);
  var color1_blue = formatNumbers(rgb1[2] / 255);
  document.getElementById("c1-d-r").innerHTML = color1_red;
  document.getElementById("c1-d-g").innerHTML = color1_green;
  document.getElementById("c1-d-b").innerHTML = color1_blue;

  var color2_red = formatNumbers(rgb2[0] / 255);
  var color2_green = formatNumbers(rgb2[1] / 255);
  var color2_blue = formatNumbers(rgb2[2] / 255);

  document.getElementById("c2-d-r").innerHTML = color2_red;
  document.getElementById("c2-d-g").innerHTML = color2_green;
  document.getElementById("c2-d-b").innerHTML = color2_blue;

}

function displayGC(rgb1, rgb2) {
  // Display results for html viewing. (not part of final calc)
  
  //column/color - gammacorrect - colorpart
  var color1_red = rgb1[0] / 255;
  var color1_green = rgb1[1] / 255;
  var color1_blue = rgb1[2] / 255;
  
  var color2_red = rgb2[0] / 255;
  var color2_green = rgb2[1] / 255;
  var color2_blue = rgb2[2] / 255;

  document.getElementById("c1-gc-r").innerHTML =  color1_red  < 0.03928
      ? formatNumbers(color1_red  / 12.92)
      : formatNumbers(Math.pow((color1_red  + 0.055) / 1.055, 2.4));
  
  document.getElementById("c1-gc-g").innerHTML = 
    color1_green < 0.03928
    ? formatNumbers(color1_green / 12.92)
    : formatNumbers(Math.pow((color1_green + 0.055) / 1.055, 2.4));
  
  document.getElementById("c1-gc-b").innerHTML = color1_blue < 0.03928
    ? formatNumbers(color1_blue / 12.92)
    : formatNumbers(Math.pow((color1_blue + 0.055) / 1.055, 2.4));

  document.getElementById("c2-gc-r").innerHTML = 
   color2_red  < 0.03928
      ? formatNumbers(color2_red  / 12.92)
      : formatNumbers(Math.pow((color2_red  + 0.055) / 1.055, 2.4));
    
  document.getElementById("c2-gc-g").innerHTML = 
    color2_green < 0.03928
    ? formatNumbers(color2_green / 12.92)
    : formatNumbers(Math.pow((color2_green + 0.055) / 1.055, 2.4));
    
  document.getElementById("c2-gc-b").innerHTML = color2_blue < 0.03928
    ? formatNumbers(color2_blue / 12.92)
    : formatNumbers(Math.pow((color2_blue + 0.055) / 1.055, 2.4));
}

function resultHearts(score) {
  var check = score;
  if ((score = "")) {
    console.log("need 2 colors");
    return;
  }
  var aaa = `<div class="icon-list">
              <i class="nes-icon is-medium heart is-full"></i>
              <i class="nes-icon is-medium heart is-full"></i>
              <i class="nes-icon is-medium heart is-full"></i><br/>
              <i class="nes-icon is-medium star is-full"></i>
    </div>`;
  var aa = `<div class="icon-list">
              <i class="nes-icon is-medium heart is-full"></i>
              <i class="nes-icon is-medium heart is-full"></i>
              <i class="nes-icon is-medium heart is-empty"></i><br/>
              <i class="nes-icon is-medium star is-empty"></i>
    </div>`;
  var aaL = `
<div class="icon-list">
              <i class="nes-icon is-medium heart is-full"></i>
              <i class="nes-icon is-medium heart is-full"></i>
              <i class="nes-icon is-medium heart is-full"></i><br/>
              <i class="nes-icon is-medium star is-empty"></i>
    </div>`;
  var fail = `<div class="icon-list">
              <i class="nes-icon is-medium heart is-empty"></i>
              <i class="nes-icon is-medium heart is-empty"></i>
              <i class="nes-icon is-medium heart is-empty"></i><br/>
              <i class="nes-icon is-medium star is-empty"></i>
    </div>`;
  //set all ratio ids to normal weight
  // document.getElementById("wcagChart").id[0].style.fontWeight = "normal";
  const listItems = document.querySelectorAll("#wcagChart li");
  for (let i = 0; i < listItems.length; i++) {
    listItems[i].style.fontWeight = "normal";
    // console.log( listItems[i]);
  }
  console.log(check);
  if (check >= 7) {
    document.getElementsByClassName("resultHearts")[0].innerHTML = aaa;
    // document.querySelector('[id^="ratioA"]').style.fontWeight = "normal";
    document.getElementById("ratio-AAA").style.fontWeight = "900";
    console.log("GOLD STAR: AAA");
  } else if (check >= 4.5) {
    document.getElementsByClassName("resultHearts")[0].innerHTML = aaL;
    document.getElementById("ratio-AAL").style.fontWeight = "900";
    console.log("Score: AA for large text");
  } else if (check >= 3) {
    document.getElementsByClassName("resultHearts")[0].innerHTML = aa;
    document.getElementById("ratio-AA").style.fontWeight = "900";
    console.log("Score: AA");
  } else if (check < 3) {
    document.getElementsByClassName("resultHearts")[0].innerHTML = fail;
    // document.querySelector('[id^="ratioA"]').id.style.fontWeight = "normal";
    document.getElementById("ratio-Fail").style.fontWeight = "900";
    // document.getElementById("ratioAAA").style.fontWeight = "400";
    console.error("CONTRAST FAILURE");
  }
}

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