image orientation on the screen

[Michael Newberry]

On 20 Jan 1997 18:34:44 GMT, hodge at bowline.stsci.edu (Phil Hodge)
wrote:

>Michael Newberry (mnewberry at axres.com) writes:
>
>> It would be really useful if there were two keywords that would go
>> into an image header at the point of origin that could later be used
>> to give the correct display in the "mirror" sense. This would leave
>>	...
>> Personally, I like XREVERSE and YREVERSE.
>
>I don't think it's a good idea to create FITS keywords which depend
>on the convention for displaying images.  A separate keyword for each
>axis is also not very useful for images taken from spacecraft.  The
>parity of an image is easily obtained from the sign of the determinant
>of the CD matrix (or the PC matrix multiplied by the CDELTi, in Greisen
>and Calabretta's notation).  The interpretation of the sign depends on
>how you display the image, of course.  The sign is negative for images
>that will be displayed correctly (i.e. without mirror reversal, but
>possibly rotated) if the first pixel is at the lower left, with the
>sample direction running from left to right.
>
>						Phil

Phil,

You are right about using the signs of CRDELT1 and CRDELT2.  But you
are talking about World Coordinate keywords whereas I'm talking about
keywords that do not require an world coordinate calibration.  Most
CCD users do not WCS calibrate their images---all I want is North
almost up and East almost left for *display* purposes.

Mike Newberry.