Dust Removal with Flatfield
Dust can get into camera optics. In panorama images created from such photos the repetitive pattern can be particulary irritating, as a dust speck or a dust speck pattern tends to show up multiple times.
One can prepare a flatfield for correction. While one may correct the input files prior to stitching precessing each with the gimp for example, a much smoother workflow results if the stitching software itself provides for the use of a flatfield.
Flatfields may also be used to correct vignetting, or vignetting correction and dust removal combined. But even with hugin 0.7 using a parametrisation approach for vignetting, a suitably prepared flatfield is useful for dust removal. -- Klaus 19:11, 2 December 2007 (CET)
Plea for Flatfield
<img width=640 height=480 alt="foto" src="flatfield3.jpg">
This is a photo with dust in the lense. Here it is rather visible,
as my camera sometimes had the habit of showing a focussed image
of the dust specks as well. I have downscaled the images.
Shortly later the camera fell into that habit again, and I had a whiteboard nearby, so I set the lense aperture to the same value and took a photo of the featureless whiteboard.
<img width=640 height=480 alt="foto" src="flatfield1.jpg">
It is rather dark, but it shows the same dust specks
as the photo I want to correct.
<img width=640 height=480 alt="foto" src="flatfield2.jpg">
After some brightness adjustment and blurring (details on request)
with the gimp here is now the flatfield as used in the layer
with the mode set to "divide".
<img width=640 height=480 alt="foto" src="flatfield3.jpg">
Once again the uncorrected photo.
<img width=640 height=480 alt="foto" src="flatfield4.jpg">
And here the corrected photo. If one looks closely I did not get the flatfield completely right in the upper half of the image, as the white is blown the dust specks in the sky are under-corrected. See in particular the top left corner. with a little more time I probably could produce a better flatfield version. But the specks on the runway are pretty much gone.
I show this example with a view towards hugin, where recently a flatfield image was available but the most recent beta release does no longer have it in the GUI. In particular within a panorama stitch the dust specks have the nasty habit of showing up several times and forming a repetitive pattern which your eye is faster to spot than a single spot positioned at random.