supportsCharacter.js

//A somewhat conservative test for supporting characters.

//Actually, it's a test to see if the glyphs of two characters
//are identical. This tends to work, however, some implementations
//may instead replace the text with a box with the hex code
//and this wouldn't work in those cases. One way might be to detect
//the presence of a rectangular box, but that may cause issues with
//glyphs that are rectangluar boxes (eg. Mail symbol).

//Basically, it compares pixels of the character at num and num+1
//if the glyphs are identical, it assumes its non-supported.
function supportsCharacter(num){
  function text(chrnum){
    //convert a character number into a ImageData.data array
    var x = document.createElement('canvas').getContext('2d');
    x.fillText(String.fromCharCode(chrnum),2,8);
    return x.getImageData(0, 0, 11, 12).data;
  }
  var x = text(num), y = text(num+1); 
  for(var i = x.length; x[i] == y[i] && i--;); //loop while pixels are equal or reached end.
  return i != -1 //if it reached the end, then the characters are the same
}