PanoTools:
Re: My attempt to eliminate vignetting effect
JD Smith 2004-Nov-30 16:26:25
On Tue, 30 Nov 2004 00:22:28 +0000, smensch01 wrote:
>
>
> Hi,
>
> If I'm not totally wrong, vignetting should not change along a series
> of photos with the same settings, right? So would it be an option to
> embed a 'vignetting profile' for the current setting? This way
> vignetting doesn't need to be guessed.
>
That's correct. The best way to estimate the flat field, which
include vignetting, pixel to pixel responsivity variations, etc., is
to photograph a flat, evenly illuminated surface (possibly out of
focus), median combine a large number of these images, and normalize
to the mean (possibly after some spatial sigma-clipping to remove
bad/hot pixels outliers). This gives you a nice function, which will
likely fall off roughly radially, and, as Pablo mentions, may not be
centered on the sensor center. This could be used as is to divide
into images before they are ever seen by PanoTools (or better during a
floating point or 16bit workflow inside of PT), or could be fit with a
radial profile, the parameters of which are included in, e.g. hugin
for a noiseless correction before image transformation. Ideally this
would all be done in linear response space, which means undoing the
gamma correction built into the camera JPG encoder, or using RAW, pre
tone-mapping (and white-balance correction, sharpening, etc.).
In reality, most people won't want to do this, so some in situ
estimation of a rough radial vignetting correction function may still
be appropriate. For typical 30% overlap, the exact center of the
function probably won't matter too much.
JD
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