[Radiance-general] Fwd: errors from hdrgen

[Greg Ward]

Hi Martin,

I believe that hdrgen isn't coming up with a good response function for 
your camera with this scene, probably due to the strong color cast of 
the walls.  Since hdrgen (and Photosphere) compute the responses for 
each of the RGB channels, using a lot of colorful patch samples can 
skew the result.  I've never seen such an extreme example before, 
though -- most walls are off-white so this hadn't come up.  Come to 
think of it, the tendency of cameras to super-saturate their colors 
could spell trouble in such cases, and I probably should modify my 
algorithm to avoid colorful patch samples.

Thanks for the image set -- I'll run some experiments on my end to see 
if I can improve on this.  It is important to fix the white balance 
whenever you take an HDR sequence, as the camera in auto white mode 
will alter the coefficients between exposures otherwise, making it 
impossible to get a consistent result.

-Greg

> From: Martin Matusiak <alex at juventuz.net>
> Date: June 11, 2004 5:17:29 AM PDT
>
>> I hdr'ed all ouf the 13 images in one go, and apart for some 'poor
>> convergence' (orders 1, 3, 4 and 5) warnings, it worked fine. There is
>> no apparent distortion.
>
> I repeated the process using the manual mode of the camera rather than
> aperture priority. Again 13 images (same room, scene was a bit 
> different as I
> eliminated the sky in the framing) and I was able to compute a hdr. 
> But the
> alignment problem still persists, I use the -a switch to turn it off.
>
> Later I took a couple more images and used the calibration data to 
> compute a
> hdr and read the luminance values off it. It turns out that those are
> somewhat accurate so that's a good sign!
>
> However, I'm still not quite satisfied. Obviously, the calibration 
> procedure
> is meant to give a most accurate static description of the camera, but 
> I used
> the camera's automatic whitebalance correction, so that's a variable. 
> My
> question is a complete novice one: how does the whitebalance affect the
> luminance? What can I do to eliminate this variable (if it is 
> significant).
>
> Martin