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Revision as of 23:26, 10 July 2020

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Orientation of FITS images

The FITS standard is a container that describes how to store image data and meta-data. Professional tools, from the early age of the FITS format, like ds9 (Harvard Smithsonian Center for Astrophysics), fv (FITS viewer from NASA), store images bottom-up. We might be tempted to say that it does not really matter, but when demosaicing or astrometry is involved, problems arise. For example, the usual RGGB Bayer pattern becomes GBRG if the image is upside-down.

Nowadays, despite this, most camera drivers are writing data in the top-down order and we have to cope with it.

For these reasons, we recently have introduced, together with P. Chevalley of CCDCiel, a new FITS keyword. We encourage all data producers, INDI and ASCOM developers, to use it in order to make things easier for everybody.

This keyword is ROWORDER of type TSTRING. It can take two values: BOTTOM-UP and TOP-DOWN.

Siril will always read and display images in the bottom-up order, however if the top-down information is specified in the keyword, then Siril will demosaic the image with the corrected pattern.

Why would some programs write images bottom-up in the first place? The reason is: mathematics do it that way.