[fitsbits] CRPIX clarification

[Eric Greisen]

Jonathan McDowell wrote:

>  However, I think the statement that I was making is nothing to do with
> your very valid concerns. My point is that we have integer pixel numbers
> (P1,P2,...) like  (241,32,81) and fractional (real-valued) pixels (X1,X2,...)
> like (241.3, 32.1, 81.9). Different software systems use different
> schemes to map between these; I have seen both
>  [A]    Xi = Pi              Pi = (int)Xi
> and
>  [B]    Xi = Pi + 0.5        Pi = (int)(Xi-0.5)
> and of course this is an independent choice from the schemes
>  [F]    i = 1,....N
>  [C]    i = 0,....N-1
> 

People may prefer the latter although I cannot see why, but the latter 
is WRONG.  If people use it then they will LIE to the recipient about 
the WCS of their data.  The fact that the center of the VOXEL (we are > 
2 dimensions a lot) is the reference point of the VOXEL is part of the 
WCS papers and has been approved at all levels by the FITS committees. 
It was certainly understood from the very beginning of FITS and 
explicitly stated at least some places.  The argument about this that 
appeals to me most is to think if the VOXEL in real space as a cube. 
The only point in that cube that does not change under rotations is its 
center.  In all other cases, if you for example transpose an image, you 
will change the "natural" (think left side) reference point of every 
pixel on that axis.  Since one cannot change the reference point of 
projective geometries without regridding the image, this is an 
unpleasant concept.

Guys - the correct way to clarify some misunderstandings in the written 
standard needs to be determined.  BUT, the issue of reference point was 
resolved long ago.  Let us not reawaken it and destroy the good work we 
have done to convey coordinates with data.

Eric Greisen