Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-09-05 Thread Nikos Alexandris 

Nikos Alexandris:


> Updated in sandbox, see: https://trac.osgeo.org/grass/changeset/69365
> Is it though correct?


Vaclav Petras wrote:


I was not looking at the logic but

if (hue == -1.0) {

is not safe. Although it will probably work most of the time if you set it
to the exactly same value (`hue = -1;`),


Glynn Clements  [2016-09-05 16:46:31 +0100]:


There's no "probably" about it. If you execute "hue = -1;", a
subsequent "if (hue == -1)" comparison *will* succeed.

Rounding error only occurs when the correct result isn't exactly
representable, which isn't the case here. It doesn't occur randomly.


If using -1 is good, it seems simpler (to me, the ignorant) than trying
introducing yet another boolean flag.

Nikos
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Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-09-05 Thread Glynn Clements 

Vaclav Petras wrote:

> > Updated in sandbox, see: https://trac.osgeo.org/grass/changeset/69365
> >
> > Is it though correct?
> >
> 
> I was not looking at the logic but
> 
> if (hue == -1.0) {
> 
> is not safe. Although it will probably work most of the time if you set it
> to the exactly same value (`hue = -1;`),

There's no "probably" about it. If you execute "hue = -1;", a
subsequent "if (hue == -1)" comparison *will* succeed.

Rounding error only occurs when the correct result isn't exactly
representable, which isn't the case here. It doesn't occur randomly.

-- 
Glynn Clements 
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Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-09-05 Thread Nikos Alexandris 


Nikos Alexandris:


Updated in sandbox, see: https://trac.osgeo.org/grass/changeset/69365
Is it though correct?



Vaclav Petras:


I was not looking at the logic but

if (hue == -1.0) {

is not safe. Although it will probably work most of the time if you set it
to the exactly same value (`hue = -1;`), but it is not portable because it
is exact comparison for floats/doubles. Generally, you need to do something
like

if (fabs(hue - (-1.0)) < GRASS_EPSILON) {

However, here it might be more appropriate to signal the good or bad state
of hue by some other variable:

hue_good = FALSE;

/* setting hue here together with hue_good = TRUE */

if (!hue_good) {

/* do the null set */


Thanks Vaclav. Will try that out -- a quick run tells me that I didn't
understand yet how to do it. Will get to it.


Also, it seems that the code is not consistent with FCELL versus DCELL. I
can see DCELL *rowbuffer and Rast_is_d_null_value but also
(FCELL)saturation as well as floats and doubles. Since the input claims to
be always DCELL, use DCELL/double everywhere.


See my other post, if you have some time. Ideally, it would be smart to
have the input be autodetected, be it CELL or DCELL, ranging in [0,
2^bitness-of-input-data) while the output will be always FCELL (hue in
[0.0,360.0] and saturation/lightness in [0.0,1.0].


Related to that, although I'm never sure about where is its appropriate to
use DCELL and where double (DCELL currently gets evaluated to double:
typedef double DCELL;). I would probably use DCELL everywhere in this
function; just to make it clear that we deal purely with cell values and
there is no need to switch to anything else.


But using DCELL everywhere is a bit of waisting, if I understand
correctly. I'd like to keep things clean, even if it appears to be
unnecessary to do so on first sight.

Nikos
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Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-09-04 Thread Vaclav Petras 
On Sun, Sep 4, 2016 at 9:45 AM, Nikos Alexandris 
wrote:

> Updated in sandbox, see: https://trac.osgeo.org/grass/changeset/69365
>
> Is it though correct?
>

I was not looking at the logic but

if (hue == -1.0) {

is not safe. Although it will probably work most of the time if you set it
to the exactly same value (`hue = -1;`), but it is not portable because it
is exact comparison for floats/doubles. Generally, you need to do something
like

if (fabs(hue - (-1.0)) < GRASS_EPSILON) {

However, here it might be more appropriate to signal the good or bad state
of hue by some other variable:

hue_good = FALSE;

/* setting hue here together with hue_good = TRUE */

if (!hue_good) {

/* do the null set */

Also, it seems that the code is not consistent with FCELL versus DCELL. I
can see DCELL *rowbuffer and Rast_is_d_null_value but also
(FCELL)saturation as well as floats and doubles. Since the input claims to
be always DCELL, use DCELL/double everywhere.

