[vtk-developers] Charts and float/int conversion

[Marcus D. Hanwell]

On Tue, Sep 21, 2010 at 7:11 PM, David Gobbi <david.gobbi at gmail.com> wrote:

> On Tue, Sep 21, 2010 at 4:53 PM, Jim Peterson <jimcp at cox.net> wrote:
> > David Gobbi wrote:
> >>
> >> inline vtkContext2D::FloatToInt(float x)
> >> {
> >>  return static_cast<int>(x + 1.0625) - 1;
> >> }
> >>
> >> This provides a tolerance of 1/16 of a pixel for the float-to-int
> >> conversion. The "1" and "- 1" ensure that float values in the range
> >> [-1.0, 0.0] are rounded down, instead of being rounded toward zero.
> >>
> >>  David
> >
> > David,
> > pardon my ignorance, but if we are rounding float to int, why do we want
> > -0.1 to round to -1 and not 0? in other words, why not x + 1.5 instead of
> x
> > + 1.0625?
> >
> > Thanks,
> > Jim
>
> It all depends on what the rounding is being used for.  When
> rasterizing floating-point coordinates to the screen (i.e. to pixels),
> it is convention to round down.
>
> The IEEE 754 standard describes five rounding modes, each of which is
> valid for its intended uses:
> 1) Round toward 0, which is C standard float-to-int
> 2) Round towards -inf, which is like floor()
> 3) Round towards +inf, which is like ceil()
> 4) Round to nearest, towards even number if tie, default mode on many CPUs
> 5) Round to nearest, away from zero if tie
>
> My favorite mode is round to nearest, and towards the higher of the
> two integers in case of a tie.  Unfortunately that isn't in the
> standard. :P
>
> Thanks for the tips, I had been meaning to look into some of the rounding
issues. I will test this out and seeing about getting it in.

Marcus
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