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I have this nice Lappland Autumn morning panorama, except the polariser effect is obviously too big just to the right of the small top on the right.

panorama

I would like to try to apply a kind of mask with a softly shaped triangular wedge covering the deepest blue part, but with a gradient around it, so that the corrections blend in better.

Once I have that, I can selectively manipulate the blue in GIMP.

So the first iteration of the question is: How do I do a mask with a gradient around a freehand shape in GIMP?

Alternatively, does anybody know a better way of correcting the sky? Thank you!

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    Panoramas plus polarizers perpetuate problems pending post processing. Prefer pre-processing. Preclude polarized panorama photographs. – user13451 Apr 6 '15 at 2:53
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one way to reduce the contrast of polarised areas in sky is to do as follows (applies equally to PS and GIMP):

  1. Using the freehand lasso tool, select the area(s) of highest contrast sky, including a reasonably wide margin. It is important that this margin extends somewhat into the non-polarised sky, and is of consistent width.
  2. Feather the selection by a large amount - e.g. 120-250px (depending on resolution).
  3. Using the Levels tool, adjust the mid-tones to the right (decrease contrast) by 5-20%; only as much as you think looks natural. You want to avoid creating a new light band at the edge of your feathered selection.
  4. Invert the selection. The non-polarised section of sky is now selected.
  5. Using the Levels tool again, move the mid-tones slider to the left this time (increase contrast) by 5-10%; again only as much as you can get away with without making the image look too 'shopped.

Done.

The overall effect here is to slightly lighten the dark polarised sky and slightly darken the rest of the sky, thereby reducing the polarising effect slightly. There is no doubt a more effective way to do this, but the above method is quick (10 seconds) and can often make the difference between rejecting and keeping a panorama.

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    While the basic process is sound, it would probably be a better idea to use a luminosity mask (Gimp version here, Photoshop equivalent all over the web) to allow the problem area of the sky to essentially select itself, so as to avoid incongruous edges and other artifacts. Manual selection/erasure/what have you will work well to eliminate other areas of the picture from the correction, but if you can get the problem to do its own heavy lifting, you win. – user38275 Apr 6 '15 at 11:48

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