Subject: ImageBaseColor's photometry: Redefine to be simple, convenient, practical

From: "Dale Knutsen" <[email-masked]>
Date: Sun, 29 Jul 2001 01:25:29 +0800

I am working on software implementing a TIFF-FX reader, and I
have found the current definition of ImageBaseColor in TIFF-FX
to be rather impractical, and I recommend changing it as
described below.

    The Problem

The color sample data contained in the ImageBaseColor field is
specified to be in photometry ITU L*a*b*. Notice that this
photometry may be DIFFERENT than that of the image that is
being represented. 

For example, consider TIFF generator software which has an
input image with photometry CMYK; the software is rendering
this image into a TIFF-FX data stream. If the generator notices
a strip whose color is constant, the generator could achieve
excellent compression by representing the constant color in
ImageBaseColor and setting the strip byte count to zero.

HOWEVER, the generator is faced with the daunting task of
representing the CMYK (in this example) constant color in ITU
L*a*b* color space. This requires:

(a) the generator must have the mapping from device-dependent
CMYK photometry to the colorimetric ITU L*a*b* photometry;
typically this is done by having the appropriate ICC profile
and applying it to the CMYK. However, the generator may not
have the profile data available for a variety of reasons.

(b) the generator must incorporate the complicated algorithms
(color space conversion, ICC profile processing) -- which are
not required otherwise of the generator.

Similar problems occur to software reading a TIFF data stream. 

    Analysis and Proposal

It seems to me that the motivation behind of ImageBaseColor is
to provide an efficient way to represent strips of constant
color. Of course we want it to be simple and practical too --
which I think the present definition is not. 

I propose that that the photometry and interpretation of the
ImageBaseColor field data be changed to be IDENTICAL TO that of
the containing IFD (including all applicable fields which
modify the representation and photometric interpretation of
color sample data). This means that the representation of
ImageBaseColor sample data would be identical to the
representation of the ordinary color sample data that it
represents (stands in place of). This would be extremely simple
and convenient, both for the creator of a TIFF data stream and
for the reader of a TIFF data stream! There would be NO
difference in the treatment of ordinary color sample data and
the color sample data in the ImageBaseColor field. 

    Dale Knutsen

-- 

    Dale Knutsen

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