[GRASS-dev] Fwd: Re: Upcoming 7.2.0: review which addons to move to core

Vaclav Petras wenzeslaus at gmail.com
Thu Oct 6 14:34:40 PDT 2016

On Tue, Oct 4, 2016 at 4:55 PM, Anna Petrášová <kratochanna at gmail.com>

> [...]
c) test correctness of results.
> It just depends how you write them, and yes, for some modules c) is
> more difficult to implement than for others.

On Thu, Oct 6, 2016 at 5:00 PM, Anna Petrášová <kratochanna at gmail.com>

> There is correct result in the sense "as author intended". So you can
> use pen and paper and solve very small example. That doesn't mean it
> will cover all options, but better than nothing. [...]

For r.forestfrag, I wrote a test which was based on an example in the
original paper which was computing value of one cell in 3x3 window. It is a
really trivial example which tests 1 number and two other numbers which are
intermediate results. However, by writing it, I actually discovered that
although the results for a large area look good, this small example, which
I was able compute by hand, is failing. After investigation, it turned out
that the error is actually in r.mapcalc. Computing the result outside of
GRASS was crucial in this case. It was an index and I was able to compute
it by hand (and check it with the example in the paper). For some other
modules it could be more difficult and I don't want to compute it without
GRASS for more than one cell even in this case, but it was certainly
possible to write a test of correctness for this module. Note that the test
doesn't show if the (forest fragmentation) index makes sense or if it is
useful, but it shows that the module gives the result it promised.

This is the original test:


This is the r.mapcalc bug:


And this is test which specifically shows the bug (would show if it would
be reintroduced):

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