[GRASS-dev] r.profile tests

[Anna Petrášová]

On Mon, Oct 20, 2014 at 9:45 PM, Glynn Clements <glynn at gclements.plus.com>
wrote:

>
> Anna Petrášová wrote:
>
> > I was wondering, if r.profile shouldn't use atan2
> > instead of atan? It seems that the division in atan could possibly lead
> to
> > overflow? But nobody complained about it yet... Tests work with atan2.
>
> Using atan2() shouldn't be necessary, as each quadrant is handled
> separately. However, the due south case is problematic:
>
>             if (rows > 0 && cols >= 0) {
>                 /* SW Quad or due south */
>                 AZI = atan((rows / cols));
>
> as "cols" could equal zero.
>
> Floating-point division by zero won't result in overflow, it will
> result in infinity (which is representable). The problem is that it's
> not clear whether the argument to atan() will be positive infinity
> (resulting in AZI=pi/2) or negative infinity (resulting in AZI=-pi/2).
>
> If cols is -0.0, the argument will (typically) be negative infinity
> (IIRC, C implementations aren't required to preserve the sign of a
> zero).
>
> Given that AZI is only used for calculating X and Y, I would suggest
> using neither atan() nor atan2() but simply normalising the vector,
> e.g.
>
>         double k = res / hypot(rows, cols);
>         double X = k * rows;
>         double Y = k * cols;
>

Thanks for the suggestion, I committed it in 62327. It passed my tests.

Anna


> Note that this doesn't need a separate case for each quadrant.
>
> Technically, we shouldn't be using hypot() as it isn't part of C89 (it
> was added in C99). But it's already being used in:
>
>         lib/vector/diglib/prune.c
>         lib/vector/Vlib/line.c
>         lib/vector/Vlib/poly.c
>         lib/gmath/la.c
>         lib/gis/distance.c
>
> --
> Glynn Clements <glynn at gclements.plus.com>
>
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