[cairo-bugs] [Bug 4846] Performance problems for GTK+/Cairo on MacOS X

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https://bugs.freedesktop.org/show_bug.cgi?id=4846          
     




------- Additional Comments From bogdanni at hotmail.com  2005-10-27 16:43 -------
I took a closer look at what Bill Spitzak actually proposed, i.e. it does rounding of 65536 * val, and I 
think the code is correct. The one I put in comment #16 does actually chopping like on http://
www.stereopsis.com/FPU.html. Now I _really_ feel like an idiot.

The rounding is done in the sense of second definition in comment #16, i.e. half-way numbers are 
rounded to the closest even.

It's easy to overflow this operation, so some kind of input checking is useful. I reported I was getting 
huge numbers as input to _cairo_fixed_from_double(), it was because of the code mentioned in https://
bugs.freedesktop.org/show_bug.cgi?id=2675, comment #9:
     y_intersect = _cairo_fixed_from_double ((b2 - b1) / (m1 - m2));

I think is better to do the operation through a union, but check with a C language lawyer.
So the code would be like that:

#if __BIG_ENDIAN_
#define iman_ 1
#else
#define iman_ 0
#endif

cairo_fixed_t
_cairo_fixed_from_double (double d)
{
    union { double d; int32_t i[2]; } u;

    u.d = d + 68719476736. * 1.5;
    return u.i[iman_];
}

Sorry for any confusion I caused.
          
     
     
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