[fitsbits] top or buttom

[Eric Greisen]

Juergen Liesmann writes:
 > Hi
 > 
 > If the first pixel of a fits-image like this:
 > 
 > SIMPLE  =                    T / Standard FITS format flag                      
 > BITPIX  =                   32 / Bits per pixel                                 
 > NAXIS   =                    2 / Number of dimensions                           
 > NAXIS1  =                  496 / Row length                                     
 > NAXIS2  =                  390 / Number of rows                                 
 > END
 > 
 > at the top left corner or at the buttom left corner?
 > 
 > -- 
 > Juergen Liesmann
 > http://home.t-online.de/home/liesmann
 > Email: liesmann at t-online.de

The proposed WCS convention paper states:


\subsection{Image display conventions}

It is very helpful to adopt a convention for the display of images
transferred via the FITS format.  Many of the current image processing
systems have converged upon such a convention.  Therefore, we
recommend that FITS writers order the pixels so that the first pixel
in the FITS file (for each image plane) be the one that would be
displayed in the lower-left corner (with the first axis increasing to
the right and the second axis increasing upwards) by the imaging
system of the FITS writer.  This convention is clearly helpful in the
absence of a description of the world coordinates.  It does not
preclude a program from looking at the axis descriptions and
overriding this convention, or preclude the user from requesting a
different display.  This convention also does not excuse FITS writers
from providing complete and correct descriptions of the image
coordinates, allowing the user to determine the meaning of the image.
The ordering of the image for display is simply a convention of
convenience, whereas the coordinates of the pixels are part of the
physics of the observation.

See these papers from my home page:

   http://www.cv.nrao.edu/~egreisen

Eric Greisen