when use RGB and when use RGBA?

To: "visad-list@xxxxxxxxxxxxx" <visad-list@xxxxxxxxxxxxx>
Subject: when use RGB and when use RGBA?
From: Helen Yi <y.yi@xxxxxxxxxxxxx>
Date: Wed, 04 Sep 2002 11:57:05 -0400

Hi

I need some help on how to determine  when and where use RGB  and RGBA.

Suppose  the normal range of a flat field is between 0 and 100, but some
points are with negative values.
How can I make a color display but use grey color represent  the
negative values?


Thanks for your help.

helen

Follow-Ups:
- Re: when use RGB and when use RGBA?
  - From: Bill Hibbard