[bug #63171] Strange patterns on warped images

[Pedram Ashofteh Ardakani]

Update of bug #63171 (project gnuastro):

                  Status:             In Progress => Fixed                  
             Open/Closed:                    Open => Closed                 

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Follow-up Comment #19:

Hi all 👋

We've made the new commit [1] which discusses where these Moire patterns come
from, and some ways to mitigate/remove them in the 'Moire pattern and its
correction' section under 'Warp'.

[1] https://git.savannah.gnu.org/cgit/gnuastro.git/commit/?id=b58b49872b

Thanks to Mohammad's nice example demonstration in the Book [2] and the new
'--checkmaxfrac' option in Warp, we have a way to visually inspect the effect
- see (file jplus-warp-reg.jpg) and (file jplus-warp-reg-maxfrac.jpg), and
quantify it by interpreting the 'Statistics' of the 'MAX-FRAC' HDU which is
added to the Warped image if '--checkmaxfrac' is given, see [2].

[2]
https://www.gnu.org/software/gnuastro/manual/html_node/Moire-pattern-and-its-correction.html

Also, we can see that stacking the 'dithered' images helps resolve this issue
in the most precise way, potentially even *improving* the spatial resolution
of the image. It is worthy of note that stacking 'dithered' images can create
*deeper* images - see (file jplus-stack.jpg).

Please note that scales and limits are not exactly the same in the images
attached here, I tried to make them look similar. But you can make all of them
yourself following the instructions in [2].

Hopefully now things are making more sense. Please feel free to let us know
and continue the conversation if anything comes up.

Best,
Pedram

(file #53899, file #53900, file #53901, file #53902)

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Additional Item Attachment:

File name: jplus-warp-oversample.jpg      Size:575 KB
   
<https://file.savannah.gnu.org/file/jplus-warp-oversample.jpg?file_id=53899>

File name: jplus-warp-reg-maxfrac.jpg     Size:287 KB
   
<https://file.savannah.gnu.org/file/jplus-warp-reg-maxfrac.jpg?file_id=53900>

File name: jplus-warp-reg.jpg             Size:448 KB
    <https://file.savannah.gnu.org/file/jplus-warp-reg.jpg?file_id=53901>

File name: jplus-stack.jpg                Size:439 KB
    <https://file.savannah.gnu.org/file/jplus-stack.jpg?file_id=53902>



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