[webkit-dev] TransformationMatrix rotation code and floating-point precision

[Shawn Singh]

OK, I will do that.

Thanks!
~Shawn


OK, I will do that.
>
> Thanks!
> ~Shawn
>
>
>
> On Mon, May 14, 2012 at 10:52 PM, Ryosuke Niwa <rniwa at webkit.org> wrote:
>
>> I don't know anything about transformation matrix, but that sounds like a
>> pretty simple change. I would just upload a patch to make this change on
>> bugs.webkit.org if I were you, and cc relevant people (use
>> trac.webkit.org to see who has authored/reviewed patches in the relevant
>> files).
>>
>> - Ryosuke
>>
>> On Mon, May 14, 2012 at 7:30 PM, Shawn Singh <shawnsingh at chromium.org>wrote:
>>
>>> I'm looking at TransformationMatrix::rotate3d(rx, ry, rz).  This code
>>> does something indirect, and I don't understand why.  Instead of
>>> initializing each rotation using sin(theta), cos(theta),  the code computes
>>> theta/2, and then uses trig identities to initialize the rotation matrix.
>>>
>>> I checked really quickly with fprintf, and it seems like we could
>>> actually gain 1-2 bits of precision if we avoid doing this, and use
>>> sin(theta) and cos(theta) directly.  In the current code, more error seems
>>> to accumulate due to sin^2 (theta / 2).  Squaring that value instantly
>>> increases the error inherent in the computation.  I cannot think of any
>>> valid reason that this code uses those trig identities instead of directly
>>> using sin and cos.  Does anyone else know why?  Is this worth changing to
>>> gain some precision?
>>>
>>> On a secondary note, its also fishy that we are freely mixing floats and
>>> doubles in the rotation code.  But, I don't think that is as significant
>>> error accumulation as the sin^2.
>>>
>>> Thanks,
>>> ~Shawn
>>>
>>> _______________________________________________
>>> webkit-dev mailing list
>>> webkit-dev at lists.webkit.org
>>> http://lists.webkit.org/mailman/listinfo.cgi/webkit-dev
>>>
>>>
>>
>
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