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[bug #63171] Strange patterns on warped images

From: Raul Infante-Sainz
Subject: [bug #63171] Strange patterns on warped images
Date: Thu, 6 Oct 2022 11:59:59 -0400 (EDT)


                 Summary: Strange patterns on warped images
                 Project: GNU Astronomy Utilities
               Submitter: infantesainz
               Submitted: Thu 06 Oct 2022 03:59:57 PM UTC
                Category: Warp
                Severity: 3 - Normal
              Item Group: Output not reasonable
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any


Follow-up Comments:

Date: Thu 06 Oct 2022 03:59:57 PM UTC By: Raul Infante-Sainz <infantesainz>
In Gnuastro 0.18.65-7933d, the output of warped images have strange patterns.
I found this issue when testing the new features on some images. Let me
reproduce and describe the problem step by step.

I attach an image to star with: 'img.fits'. It is a cropped region nearby to
the galaxy NGC 493. Image of 'float32' data type. Nothing special.

I convert this image to another one of type 'uint8'.

astarithmetic img.fits uint8 -o uint8.fits

Now just warp this image and have a look.

astwarp uint8.fits -o uint8-warp.fits

astscript-fits-view uint8.fits uint8-warp.fits

There is a clear pattern across the image, like an interference pattern.

If you are wondering why I am doing such a strange things, it is just because
I was warping bigger images and I found the same pattern (on normal 'float32'
images). Attached a screenshot (warped-float32.png). Then, when I tried to
upload the image to Savannah I had to decrease the size (by converting it to
integers). On this converted images the problem is even more clear.

To confirm that it is not a problem that comes from the original image I did
the following test: create an image with pure noise, warp and check it.

astarithmetic img.fits set-i i i - 3 mknoise-sigma -o noise.fits

astwarp noise.fits -o noise-warp.fits

astscript-fits-view noise-warp.fits

File Attachments:

Date: Thu 06 Oct 2022 03:59:57 PM UTC  Name: warped-float32.png  Size: 678KiB 
 By: infantesainz

Date: Thu 06 Oct 2022 03:59:57 PM UTC  Name: img.fits  Size: 1MiB   By:



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