The Tor Browser / file revision

 function makePattern(len, start, inc) {

   var pattern = [];

   while(len) {

     pattern.push(start);

     start = (start + inc) % 256;

     --len;

   }

   return pattern;

 }

 function setPattern(imageData, pattern) {

   if (pattern.length != imageData.data.length)

     throw Error('Invalid pattern');

   for (var i = 0; i < pattern.length; ++i)

     imageData.data[i] = pattern[i];

 }

 function checkPattern(imageData, pattern) {

   if (pattern.length != imageData.data.length)

     throw Error('Invalid pattern');

   for (var i = 0; i < pattern.length; ++i)

     if (imageData.data[i] != pattern[i])

       return false;

   return true;

 }