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Re: convertToSpecifiedUnits and zero units

From: Nikolas Zimmermann <zimmermann@physik.rwth-aachen.de>
Date: Sat, 27 Aug 2011 11:56:29 +0200
Message-id: <DF535EC3-2ACC-4FF2-B0A8-2F6487CAA329@physik.rwth-aachen.de>
Cc: Řyvind Stenhaug <oyvinds@opera.com>, www-svg@w3.org
To: Dean Jackson <dino@apple.com>

Am 27.08.2011 um 03:36 schrieb Dean Jackson:

> On 24/08/2011, at 10:49 PM, Řyvind Stenhaug wrote:
>> Units
>> This doesn't seem to consider what happens if the unitType argument is a zero length, e.g. SVG_LENGTHTYPE_EMS when 1em=0px.
>> <svg xmlns="http://www.w3.org/2000/svg" font-size="0">
>>  <rect width="100px"/>
>>  <script type="application/ecmascript">
>>     onload = function() {
>>        try {
>>           var width = document.querySelector('rect').width.baseVal;
>>           width.convertToSpecifiedUnits(width.SVG_LENGTHTYPE_EMS);
>>           alert(width.valueInSpecifiedUnits + ' ' + width.value);
>>        } catch(e) {
>>           alert(e);
>>        }
>>     }
>>  </script>
>> </svg>
>> Gecko and Presto give "Infinity NaN", failing to "[p]reserve the same underlying stored value".
>> WebKit throws NOT_SUPPORTED_ERR, although the spec only says it's "[r]aised if unitType is SVG_LENGTHTYPE_UNKNOWN or not a valid unit type constant (one of the other SVG_LENGTHTYPE_* constants defined on this interface)".
>> (When trying to convert a zero value, Presto gives "0 0", Gecko "NaN NaN" and WebKit still throws.)
> The WebKit behaviour here is a bug which is easily fixed, but what values do you suggest we return?

I don't think it's a bug. This was my intention, if no font-size is available then throw here.
In general WebKit is much more strict about the SVG DOM primitives like SVGAngle, SVGLength, etc, when it comes to corner cases.
I think it's a bad idea to silently fail/ignore these things.

The scope of this problem is much larger. Think of handling percentage viewports on eg. <rect>'s rect.x.baseVal if the <rect> is not living in the document, only created dynamically.
I think failing silently, when someones tries to convert eg. 100px to percent, if the <rect> is not in the document, is a bad idea.

I wrote several tests for the SVG DOM primitives that nail down the WebKit specific throwing behaviour. (IIRC Gecko also throws in lots of cases).

Mostly interessting is eg this snippet form SVGLength-px.js: 
debug("Try converting from px to ems, should fail as the SVGLength is not associated with a SVGSVGElement, and thus no font-size information is available");

My opinion is that the SVG DOM primitives should be _much_ more strict and throwy. We've changed to this in WebKit trunk sth. like 10 months ago, and when I tested it back then, I saw FF also throw a lot more than speced, and it's a good thing IMHO.

Received on Saturday, 27 August 2011 10:00:57 UTC

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