[whatwg] Canvas API: What should happen if non-finite floats are used

Tue Sep 7 21:41:09 PDT 2010

Consider this testcase:

<!doctype html>
<html>
   <body>
     <canvas id="c" width="200" height="200"></canvas>
     <script>
     try {
       var c = document.getElementById("c"),
       t = c.getContext("2d");
       t.moveTo(100, 100);
       t.lineTo(NaN, NaN);
       t.lineTo(50, 25);
       t.stroke();
     } catch (e) {alert(e); }
     </script>
   </body>
</html>

Behavior in the spec seems to be undefined (in particular, no mention is 
made as to what the canvas API functions are supposed to do if 
non-finite values are passed in).  Behavior in browsers is:

Presto: Throws NOT_SUPPORTED_ERR on that lineTo(NaN, NaN) call.
Gecko: Throws DOM_SYNTAX_ERR on that lineTo(NaN, NaN) call.
Webkit: Silently ignores the lineTo(NaN, NaN) call, and then
         draws a line from (100,100) to (50, 25).

Seems like the spec needs to define this.

-Boris

P.S.  This isn't a hypothetical issue; this came up in a page that was 
trying to graph things using canvas and ending up with divide-by-0 all 
over the place.  It "worked" in webkit (though not drawing the right 
thing, so much).  It failed to draw anything in Presto or Gecko.