[bug #63171] Strange patterns on warped images

[Mohammad Akhlaghi]

Follow-up Comment #3, bug #63171 (project gnuastro):

This bug inspired me to define task #16272. It will be very useful in
understanding the distortion of a camera, and visualizing things like this.

While writing that bug, the following test occurred to me (making an image
with all input pixels equal to one and then applying the warp. This should
show how many input pixels have been used in each output pixel 


$ astarithmetic img.fits set-i i i - 1 + -o one.fits

$ astwarp one.fits


Here is the output: as we see, we don't see any such pattern in the number of
pixels that are used in the output pixel grid. 

(file #53808)

So this is indeed, very confusing for me! We don't see this effect in the
noise of other images with distortion that we have tested! 

The strong change in smoothing level that we see in the second image of
Comment #1 hints that a significantly different number of input pixels are
being used, but the test above doesn't show that! Either the test is wrong, or
there is another cause for this strongly different smoothing!)

We have to investigate what is happening here!


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