[Radiance-general] pcomb-masking color channels

[Humann Chris]

Thanks Greg and Ieble,

Iebele, that's a great start for me to play around with.  Ultimately  
I'm trying to isolate the blue sky from white clouds in a single HDR  
image to get an approximation of how overcast the sky is.

Kind regards,
Chris

Christian Humann ~ Associate
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On Mar 30, 2012, at 4:11 AM, Iebele Abel wrote:

> Hi Chris,
>
> I made a mistake in last my mail, here the corrected line:
>
> If the value is too high (not too low), the result will be just a  
> copy of blue_sky.hdr.
>
> Iebele
>
>> Hi Chris,
>>
>> I had something similar a few weeks ago, and figured out this:
>>
>> pcomb -e 'sens=1.5;difr=(ri(1)-sens)-ri(2);difg=(gi(1)-sens)-gi(2);  
>> difb=(bi(1)-sens)-bi(2); ro=if(difr,ri(1),ri(2));  
>> go=if(difg,gi(1),gi(2)); bo=if(difb,bi(1),bi(2)); '  sun.hdr  
>> blue_sky.hdr   |   ximage
>>
>> You can tweak the "sens" variable to get the result you want. In my  
>> case (which was not a sky picture) the value was around 0.9 to get  
>> the result I was after. The value you need to mask the sun out of  
>> the sun.hdr image must probably be higher. If the value is too low,  
>> the result will be just a copy of blue_sky.hdr.
>>
>> The command first finds the difference between pixel values for  
>> red, green and blue in sun.hdr and blue_sky.hdr. The difference  
>> (tweaked by 'sens' ) determines from which input file the pixel  
>> component value is written to the output.
>>
>> If you are only interested in changing the luminance values of  
>> blue_sky.hdr at those spots where the sun (or clouds) are bright in  
>> sun.hdr you may try something like this:
>>
>> pcomb -e 'sens=1.5;difl=(li(1)-sens)-li(2);ro=if(difl,  
>> li(1)*ri(2),li(2)*ri(2) ); go=if(difl, li(1)*gi(2),li(2)*gi(2) );  
>> bo=if(difl, li(1)*bi(2),li(2)*bi(2) );  ' sun.hdr blue_sky.hdr |  
>> image
>>
>> Using these commands, the sun (and clouds) will appear with sharp  
>> boundary edges (not blurred) in the resulting composite image.
>>
>> Hope this helps a bit,
>>
>> Iebele
>>
>>
>>
>> Op 30 mrt. 2012, om 07:45 heeft Greg Ward het volgende geschreven:
>>
>>> Hi Chris,
>>>
>>> Seems no one answered this, but I believe there are experts out  
>>> there who have played with sky blending techniques.  I won't name  
>>> any names, you know who you are....
>>>
>>> I'm heading out on a little vacation tomorrow with a million  
>>> things to do first, otherwise I'd take a crack at it.  Maybe you  
>>> could explain a little more clearly the effect you are going for?
>>>
>>> Best,
>>> -Greg
>>>
>>>> From: Humann Chris <chris at coolshadow.com>
>>>> Date: March 27, 2012 3:19:15 PM PDT
>>>>
>>>> Dear group,
>>>>
>>>> I'd like to be able to generate two separate images from an HDR  
>>>> image of the sky vault: one with blue sky; the other with clouds  
>>>> and sun (assuming it's not occluded).  I'm not well versed in the  
>>>> usage of 'pcomb' but I'm assuming I'll need to somehow create a  
>>>> mask based on both color and brightness thresholds.  It seems  
>>>> from my archive search that a call to 'pgblur' with a high radius  
>>>> might help in defining sharper boundaries between clouds and blue  
>>>> sky. (http://www.radiance-online.org/pipermail/radiance-general/2008-May/005046.html 
>>>> )
>>>>
>>>> Has anyone been successful in achieving something similar?
>>>>
>>>> As always, thanks!
>>>>
>>>> Chris
>>>
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