[Radiance-general] Fwd: errors from hdrgen

[Greg Ward]

Hi Martin,

To get a reasonable fit to a particular camera, Photosphere (or hdrgen) 
needs more than just a few exposures.  I find that it likes at least 7 
exposures, better 9,  of a wide dynamic-range scene.  Each exposure 
should be separated by an f-stop or so by varying the shutter speed 
only.  This is explained in the Photosphere quickstart_pf.txt file, 
which I have quoted on this list recently.  (See my response to Barbara 
Matusiak on May 28 under the subject "conversion of digital pictures to 
Radiance?")  Once you have established the response function for a 
particular camera, you can then reuse it for a shorter exposure 
sequence, or even a single image (though you should not expect a high 
dynamic-range result in that case).  This storage and reuse is 
accomplished with hdrgen's -r option, or via the preferences file in 
Photosphere.

If the algorithm cannot arrive at a reasonable response function for 
your camera, Photosphere offers the option of applying a generic 
response function, instead.  Although I don't recommend this if your 
goal is accuracy, the following response file will permit hdrgen a 
similar fall-back using the -r option:

2 1 0 0
2 1 0 0
2 1 0 0

This simply assigns each channel the polynomial f(x) = x^2, which 
roughly corresponds to a standard gamma curve.  It's really a crude 
approximation, but since you are taking overlapping exposures, the 
global errors in the mid-exposure region are reasonably small.

By the way, I managed to convert both your sequences without complaints 
in Photosphere, though the three-image sequence is a bit blurry.  
Obviously, the alignment algorithm didn't quite work on this one.  
Unfortunately, it doesn't know when it's failed, so no errors or 
warnings are issued in most cases.

-Greg

P.S.  In response to Pillo's inquiry, the luminance reported by ximage 
should correspond roughly to the luminance you would measure with a 
photometer, accounting for the photopic response of the probe's filter. 
  However, you are definitely better off measuring something as close to 
white as possible, and preferably not a light source -- something more 
in the middle of the exposure range, like a grey card.  The latest 
version of Photosphere even includes a calibration option to make this 
work easier.

> From: Martin Matusiak <alex at juventuz.net>
> Date: June 10, 2004 5:20:10 AM PDT
>
> Terribly sorry for sending that mammoth attachment to the list, I 
> didn't
> realize it was that big before it was too late. :(
>
> Here they are instead:
> http://www.juventuz.com/_temp/good_set.tar.gz
> http://www.juventuz.com/_temp/bad_set.tar.gz
>
> Martin
>
> Subject: errors from hdrgen
> Date: Thursday 10 June 2004 13:11
>
> I seem to have trouble using hdrgen to generate hdr's.. The other day 
> it
> seemed to be working fine but now I'm getting errors like
> "Poor covergence of order 1 fit"
> "Cannot solve for response function"
>
> Ultimately it won't generate the image. Perhaps you could tell me what 
> they
> mean and what I'm doing wrong? I've attached the five images that gave 
> this
> error. I've also attached another set of 3 that gave no error or 
> warning at
> all, even though the resulting hdr is quite blurry.
>
> Thanks a lot!
>
> Martin