[Radiance-general] HDRI - Camera Response Curve

[Gregory J. Ward]

Hi Jack, Axel & Rob:

> From: Jack de Valpine <jedev at visarc.com>
>
> Axel Jacobs wrote:
>>
>>> So as to scene, clear sunny sky conditions would be ideal I  
>>> suppose with a range from shadowed features to direcly  
>>> illuminated features... Sorry for this next... So how do you use  
>>> the luminance measure from the reference card to further  
>>> calibrate the response curve?
>> Have a look at http://luminance.londonmet.ac.uk/webhdr/ 
>> calibrate.shtml In the header of the RGBE image you'll find an  
>> exposure value. Multiply this with your calibration factor. This  
>> CF will be roughly one, but not quite.
> Hey this is great! Though I am still not exactly clear what to do  
> with the CF if it is other than 1.0? Am I using this multiplier to  
> correct the exposure value of the image?

You can actually edit the header using the getinfo command in the  
following sequence:

getinfo < orig.hdr > fixed.hdr
vi fixed.hdr
getinfo - < orig.hdr >> fixed.hdr

In your editor (vi in the example above), either replace the  
EXPOSURE= line with a corrected value, or add a new EXPOSURE= line  
with the correciton, equal to image_value/measured_value.

Photosphere makes this easy by allowing you to select a region and  
enter the correct luminance value via the pop-up "Calibration" menu  
item on the "Apply" button of the image display window.  You also  
have the option of applying this calibration factor to the camera's  
recorded response function, so it will adjust future exposure  
sequences from the same camera.

>>> In the book (the new one;-) you also indicate that the darkest  
>>> exposure should have no RGB values greater than ~200 and the  
>>> lightest no values less than ~20. I could certainly figure out a  
>>> filter routine with pcomb, however there must be a simple way to  
>>> do this with ImageMagick. The point being to run a quick  
>>> preprocess check on the the bounding images. Any takers or  
>>> suggestions for how to do this easily?
>> The netpbm utilities allow you to easily create histograms. I use  
>> this in WebHDR: For each uploaded image, the histogram is plotted,  
>> and the exposure value given. Give it a try, if you like.
> Great! I will check it out.

Photosphere also has a histogram feature, as well as live false color  
luminance display.  It might be worth $500 to get a Mac mini --  
cheaper than a copy of Photoshop.

-Greg