Pixel list format

Frank Valdes wrote on Jun 15, 1998

Q: Pixel lists do not seem to allow negative values,
i.e. and imcopy of an image to a .pl file replaces
all negative values with zero. Why is this so?
Where can I get some detailed info re. masks
and pixel lists?


A: You are correct that the pixel list format does not allow negative values.
Why this is so was a design decision which I think is due to the concept
that the data consists of bit-flags using all bits; i.e. no sign bit.
The pixel list format is a very compact description of regions with
constant non-negative integer values.  Zero is normally expected to be
the most common value to provide the most compact description.  It is
intended for uses as pixel masks or region descriptions but can be used
in other applications including a compact non-negative integer image.
The conversion from other image formats to a pixel list "image" enforces
a non-negative conversion as well as truncation of real image values.

To get more info about pixel lists type:

	cl> phelp plio$PLIO.hlp f+
	
This is mostly a programmer's document but it does contain much interesting
information.  There isn't too much currently for users other than what
is in the task help descriptions or the newsletters.  The X-ray package
has some interesting applications for creating masks as regions and some
general help about the concept of masks as they apply to x-ray data.
If you have some general questions you can write to iraf@noao.edu or
the newsgroup "adass.iraf.system" or "adass.iraf.applications".

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Last post on Jun 15, 1998