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removing the bright sun from an image

Posted: 30 Jul 2020, 19:10
by cardanoc
Hi

I took this photo of the sun and overexposed it in order to view the prominences.

Is there a method that I can use to remove the full disk of the sun and leave only the prominences.

I tried using masks, but could not figure it out.

Thank You.

Conrad

Re: removing the bright sun from an image

Posted: 31 Jul 2020, 14:15
by fabdev
Hi, it can be done both via image processing and via masks. Here are a couple of examples:

1) Make a duplicate of the image (F2) then Filter / Histogram Stretch to attenuate the prominences and keep only the disk. If the image is 8 bit write "255" as New Range. Apply, then OK.
2) Filter Gauss with Sigma 20 to smooth the disk:
3) Again filter Histogram Stretch to reduce the faded border to 2-3 pixels. "255" for 8 bit range. Center the ramp in the middle of the window.
4) Filter Statistical / Erosion with Cell = 3 to decrease the size of the disk by 2-3 pixels.

5) Now select the original image and subtract the disk. Here's the result:
If the white border is too big or too small, then Undo, and use Erosion/Dilation in in the disk image to adjust its size. Then repeat the subtraction.

Re: removing the bright sun from an image

Posted: 31 Jul 2020, 14:31
by fabdev
Using masks, the procedure is the same of graphical programs (for example Gimp).

1) Open the mask window. Place the mouse cursor at position 465,943 (these are the Left and Top coordinates of the sun, they can be measured moving the mouse there) and use the Ellipse tool to draw the mask on the disk. There's no need to be precise because you will move the mask at point 3.

2) Smooth the mask, Sigma = 3.


3) Move the mask, with the "One pixel" buttons so it's centered on the image.

4) Menu Image , Fill Inside with value = 0.
Here's the result:

If you have a normal exposure of the sun, you could insert it into the saturated image, instead of making the disk black.
1) Same procedure as above, but on the normal exposure.
2) Again smooth the mask.
3) Copy to clipboard.
4) Paste, Into Image, then use the arrows to position correctly the new disc into the image.

Re: removing the bright sun from an image

Posted: 31 Jul 2020, 18:40
by cardanoc
Thank You!

It worked well!

Conrad
Rhode Island
USA

Re: removing the bright sun from an image

Posted: 03 Aug 2020, 23:59
by merlin66
Conrad/ Fabio,
There's an even easier way...
I use Sylvain's CORONO software.
http://sweiller.free.fr/Softwares/Corono/corono.html