[Radiance-general] HDRI - Camera Response Curve

[Gregory J. Ward]

Hi Jack,

I've just been playing around with this, myself.  To get a good  
response curve, it is best to start with a scene that has a daylight  
color balance, as this is the design point for most cameras, and some  
models end up boosting the blue too much if you calibrate under  
incandescent lighting.  For this reason, I prefer calibrating with  
natural light, as opposed to a controlled condition with incandescent  
or fluorescent lighting.  In any case, you should fixed a white  
balance setting on the camera appropriate to the test lighting.   
(Memory aid:  If the white balance isn't fixed, then it's broken.)

Color charts are of limited use, except as a means to verify your  
calibration.  The more images you take in a sequence, and the more  
closely they are spaced, the less you rely on the recorded camera  
response function.  This is because the overlap of the images serves  
(together with the known shutter speeds) to give you an accurate  
result, regardless.  It is more important to have an absolute  
calibration value if you are after real numbers, and for this a  
luminance measurement on a reference card of known reflectance is  
invaluable.

-Greg

> From: Jack de Valpine <jedev at visarc.com>
> Date: January 9, 2006 9:19:54 AM PST
>
> Hi All,
>
> Happy New Year first off.
>
> I am making a first pass through the "High Dynamic Range Imaging"  
> book that Greg co-authored. I am wondering about methods for  
> generating a good response curve for a given camera. The book gives  
> a variety of tips and suggestions, however I am curious about good  
> scenes.
>
> In a prior email footnote from Greg (a faq item relating to camera  
> response curves and Photosphere), he suggests shooting a scene  
> looking out a window during daytime with about 10 exposures. What  
> about shooting a controlled scene such as a Macbeth Color Checker  
> or a Kodak gray scale chart (can't remember the name of this)  
> outdoors under daytime conditions or using some type of interior  
> fixed lighting setup?
>
> -Jack