[gnuastro-devel] [task #14775] Convolution with varying PSF/Kernel over the image

[Mohammad Akhlaghi]

Follow-up Comment #2, task #14775 (project gnuastro):

After re-reading the suggestion, I thought of adding some further
clarifications to the suggestion before:

1. By "If the kernels are all the same size ...", I meant the size of the
kernel image (number of pixels it contains), not its FWHM. So for example, a
kernel with a smaller FWHM would be padded with zeros to have the same image
pixel "size" as that of a larger FWHM kernel. Ofcourse, it would be much more
efficient if each image only has the number of pixels necessary to contain it.
But that needs a little work (in checking the edges especially).

The kernel can also be larger than the image, although edge effects might not
be negligible in such cases (see the edge effects in the spatial
<https://www.gnu.org/software/gnuastro/manual/html_node/Edges-in-the-spatial-domain.html>
or frequency
<https://www.gnu.org/software/gnuastro/manual/html_node/Edges-in-the-frequency-domain.html>
domain).


2. In the suggested scenario (which is based on Gnuastro's modular nature),
the PSF interpolation (what region of the image needs what kernel) should be
done by another program. Such an interpolation is beyond the scope of the
Convolve program (for modularity). 

A separate program (maybe called "Interpolate" and called on the command-line
with `astinterpolate') can be added for such interpolations which can also do
things like interpolating over blank pixels and generally any kind of
interpolation/extrapolation. 

As in any interpolation, we can go to the highest resolution (estimate a
kernel for each pixel). But the basic idea of this scenario is this: how
realistic is such (infinite) accuracy? Given all the error sources we have and
also in the distant wings of the PSF which are the main concern, and our
limited computing power, certain regions can reliably be considered as
having/needing the same kernel.

But the important thing is that if this hypothetical interpolation program
doesn't do what a user wants, the user can always define their own labels and
kernels using their own tools (to the maximum resolution, if they want) and
feed them into Convolve.

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