[Radiance-general] getting pixel value at x,y

[Greg Ward]

Hi Lars,

So, this is why being specific about your problem helps -- we avoid a  
lot of spurious suggestions that don't apply.

The best/fastest solution for rectangular areas is pcompos piped into  
pvalue and total, like so:

	pcomb -o captured.pic \
		| pcompos -x ${box_width} -y ${box_height} - -${left} -${lower} \
		| pvalue -h -H -df | total -if3 -m

The initial pcomb command is necessary to apply the exposure values  
correctly.  If you want luminance (rather than radiance) output, use:

	pcomb -s 179 -o captured.pic \
		| pcompos -x ${box_width} -y ${box_height} - -${left} -${lower} \
		| pvalue -h -H -b -df | total -if -m

Either of these commands will be very fast compared to any script you  
might concoct.

-Greg

> From: "Lars O. Grobe" <grobe at gmx.net>
> Date: May 29, 2008 8:52:51 AM PDT
>
> First thanks for the replies and suggestions. What I actually want to
> do, is to sample pixel values over an area and get the average.  
> This is
> done for a HDR image assembled of photographs, so I cannot use vwrays.
> Pfilt also does not work, as I need the samples to hit a light  
> source in
> a black (dark) surrounding. What I did now is writing a script in perl
> which filters the output of pvalue, but still, as these files are  
> large
> and I am working on batches of such images, it is not that comfortable
> to do so (it takes time). I am taking these values into my script
> anyway, so writing this was not the problem, it is more about the
> unnecessary lookups of all those pixels that are just not that  
> interesting.
>
> CU Lars.