color correction & vignetting [Re: [ptx] Hugin wishlist, RFC]

[Glenn Barry]

Hi All,

I've been lurking on this list for some time, I'm not a coder and currently
use PTAssembler, but the stuff you guys have been talking about coming up
with amazes me, especially the control point automation.

On the subject of colour correction I have a Fuji S1 which has uneven colour
from one side of the CCD to the other, I've made photoshop actions to
correct for this and vignetting when using the Sigma 8mm fisheye, but the
way I did it was to photograph a white evenly lit piece of paper and use the
RGB channels as masks for the colour correction.

I don't know how prevalent uneven colour across CCD's is but what would be a
great feature would to be take a calibration shot of a piece of evenly lit
white paper and use the light falloff/vignetting to calculate values for a
particular lense aperture combination and save those settings for radial
luminance correction.

Using this technique may help with the colour correction as well. With my
setup, before I made my photoshop actions I couldn't use the auto correction
in the original Panotools as from the centre the bad colour balance from one
edge of the CCD to the other would give a cast that went from yellow green
to magenta red from one edge of the panorama to the other because the centre
reference image had that transistion on the CCD. The colour corection merely
increased this progressively towards the edges of the panorama.

Now sorry for the rant, but if you have a calibration shot for a certain
lens/aperture you can adjust for any differences in light falloff and
vigentting from a known reference point. This may also help with calculating
colour correction for shots all taken with the same settings for any given
panorama. I know this adds one more step to maing panoramas, but given that
FOV, A, B and C calibration is a good idea anyway, the extra step to record
colour variation may make totally seamless output a reality.

What does anyone think?

BTW I am going to try and Port Hugin to SGI Irix as I've got an SGI Box here
that I want to make more use of(meaning fumble around and try to get it to
compile, about which I know next to nothing, but you have to start somewhere
;-)

Glenn

----- Original Message -----
From: "Pablo d'Angelo" <pablo at mathematik.uni-ulm.de>
To: "PTX mailing list" <ptx at email-lists.org>
Sent: Wednesday, February 04, 2004 7:30 PM
Subject: color correction & vignetting [Re: [ptx] Hugin wishlist, RFC]


> On Wed, 04 Feb 2004, Pablo d'Angelo wrote:
>
> > Before we start with exposure compensation we have to tackle another
lens
> > defect, namely vignetting. I found that my camera produces gray level
> > differences of ~ 40, if zoomed out.
> >
> > The pano tools plugins have a simple p = b*r^2 * p  (where p=pixel
value)
>
> Ehm, sorry, pano tools uses additive instead of multiplicative correction
>  a*R^2+b  (b=brighness), a=coefficent. See also:
> http://www.email-lists.org/pipermail/ptx/2003-October/000721.html
>
> I'm not sure if we should go with a formula (simple for the user), or if
we
> should use a flatfield, like it's done in astronomy. Comments welcome.
>
> I think the flatfield is harder to create. on the other hand, maybe its
> feasible to fit the parameters for the correcting, radial function if
images
> with high overlap and without scene changes are used.
>
> ciao
>   Pablo
> --
> http://wurm.wohnheim.uni-ulm.de/~redman/
> Please use PGP
>