[Radiance-general] Fwd: errors from hdrgen

Thu Jun 10 19:34:30 CEST 2004

Hi all,

I`m not sure about this, but the way I understand the calibration procedure,
you have three curves (one for each rgb color) that relate the pixel values
in the jpegs to the luminance values. On the other hand, most cameras have
some built in correction functions for different light sources, so I guess
each correction function (daylight, tubes, etc.) will have different
response curves for the three colors (in order to compensate for the
different source spectra). However, if you use always the same correction
function in the camera (not the auto-correction, which is changing all the
time, this really got me confused for a while), you`ll get good results with
any light source (probably there is still some error caused by the rgb being
different from the CIE observer... is it?)

So, for example, if you make your calibration with the camera set up for
daylight, then you should use it always with the daylight correction, even
if you are measuring a scene lit up with tubes. The images will look
greenish, but the values will be calculated according to the correct
calibration curves. I`ve tried some measurings (not with such wide range of
conditions, but) using daylight compensation for an indoor scene, with good
results.
I hope this makes some sense, got too long...
Saludos,

Santiago

> -----Original Message-----
> From: radiance-general-bounces at radiance-online.org
> [mailto:radiance-general-bounces at radiance-online.org]On Behalf Of
> Giulio Antonutto
> Sent: Friday, June 11, 2004 1:06 AM
> To: 'Radiance general discussion'
> Subject: RE: [Radiance-general] Fwd: errors from hdrgen
>
>
> 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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