[Radiance-general] Fwd: errors from hdrgen

[Giulio Antonutto]

just a quick extra question about HDR and light sources:

supposing that with a camera we have got some images,

1 we calculate the HDR image
2 we measure on the mac screen the luminance with ximage
3 we measure the luminance in the real scene.
4 we compare the numbers and find a factor ( k=lum(hdr):lum(real) ).


is this factor independent from the spectrum of the light source once that
the camera is 'calibrated' or it is related to it?
like in the case of: sun, low pressure sodium, MH, mercury lamps, theatre
blue filtered light....
is the correction factor (k) constant for these different conditions?
if not is there a best calibration environment (D65 or something else)?

many thanks,
giulio

-----Original Message-----
From: radiance-general-bounces at radiance-online.org
[mailto:radiance-general-bounces at radiance-online.org]On Behalf Of Axel
Jacobs
Sent: 10 June 2004 15:26
To: Radiance general discussion
Subject: Re: [Radiance-general] Fwd: errors from hdrgen


Martin Matusiak wrote:

>"Poor covergence of order 1 fit"
>  
>
Ignore this one, this is a debug message and only means that things are 
not running as smoothly as they could.

>"Cannot solve for response function"
>  
>
That is more of a problem. Essentially, the dynamic range of the images 
has gaps between them or is clipped on either side of the range. Try:

- Taking images at smaller f-stop intervals
- remove images that are too bright or too dark
- include darker or lighter images.

We keep having this problem with webhdr, but here doesn't seem to be a 
rule of thumb for getting it right. If the worst comes to the worst, you 
can disable hdrgen's built-in auto exposure routine and do a hand-jobby, 
but this is more difficult.
https://luminance.londonmet.ac.uk/webhdr/

You should also save the coefficients from a run that doesn't create any 
errors/warning and re-use them when you do get them. Please note that 
the response curve is/can be different even between two cameras of the 
same model/make, so don't mix-n-match them.

Axel


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