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Simple removal of gradients and vignetting

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I'm sure many experts know about this function within PixInsight already, but I just wanted to mention this as a very quick and simple method using this free software from:

PixInsight — Downloads — PixInsight LE

I've only just downloaded this software and given it a go and found this particular function very quick and easy. There are more detailed instructions on the website and various other applications that will perform this funtion, but here's a few quick steps to using this software.

Open the image file containing the vignetting or gradient (see example below)

Press ALT+B to enter dynamic background subtration mode

Click anywhere on the image to open the dynamic background subtration window.

Select the GLOBAL tab and set the auto intervals to 15-20 for a typically sized DSLR image (the more the better, but longer processing times) and click GENERATE next to this number. You will see an even grid of points appear on the image.

Select any that are NOT part of the background and press delete to remove them.

Then click GENERATE at the bottom and wait for it to extract the background (see second attachment).

Select DONE twice. You can switch between the original image and tghe background under the Window menu. The background can be saved as any image file.

Click the pixel math button on the bottom left tool bar.

Select NEW in the PIXEL MATH window.

In the OPERATION pull down menu, select DIVIDE

In the OPERAND pull down menu, select your background image.

Check the 'create new image' box and click apply and then select your original light image from the options

You should end up with a similar result to the last image attached here.

I hope this helps some newbies, I certainly found it a great feature.




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It's a very useful program Rikyuu and thanks for posting the set of instructions. I have been using it for over a year and it a fantastic program. It's also worth experimenting with the subtract or absolute difference function instead of the divide which works better on some images. You may need to modify the operand R term to around 0.7 for each channel to avoid the image being too clipped



Edited by BeyondVision
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oh my goodness, I got a sticky! :) That's made my day now!

Back to topic; in the more detailed tutorial, there's some mention of setting the RGB values in the RESULT section of the PIXEL MATH window (although I don't really understand the reasons why, but it may be related to what you mentioned earlier Kevin). Sometimes this does not need to be done, but on a couple of occasions, the resulting image comes out strange, eg. totally white. If this happens, editing the RGB numbers as described seems to correct the issue.

It's quite simple to do;

With the original image active, select IMAGE > STATISTICS from the menu bar.

Using the median figures given for R, G and B in the statistics window, enter them into the R, G and B boxes in the PIXEL MATH result section, then proceed as before.


I briefly compared this technique with NC's actions in Photoshop, using the same image. This technique seems to do a better job of removing the gradient, but the results will probably vary from image to image. However, it's quite satisfying to see the actual background in it's extracted form before it gets removed.

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I cant get this to work. First: alt+B does nothing whatsoever here, and I can't find the global tab you refer to.

What version are you referring to?

Any chance of screen shots?

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On 24/06/2009 at 11:05, rikyuu said:

....not so useful then I guess!

This is one of those 'can't do without' bits of knowledge.

I note from later comments that the software is no longer available.
However, my guess is that this operation can be completed in Gimp.
... the only problem is figuring it out :grin:

However, your examples provide good clues.

I'll download the original image and the subtraction image, and figure it out from there
There's always Gimp Forum ;)


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