Text:<br/> <textarea id="text" rows="7" cols="100">In a little district west of Washington Square the streets have run crazy and broken themselves into small strips called 'places'. These 'places' make strange angles and curves. One Street crosses itself a time or two. An artist once discovered a valuable possibility in this street. Suppose a collector with a bill for paints, paper and canvas should, in traversing this route, suddenly meet himself coming back, without a cent having been paid on account!</textarea>
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Secret: <input id="secret" style="width: 300px" type="text" value="Washington paints himself"/> <input type="button" value="Create" onclick="create()"/>
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<a href="https://github.com/trungdq88/ASCII-stereogram-generator" target="_blank">Github</a>
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<h3 style="text-align:center">Result</h3>
<pre id="result" style="text-align:center"></pre>
<script>
function create() {
  var arr = document.getElementById("text").value.split(' ');
  var secret = document.getElementById("secret").value.split(' ');
  var i = -1;
  var MAX_WIDTH = 25;
  var lines = [[]];
  var _l = 0;

  var result = '';

  while (i++ < arr.length - 1) {
      if ((_l + arr[i].length) < MAX_WIDTH) {
          if (secret.indexOf(arr[i]) > -1) {arr[i] = (_l?'  ':' ') + arr[i]}
          lines[lines.length - 1].push(arr[i]);
          _l += arr[i].length + 1;
          
      } else {
          _l = 0;
          i--;
          lines.push([]);
      }
  }

  for (var i = 0; i < lines.length - 1; putText(lines[i++].join(chars))) {
    // Align text
    var chars = ' ';
    while (lines[i].join(chars + ' ').length < MAX_WIDTH) {
      chars += ' ';
    }
  }
  putText(lines[i].join(' ') + ' ');
  function putText(line) {
    while (line.length < MAX_WIDTH) {
      line = line.replace(' ', '  ');
    }
    // Make the illusion
    result += line;
    for (var val in secret) {
      line = line.split('  '+secret[val]).join(' ' + secret[val] + ' ');
    }
    result += ('   ' + line) + '\n';
  }
  document.getElementById('result').innerText = result;
}
</script>