Related to that, although I'm never sure about where is its appropriate to
use DCELL and where double (DCELL currently gets evaluated to double:
typedef double DCELL;). I would probably use DCELL everywhere in this
function; just to make it clear that we deal purely with cell values and
there is no need to switch to anything else.

And of course, congrats and kudos for your C efforts, Nikos!

Vaclav
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Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-09-04 Thread Nikos Alexandris 

Nikos Alexandris:


   if (chroma == 0.0) {

   saturation = 0.0;

   /* undefined -- (how to) set to NULL? */
   hue = 0.0;



I'd like to set hue to NULL, whenever the condition is met. How do I do
that?


Markus Neteler:


Something like:

   for (row = 0; row < nrows; row++) {
   G_percent(row, nrows, 5);
   for (col = 0; col < ncols; col++) {
   cell_buf[col] = bas[SEG_INDEX(ba_seg, row, col)];
   if (cell_buf[col] == 0)



   Rast_set_null_value(_buf[col], 1, CELL_TYPE);


Alright, that is:  anything that `_buf[col]` points to, set the
number of values `1` to NULL.


Updated in sandbox, see: https://trac.osgeo.org/grass/changeset/69365

Is it though correct?

Nikos
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Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-09-04 Thread Nikos Alexandris 

* Markus Neteler  [2016-08-27 15:37:35 +0200]:


On Sat, Aug 27, 2016 at 3:26 PM, Nikos Alexandris
 wrote:

While modifying i.rgb.his,

   if (chroma == 0.0) {

   saturation = 0.0;

   /* undefined -- (how to) set to NULL? */
   hue = 0.0;

   rowbuffer[0][sample] = (FCELL)hue;
   rowbuffer[1][sample] = (FCELL)intensity;
   rowbuffer[2][sample] = (FCELL)saturation;
   }


I'd like to set hue to NULL, whenever the condition is met. How do I do
that?

   hue = NULL;

does not work.


No, it is done withRast_set_null_value()

Something like:

   for (row = 0; row < nrows; row++) {
   G_percent(row, nrows, 5);
   for (col = 0; col < ncols; col++) {
   cell_buf[col] = bas[SEG_INDEX(ba_seg, row, col)];
   if (cell_buf[col] == 0)



   Rast_set_null_value(_buf[col], 1, CELL_TYPE);


Alright, that is:  anything that `_buf[col]` points to, set the
number of values `1` to NULL.

I need a few more examples to understand.  What is the "number of values"?
Could it be a series of multiple values?

Nikos



   }
   Rast_put_row(basin_fd, cell_buf, CELL_TYPE);
   }

(random example, taken from r.water.outlet/main.c)

HTH,
Markus


--
Nikos Alexandris | Remote Sensing & Geomatics
GPG Key Fingerprint 6F9D4506F3CA28380974D31A9053534B693C4FB3 
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Re: [GRASS-dev] How to: if undefined, set to NULL?

2016-08-27 Thread Markus Neteler 
On Sat, Aug 27, 2016 at 3:26 PM, Nikos Alexandris
 wrote:
> While modifying i.rgb.his,
>
>if (chroma == 0.0) {
>
>saturation = 0.0;
>
>/* undefined -- (how to) set to NULL? */
>hue = 0.0;
>
>rowbuffer[0][sample] = (FCELL)hue;
>rowbuffer[1][sample] = (FCELL)intensity;
>rowbuffer[2][sample] = (FCELL)saturation;
>}
>
>
> I'd like to set hue to NULL, whenever the condition is met. How do I do
> that?
>
>hue = NULL;
>
> does not work.

No, it is done withRast_set_null_value()

Something like:

for (row = 0; row < nrows; row++) {
G_percent(row, nrows, 5);
for (col = 0; col < ncols; col++) {
cell_buf[col] = bas[SEG_INDEX(ba_seg, row, col)];
if (cell_buf[col] == 0)
Rast_set_null_value(_buf[col], 1, CELL_TYPE);
}
Rast_put_row(basin_fd, cell_buf, CELL_TYPE);
}

(random example, taken from r.water.outlet/main.c)

HTH,
Markus
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