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[task #15128] TPV and SIP keywords when WCS distortions are necessary

From: Sachin Kumar Singh
Subject: [task #15128] TPV and SIP keywords when WCS distortions are necessary
Date: Mon, 18 May 2020 19:12:36 -0400 (EDT)
User-agent: Mozilla/5.0 (X11; Ubuntu; Linux x86_64; rv:75.0) Gecko/20100101 Firefox/75.0

Follow-up Comment #20, task #15128 (project gnuastro):

I read the wcslib's official documentation and now have an understanding of
it's working, at least for the current task. I am enlisting the points below:

* the `dis.h` the header has all the necessary structures and functions for
distortion corrections.

* The main structure, `dispram`, has the first element flag whose value is to
be set as 0 when initialising the `dp` dpkey structure. The initialisations
are done through an internal function `disinit()` whose value is to be set as
-1 for first initialisation only. 

* The keywors for TPD distortion like DPja or DQia are all placed inside the
`dpkey` structure.
* The initialisation in the the `dispram` is done through the function
`disset()`. For this particular task, it will automatically be invoked by
disp2x() function.

* The dishdo() function tells wcshdo() to write FITS headers in the form of
the TPD translation used internally, but I still have to figure out if it will
be of any use.

* As wcspih() is a more general function to load the TPV and SIP keywords into
`dispram`, it makes more sense to use it rather than specific ones(like
wcsset() for TPV to TPD conversion of keywords).

* To specify the particular distortion to be converted into TPD, we can use
the `dpram` array in `dispram` to store specific coefficients of distortions
in the general TPD coefficients for automatic detection and conversion. I
still have to figure out how to go about it.

This is a general overview of how the read and write function's programmatic
workflow. If you find any improvements or changes please feel free to


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