[cairo] Reduce number of floating point operations

[Behdad Esfahbod]

[CCing cairo list back]

On Wed, 2006-09-20 at 20:03 +0000, Aivars Kalvans wrote:
> Behdad Esfahbod wrote:
> > Hi Aivars,
> >
> > I like most of your patch.  Comments below:
> >
> > On Thu, 2006-09-14 at 18:30 -0400, Aivars Kalvans wrote:
> >   
> >>  _cairo_matrix_is_identity (const cairo_matrix_t *matrix)
> >>  {
> >> +#if defined (__arm__)
> >>     
> >
> > We should do something about this.  Either a configure test or option to
> > detect FPU, or hardcode one way or the other.  Anybody sees any reason
> > why hardcoding memcmp is a bad idea?
> >   
> No real reason, it should be memcmp.
> (I was thinking about 4 fp instructions vs. function call, but compiler
> should be able to inline this)

The inlined memcmp is faster on all systems.


> >> +    /* memcmp() should be faster than 4 soft float operations */
> >> +    static const cairo_matrix_t identity = {
> >> +           1.0, 0.0,
> >> +           0.0, 1.0,
> >> +           0.0, 0.0
> >> +    };
> >> +    return (memcmp (matrix, &identity, 4 * sizeof(double)) == 0);
> >> +#else 
> >>      return (matrix->xx == 1.0 && matrix->yx == 0.0 &&
> >> -           matrix->xy == 0.0 && matrix->yy == 1.0 &&
> >> -           matrix->x0 == 0.0 && matrix->y0 == 0.0);
> >> +           matrix->xy == 0.0 && matrix->yy == 1.0);
> >> +#endif
> >>     
> >
> > Why is it called is_identity if it doesn't check for translation?
> >   
> According to wikipedia (http://en.wikipedia.org/wiki/Identity_matrix)
> identity matrix is n x n, so third row in cairo_matrix_t actually is a
> translation vector.

Right, so the wikipedia article just confirms that a 6-item matrix
cannot be identity.  However, it can be an identity transformation:

  http://en.wikipedia.org/wiki/Identity_transform

and that's most of the time what the intention is.

As for your case, I suggest cairo_matrix_is_translation().

-- 
behdad
http://behdad.org/

"Commandment Three says Do Not Kill, Amendment Two says Blood Will Spill"
        -- Dan Bern, "New American Language"