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From: planting@hobbes.cs.pittsburgh.edu (Dr. Harry Plantinga)
Newsgroups: comp.graphics
Subject: Image file format discussion
Summary: We need a new format
Message-ID: <6040@pitt.UUCP>
Date: 20 Oct 89 14:52:59 GMT
References: <1198@clinet.FI> <390039@hpfcdq.HP.COM> <4608@mentor.cc.purdue.edu>
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Reply-To: planting@cs.pitt.edu
Organization: Computer Science Dept., Univ. of Pittsburgh
Lines: 57

In article <4608@mentor.cc.purdue.edu> ahg@mentor.cc.purdue.edu (Allen Braunsdorf) writes:

>GIF	A format for storing images with color look up tables.  No aspect
>	ratio is saved, so fidelity cannot be insured.  The file consists
>	of a header of fixed fields, an optional global color map,
>	and one or more images (which each have a header and optional
>	local color map).  The format is extensible through ! blocks.  
>
>IFF	A general interchange format for data.  The part you want to look
>	at is the FORM ILBM.  This is a fairly simple digital image
>	structure that describes the attributes of an image and how
>	to decipher it and then has the image itself.  The format is
>	extensible by the addition of chunks.
>
>TIFF	A very flexible image storage format.  Files contain a small header
>	and then a directory of tags.  These tags describe attributes
>	of the image.  By adding tags, the format can be extended.

There is other information that should be stored with images but which
I have not heard anyone mention.

_Big-endian vs. Little-endian_:  does 255 represent white or black?

_Primary colors_:  the wavelengths of the (R, G, and B) primaries used
in representing the image.  Usually the NTSC primaries are used, but
some devices such as color printers may use other primaries.
Furthermore, the actual color of the pigments may vary from monitor to
monitor, and if the information is known for a particular monitor then
color correction can be performed.  Primary color information is is
essential in matching colors between devices with different primaries.
[See Roy Hall's book "Illumination and Color in Computer Generated
Imagery."]

_Gamma correction factor_:  Monitors, video cameras, and other
equipment vary in their "gamma correction" value.  Typically, the
brightness output of a monitor is not linear with the input value, but
exponential.  It typically depends on the input as the function

lookup value = intensity ^ (1/gamma)

It is possible to perform gamma correction before storing the data in
a standard file format, but much information is lost.  For a monitor
with gamma 2.2 and linear input, only about 190 of the 255 different
possible output values can be used for an 8-bit image [from Hall].

=================

Is there other information that should be stored with an image?

Is there a file format that stores this information?

If not, perhaps this group should define standard extension to one of
the existing file formats for storing this information.


-Harry Plantinga
planting@cs.pitt.edu