Write access in C library

[danielgruen]

Hi Erin,

do I understand correctly that the C library is currently used for reading MEDS filed, while write access is only implemented in Python? A C method to write a modified meds object would be of great use (well, at least to me), so unless you're working on it I probably would look into it (which may take a while due to my lack of understanding of the code).

Cheers,
Daniel

[esheldon]

Hi Daniel -

My code to write the MEDS files is in C++ and is in the shapelets svn
codebase. I put it there because it relies on some C++ classes written by
Mike. The svn archive is controlled by DESDM and is hidden away from the world.

Mike has been meaning to move our code over to github so people can look at
it. I can send you a snapshot of my MakeCutouts.cpp code if you want. . We
should bug Mike again to move the shapelets code.

As for this C library writing MEDS files, that would be pretty difficult to
do in full from DES data, because it involves dealing with all the WCS
transformations and book keeping that I've put in the C++ code. But if you
had already assembled the necessary pieces, as you would in a simulation, then
it would be straightforward to just dump it to a file.

But that sort of thing would be much easier in python (especially using the
fitsio package); in fact looking back at this now I should have written the
code in python. This would have required translating Mike's WCS class to
python, but that might not have been so bad.

-e

[danielgruen]

What I'd like to do is modify the image arrays read with meds.c in place and write the modified version to a new file, so no WCS library is required. In case MakeCutouts.cpp uses cfitsio to write the table, this would probably be something to start from. I just don't have much experience with cfitsio.
I would prefer to keep everything in C because I suspect it'll run much faster that way, or do you think that's not the case?
D

[esheldon]

What you are proposing is not a slow operation, so I doubt it matters how you implement it.

Note you can write into images in place using cfitsio so you could just make a copy of the meds file, funpack it (if you don't it will be slow!), and then read the data, modify it, and write it directly back.

[esheldon]

(you can also use the python package fitsio to write in place, not sure about pyfits)