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

[Pablo d'Angelo]

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).  

Yes, thats the difficult part about it. One has to create the flatfields.
As you noted below, the good thing is that they handle arbitrary light
falloff.

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.

> Creating a high-quality flat field is usually best achieved with a
> diffuse (non-spotlight) lighting source on a neutral white screen
> (poster paper, etc.).  Take many (10 or more) images at each f/#
> setting, and average them together.  You might also adjust the lighting
> somewhat between images to average over any illumination gradients. 
> White balance setting on your camera may also affect the measured flat.

I'll play around with that some time in the future.

> Once correctly flat fielded, variations in intensity and color are due
> to differences to true lighting (a cloud covered the sun), exposure
> times (left camera on auto-exposure), and possibly spatial variations on
> the detector itself (green increasing from left to right, etc).  A paper
> that Pablo pointed out seems to have the best approach to blending I've
> encountered, and is worth considering for implementation:
> 
> http://leibniz.cs.huji.ac.il/tr/acc/2003/HUJI-CSE-LTR-2003-82_blending.pdf

I have found another paper of interest:

"Mapping Colour in Image Stitching Applications":
http://ivrgwww.epfl.ch/publications/hs03_JVCIR.pdf

It doesn't really take vignetting into account (they assume better cameras
than I have ;). But its more concerned with recovering the transfer function
of the camera, a bit similar to HDR stuff we briefly mentioned earlier.

ciao
  Pablo
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