[Radiance-general] Fwd: errors from hdrgen

[Giulio Antonutto]

My question arises from some experiments measuring the same paper surface
under different light conditions:

Unfortunately it appears that if the source has a really narrow spectrum
this affects the results within a 40% with my camera (which I cannot give
the name... 001DNOKIN ;-) ).

I am stressing the methodology to see if it is reliable enough to substitute
luminance readings for a wide range of conditions with consumer digital
cameras (street lighting - interior lighting - day lighting ).
I really underline the concept of 'wide use' (wide = cheap ;-) ).

With this sort of equipment I would like to achieve an error within +-10%
and therefore a correction factor quite constant (within +-10%); is it
feasible?


I found a very interesting product where the question is somehow explained:
http://www.technoteam.de/pdf/lmkrollei_e.pdf


"
Measuring variations1 
halogen, D65,...<5%
fluorescent lamp, MHN-T,...<10%
1 Due to spectral distributions deviating from CIE standard illuminant A
within the scene.
"

and also

"
Under certain circumstances, spectral distributions
deviating from the CIE standard illuminant A may
cause bad measuring errors.The value displayed can be corrected by applying
an additional factor (Colour Correction Factor, ccf) during recalibration
in the software. The calculation of the ccf value requires to know the
spectral distributions within the measuring scene or also to use a mean
correction factor...

...In addition, the user will also be provided with the relative spectral
sensitivity
of the single colour channels and the resulting relative spectral
sensitivity of standard matrix formation. Thus, he
will be able to calculate other ccf values or also other weighting factors.
"

This suggest to me, I am guessing, that calibration is really important,
that must be performed under very strict and controlled conditions and that
all the measurement should be within a reasonable range of conditions
similar to the calibration environment( if I calibrate with D65 I cannot
expect to measure accurately LPS or HPS, isn't it?).

Do you have any field experience about this?

thanks a lot,
giulio

PS - sorry for this long and labyrinth style email!

-----Original Message-----
From: radiance-general-bounces at radiance-online.org
[mailto:radiance-general-bounces at radiance-online.org]On Behalf Of Greg
Ward
Sent: 10 June 2004 16:23
To: Radiance general discussion
Subject: Re: [Radiance-general] Fwd: errors from hdrgen


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


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