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

[Peter Suetterlin]

JD Smith wrote:
> On Sat, 2004-02-07 at 08:51, Pablo d'Angelo wrote:
> > On Wed, 04 Feb 2004, JD Smith wrote:
> > 
> > > On Wed, 2004-02-04 at 01:30, Pablo d'Angelo wrote:
> > > 
> > > As an astronomer who creates flat-fields all the time, I can tell you
> > > that it isn't conceptually difficult, although obtaining a true flat
> > > illumination source can be challenging (we typically use a "white spot"
> > > illuminated by a special lamp in the dome of the telescope, the bright
> > > twilight sky in the evening or morning, or, at wavelengths where the sky
> > > is bright enough already, combine large numbers of "science" images by
> > > rejecting objects to produce a "sky flat" -- sometimes a combination of
> > > all of these).  

Being an astronomer, too, I started doing this with my previous
camera, a Canon A50.  That one had a real bad vignetting.

I was taking the blue-sky approach, taking many images of the zenit
while rotating the camera.  The major problem I faced was that -
contrary to most astronomical flats - you are viewing a *huge* area,
some 60 degrees instead of <1 degree for astro ones.  So you start
getting problems with source non-uniformity.  And I'm even not sure if
not also polarisation would matter (which differs across the sky).

I gave it a second try with my new camera, but there the vignetting is
much smaller, so the flats did not really improve the result...

> > One thing I was thinking about: wouldn't in be possible to create the
> > flatfield from a panorama shot with > 50% overlap, by examining the
> > overlapping areas? The drawback is the super accurate registration
> > needed, or one has to restrict the matching to regions with uniform
> > color (sky with some trees in it, to allow registration). Hmm, probably its
> > easier to be more careful and shoot the special flatfield images.
> 
> This is possible, but very difficult, thanks to distortions.  

Depends on what you want to do.  Of course you cannot create a real
flatfield that corrects pixel-to-pixel variations (as it does in the
astro case), but rather an extremely smooth variation in
illumination. 

For my 'flatfield' for the A50 I averaged all images and then fitted a
4th order polynomial to the data set.  So that's 5 parameters, plus 2
more if you don't assume the lens centered on the chip.  So by
creating an approximate flatfield like described by Pablo and then
parametrising it should be enough to correct the vignetting problems.

> Much easier, in my opinion, to take a series of photos for which
> alignment is not required to create a flat.  Most users would skip this
> step, which is fine, as a good seaming algorithm with lots of overlap
> can overcome most of the artifacts in non-flatfielded data.

See above.  With 35mm or less of equivalent focal length you're not
really getting sources that are flat enough - at least for me that
simple approach failed.  I'll check if I can improve it using Pablos
suggestion - as soon as I find the time (currently I hardly find time
to read the list at all :-(( )

   Pit

-- 
Dr. Peter "Pit" Suetterlin                 http://www.astro.uu.nl/~suetter
Sterrenkundig Instituut Utrecht
Tel.: +31 (0)30 253 5225                   P.Suetterlin at astro.uu.nl