|
|
Thread Tools | Display Modes |
#1
|
|||
|
|||
[fitsbits] CRPIX clarification
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 |
Thread Tools | |
Display Modes | |
|
|
Similar Threads | ||||
Thread | Thread Starter | Forum | Replies | Last Post |
[fitsbits] CRPIX clarification | Jonathan McDowell | FITS | 0 | May 29th 08 02:18 AM |
[fitsbits] CRPIX clarifcation | Mark Calabretta | FITS | 0 | May 29th 08 01:15 AM |
[fitsbits] CRPIX clarifcation | Jonathan McDowell | FITS | 0 | May 27th 08 07:42 PM |
[fitsbits] Clarification of EXTEND, please | Stephen Walton | FITS | 8 | May 19th 04 12:53 AM |
[fitsbits] Clarification of EXTEND, please | Thierry Forveille | FITS | 0 | May 14th 04 06:31 PM |