[Radiance-general] Luminance - second lowest value

[Greg Ward]

Hi Iris,

The easiest method is to simply exclude values below a certain lower threshold:

	pvalue –o –d –h –b –H picture.hdr | rcalc –e ‘$1=$1*179’ -e 'cond=$1-1e-6' | total –l

The "cond" variable in rcalc suppresses any records where cond<=0.  I have set the threshold here to 0.000001 watts/sr/m^2, but you can set it to whatever you like.

If you *really* want the second smallest value in a file, you can use the Unix sort command.  The following should give you the 4 smallest values (ignoring duplicates):

	pvalue –o –d –h –b –H picture.hdr | rcalc –e ‘$1=$1*179’ | sort -g -u | head -4

Finally, there is the Radiance "histo" command, which can generate a histogram of values, instead:

	pvalue –o –d –h –b –H picture.hdr | rcalc –e ‘$1=log10($1*179)’ |  histo -7 7

In the above example, I'm generating a histogram of 10-base log luminance values between 10^-7 and 10^7, and you can adjust this as desired.

Cheers,
-Greg


> From: Iris Moonen <moonen.iris at gmail.com>
> Subject: [Radiance-general] Luminance - second lowest value
> Date: December 4, 2015 4:14:47 PM GMT+01:00
> 
> Hello,
> 
> I want to calculate the second lowest value of the luminance of a
> picture in radiance.
> I used the command pvalue –o –d –h –b –H picture.hdr | total –l |
> rcalc –e ‘$1=$1*179’, to calculate the lowest value, but this gives me
> the value zero. This is indeed the lowest value in my picture, but
> therefor I'd like to get the second lowest value of the picture. Is
> there an easy way to get this, or is the only way to get this value by
> looking into the list of values for every pixel when I don't use the
> total command?
> 
> Best regards,
> Iris