[Radiance-general] pcomb photometrically correct

[Jack de Valpine]

Hi Axel,

If you want to derive light sources from an HDR you should take a look 
at mksource, this will evaluate the scene and then output as set of 
illum sources.

-Jack

Axel Jacobs wrote:
> Dear all,
>
> I'm trying to process an HDR image to pick out the light source. The image
> was taken with hdrgen:
> RGBE: http://luminance.londonmet.ac.uk/pickup/004.hdr
> falsecolor: http://luminance.londonmet.ac.uk/pickup/004_fc.jpg
> Header:
> $ head -10 004.hdr
> #?RADIANCE
> CAMERA= NIKON E990 version E990v1.1
> hdrgen created HDR image from '0129_012.jpg' '0129_011.jpg' '0129_010.jpg'
> '0129_009.jpg' '0129_008.jpg' '0129_007.jpg' '0129_006.jpg' '0129_005.jpg'
> '0129_004.jpg' '0129_003.jpg' '0129_002.jpg' '0129_001.jpg'
> Removed lens flare
> EXPOSURE=4.175030e-01
> CAPDATE= 2007:01:29 17:33:57
> PRIMARIES= 0.6400 0.3300 0.3000 0.6000 0.1500 0.0600 0.3127 0.3290
> FORMAT=32-bit_rle_rgbe
> -Y 768 +X 1024
>
> What I'm after is to black out all pixels below a certain threshold, and
> leave all others, i.e. the ones that are part of the light source
> untouched.
>
> Firstly, a quick test-run to warm up:
> $ pcomb -e 'lo=li(1)' -o 004.hdr > test.pic ; ximage test.pic &
> This does nothing but show the brightness. The -o ensures I get
> photometric units.
>
> Fine so far. BUT: If I run the same command straight into ximage, the
> picture looks the same, but hitting 'L' displays a luminance of 0.0 for
> all pixels (the square light source should have a luminance of just under
> 500 cd/m2):
> $ pcomb -e 'lo=li(1)' -o 004.hdr |ximage
> This is the first thing that I don't understand.
>
> Second question: To set the threshold to 400 cd/m2 like described above, I
> run:
> $ pcomb -e 'lo=if(li(1)-400/WE,li(1),0)' -o 004.hdr > test.pic ; ximage
> test.pic &
> WE is the white efficacy, which seems to be read-only: Setting WE=2000
> doesn't do anything.
> URL: http://luminance.londonmet.ac.uk/pickup/400.png (the light only --
> rest is black)
>
> >From pcomb(1):
> "... WE gives the white efficacy (lumens/brightness) for pixel values,
> which may be used with the -o option or the  le(n) values to convert to
> absolute photometric units (see below)"
>
> The exposure value le(n) is the same for the entire picture (in my case
> just over 0.4), so is it correct to say that I only have to worry about it
> when combining multiple images? And that WE is equally useful with li(n),
> not just li(n) and -o?
>
> So combining multiple images with different exposures would be done like so:
> $ pcomb -e 'lo=li(1)/le(1)+li(2)/le(2)' -o 004.hdr 006.hdr > test.pic
> Right?
>
> Thanks for shedding some light on the issue.
>
> Cheers
>
> Axel
>
>
>
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>
>   

-- 
# Jack de Valpine
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