[iaufwg] FITS conventions

[Doug Tody]

On Tue, 14 Feb 2006, Clive Page wrote:

> I agree that ds9 now does nearly all that users want when displaying FITS 
> images.  The problem is that having selected a sensible way to display one's 
> image, there isn't any standard way of preserving that choice in the image 
> file.  In high energy astronomy at least, we are not interested in the 
> problems of true colour represenation (since there isn't one), what we want 
> is a false colour representation which shows the important features of the 
> image.  Having found that it would be nice to preserve it somehow.

This is my point as well.  Since we always have to render a science image
to display it, it would be useful to have a standard rendering which is
defined by the data provider, especially if the data is at all unusual.
Of course one often still wants to be able to manipulate the way the
image is displayed with a display program, but it would be very useful
to have a predetermined rendering, especially for simple viewers.


> So I wonder if we could start off by just defining a simple conversion from 
> the intensity (or flux or whatever) from the primary FITS image array to some 
> common colour representation, such as RGB.  A simple look-up table may be 
> enough - and I guess that's what programs such as ds9 do internally and 
> certainly what formats such as GIF store. I think this would make it easier 
> to use programs like ds9 (as there would be a default look-up table) and also 
> to use image conversion utilities like xv automatically.

This would be one approach, although having to use a lookup table is a 
fairly serious constraint.  Also it could get a bit tricky depending upon
the image - floating point data with a high dynamic range for example could
be hard to render.  True color images such as an RGB rendering of multiband
data are not practical with this approach.  The nice thing about something
like JPEG is that it supports true color image display and high compression.


On Tue, 14 Feb 2006, Lucio Chiappetti wrote:

>  - and in the latter case which to choose ? There can also be problems
>    of proprietary rights ... I remember there was a PNG vs GIF issue for
>    instance

Right, PNG was created to replace GIF due to patent claims on GIF.
JPEG appears to be ok though.  There was an attempt by a company to claim
a patent on JPEG in 2002 which is still in litigation but it looks like it
will fail.  Aside from this JPEG is free, with free software implementations
available.  Furthermore, the newest version JPEG2000 is intended to be
licence-free - evidently it is also based on wavelet compression and can
achieve even higher compression ratios.  JPEG is best for high compression
of photographic images (this includes astronomical data).  GIF/PNG are
lossless and best for line graphics, but do not compress as well.

 	- Doug