[AstroPy] FITS image data dtype
Miguel de Val-Borro
miguel.deval at gmail.com
Wed Jul 17 16:52:35 EDT 2013
On Wed, Jul 17, 2013 at 04:28:07PM -0400, Erik Bray wrote:
> >> FITS files store arrays in big-endian order natively. Actually swapping the
> >> bytes to native order can be costly, especially when the data is being accessed
> >> by way of an mmap'd file (the default in PyFITS 3.1 and up). And it's generally
> >> unnecessary when working within Numpy--it is only necessary if passing those
> >> bytes to other software that is expecting values in native byte order.
> > The matplotlib function imshow shows the original map differently than
> > after swapping the byte order to little-endian format.
> > imshow(hdulist.data)
> > https://docs.google.com/file/d/0B3Hv-zV2q5IcSjk2amw3ZmM3Mk0/edit
> > imshow(hdulist.data.byteswap().newbyteorder())
> > https://docs.google.com/file/d/0B3Hv-zV2q5IcTWZ2WXduMWttQUE/edit
> > If I remember correctly I also had problems with numpy array operations
> > on the maps with the '>f8' dtype for this kind of image data.
> Hmmm.. This sounds really familiar but I think it's actually a bug in matplotlib.
> Just be sure why don't you make sure that np.all(hdulist.data ==
> hdulist.data.byteswap().newbyteorder()) If that doesn't return True I'll eat
> my shoe.
Yes, it returns True. It could be a bug in matplotlib and not something
related to pyfits, I will try to find more information to debug the
problem. At least it's good to know that swapping the byte order helps
to be able to display the image.
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