hdrexpo is a simple Perl script for adjusting the exposure in HDR images. This is useful for adjusting the luminance values following a spot-meter calibration.


Scripting language.
pvalue, total and falsecolor
Part of Radiance and only used for testing.


Below is the help output.

$ hdrexpo.pl --help
Usage: hdrexp.pl factor original.hdr > modified.hdr

How it works

The Radiance HDR image format consists of a text header, followed by the image body in 32-bit RLE binary:

CAMERA= NIKON E995 version E995v1.6
CAPDATE= 2006:01:07 17:14:12
PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290

-Y 960 +X 1280
<lots of binary garbage next...>

The photometric pixel value (in cd/m²) is the product of the pixel value stored in the image body and the exposure value from the header. By simply adusting the EXPOSURE= line in the header, the luminance values may be adjusted for the whole image. This is done by dividing the EV by the calibration factor.

Please note that I don't know if the implementation of the Radiange RGBE image format is correct for all applications in that it honours the image exposure. When in doubt, simply try it out.


The following example assumes that you have an HDR image, before.hdr, which requires exposure calibration like explained on WebHDR. The spot reading with a luminance meter gave a reading which is twice as high as the values taken from the HDR image, so the calibration factor is 2.0.

$ ls -l *.hdr
-rw-rw-r-- 1 axel axel 1492005 2008-11-03 18:50 before.hdr
$ hdrexpo.pl 2.0 before.hdr > after.hdr
$ ls -l *.hdr
-rw-rw-r-- 1 axel axel 1492005 2008-11-03 18:50 after.hdr
-rw-rw-r-- 1 axel axel 1492005 2008-11-03 18:45 before.hdr

To see if the result is as expected, we can use Radiance tools to compute the average image brightness...

$ pvalue -h -H -d -b -o before.hdr |total -m
$ pvalue -h -H -d -b -o after.hdr |total -m

..., or we could generate a false colour image like this:

$ falsecolor -s 2000 -l cd/m2 -n 10 -e -lw 60 -ip after.hdr > after_fc.hdr
$ falsecolor -s 2000 -l cd/m2 -n 10 -e -lw 60 -ip before.hdr > before_fc.hdr
before EV correction after EV correction
Left: False colour image with extreme values before exposure correction;
Right: The same image after running hdrexpo.



hdrexpo is published under the GNU General Public License.