[gdal-dev] Histograms without null pixels

Tue Mar 5 00:01:01 PST 2013

Caleb,

On Tue, Mar 5, 2013 at 11:53 AM, Caleb Hanger <cdhanger at gmail.com> wrote:

> On Tue, Mar 05, 2013 at 11:19:27AM +0530, Chaitanya kumar CH wrote:
>
> > Caleb,
> >
> > gdalinfo uses the GetDefaultHistogram() method which can do this faster
> by
> > fetching a precomputed histogram.
>
> What do you mean?  The histogram is only precomputed if gdalinfo has been
> run previously and stored a cache of data (the XML file) for later
> reference; please correct me if I'm wrong.  Otherwise, it seems to me that
> the data *must* be analyzed and the data computed; there is no way around
> that.  Additionally, I don't think it makes sense to say that it is faster
> to assemble a histogram that includes out-of-range values than to assemble
> a histogram that does not, because the latter is a subset contained within
> the former.
>
Some raster formats can store the histogram data as metadata. Also, there
are is a shortcut; overviews can be used to get approximate values faster.

>
> gdalinfo is a command line program. Even a default histogram of 256
> buckets
> > is hard to visualize in text mode. A much better way is to use a desktop
> > GIS like qgis to view the histogram.
>
> A desktop GIS is not practical for providing a shell script meant to
> automate a repetitive (and much larger) task, with a small chunk of data
> that it needs for a single step in the process.  That is, I don't need to
> "visualize" the data -- I just need to get the information.  The more
> primitive the format, the better, as I like things that are lean and mean,
> and I know how to instruct the script to do the required mathematics upon
> the text-only histogram.
>
You can write a simple python script to get the histogram. You can get a
good idea at
http://trac.osgeo.org/gdal/browser/trunk/autotest/gcore/histogram.py#L103

>
> > Can you justify adding this extra functionality in gdalinfo?
>
> Yes: quite simply, a histogram that does not include illegitimate values
> makes more sense than a histogram that does include them.  At least, this
> is the case in my experience; perhaps there are situations I'm unfamiliar
> with in which the histogram is desired to include values outside of the
> histogram's range, for some reason.  I'd even go so far as to say that
> gdalinfo currently *lies* about the histogram, telling the user that the
> histogram shows "256 buckets from X to Y" even though the histogram
> includes values which are *outside of that range*.
>
> Arguably an even better justification is that the machinery to exclude the
> illegitimate values is already present in GetHistogram, so *very* few lines
> of code would need to be added to gdalinfo, simply to provide a flag for
> the user and if that flag is specified, pass "false" for the right
> parameter to GetHistogram (bIncludeOutOfRange), instead of just calling
> GetDefaultHistogram.
>

That sounds reasonable. If you can, you should submit a patch or a request
at http://trac.osgeo.org/gdal/newticket
Note that the functionality should include the facility to include options
to mention the min/max values.

> > On Tue, Mar 5, 2013 at 10:56 AM, Caleb Hanger <cdhanger at gmail.com>
> wrote:
> >
> > > All,
> > >
> > > I am writing a script which is calling gdalinfo to get a histogram of
> > > pixel values in an image file.  Apparently the mechanism for obtaining
> a
> > > "default" histogram enables the "bIncludeOutOfRange" flag to dump
> > > not-in-range pixels (including null pixels, in formats that support it)
> > > into whichever bin of the histogram is closest.  I see that in the
> > > GetHistogram() function, the functionality is already coded to respond
> > > appropriately to either a true *or* false value in this flag.
>  However, I
> > > can't seem to find any way to turn that flag off through any of the
> > > user-level programs (in particular, gdalinfo), rendering the parameter
> to
> > > the function useless, unless I'm just not looking hard enough.  Hence,
> my
> > > questions:
> > >
> > > 1) Is there any standard user-level operation (preferably in gdalinfo)
> > > which disables, or at least *allows* for disabling of, the
> > > bIncludeOutOfRange flag?  If so, I can't find it.
> > > 2) If not, how come?  Has there been no apparent need for histograms
> that
> > > omit null values or other "out of range" situations?
> > >
> > > Thanks in advance for any insight,
> > > Caleb Hanger
> > > _______________________________________________
> > > gdal-dev mailing list
> > > gdal-dev at lists.osgeo.org
> > > http://lists.osgeo.org/mailman/listinfo/gdal-dev
> > >
> >
> >
> >
> > --
> > Best regards,
> > Chaitanya kumar CH.
> >
> > +91-9494447584
> > 17.2416N 80.1426E
> _______________________________________________
> gdal-dev mailing list
> gdal-dev at lists.osgeo.org
> http://lists.osgeo.org/mailman/listinfo/gdal-dev
>

-- 
Best regards,
Chaitanya kumar CH.

+91-9494447584
17.2416N 80.1426E
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