Markus Neteler wrote:
> I have an extra issue: For the particular growing degree day calculation
> I am facing the problem that the values I am comparing with a FP values
> and that GRASS_EPSILON is useless in this particular case. It works
> only with
>
> /* EPSILON = GRASS_EPSILON; */
> EPSILON = 10.
> if (fabs(tval - values[i]) < EPSILON ) {
>
> because the pixel values are growing so quickly from one map to the
> next. Since we need a generic solution, another "precision" or whatever
> parameter needs to be passed otherwise the thresholding fails completely.
> EPSILON however depends on the input maps and needs to be user
> controllable.
>
> How to deal with that?
What exactly are you trying to measure?
>From your original post, it sounds like you have a monotonically
increasing input, and you want the time (index) at which the value
first exceeds the threshold, e.g.:
for (i = 0; i < n; i++) {
if (values[i] >= threshold) {
*result = (DCELL) i;
return;
}
}
Rast_set_d_null_value(result, 1);
If you need an approximate equality comparison, the usual solution is
to use the ratio of the difference to one of the values, e.g.:
double EPSILON = 1e-6; /* 1 ppm */
if (fabs(values[i] - threshold) < EPSILON * threshold)
...
--
Glynn Clements <[email protected]>
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