[pdftex] Image transparency

[George N. White III]

On Wed, 20 Jun 2001, Prof Brian D Ripley wrote:

> On Wed, 20 Jun 2001, Wendy McKay wrote:
>
> > What is wrong with jpegs or gifs?
>
> gifs: patent issues.
> jpeg: blurring and artefacts if used on line drawings or block colour
>   diagrams.

JPEG 2000, http://www.jpeg.org/JPEG2000.htm, uses wavelets and
has greatly reduced the problem of artifacts.  I think it is very
close to becoming official -- how long does it take Adobe to
implement a new image standard?

> This is the pdftex group, and png is the bitmapped image format that pdftex
> supports best.  It does not support gif.  What have `The major browsers'
> got to do with it?  They do not display pdf at all!

Many people need to use images in both PDF and HTML documents and don't
want to deal with multiple formats.  They have a choice: either
stick to commercial applications that support GIF or support
open tools like pdftex and live with the fact that some proprietary
tools won't display the PNG format correctly.

There are, however, problems with the PNG image support in pdftex.  To be
useful in demanding image work there needs to be support for color
management, transparency, etc.  PNG has the advantage that you can store a
target gamma value, but in practice applications that generate PNG rarely
let you specify the appropriate value and applications that display PNG
generally ignore the gamma value in the file.  I'm not sure how the gamma
value in a PNG file should be used in generating PDF -- AFAIK pdftex just
ignores the value.  Until recently, phototypesetters did a much better
job if you had CMYK artwork.

There can be big differences in the appearance of PNG images in documents
created using pdftex and those created using distiller.  Were I work we
routinely generate false color remote sensing images. The original
workflow uses GIF--> Photoshop --> EPS (CMYK) --> TeX --> dvipsone -->
phototypesetter, and has evolved to now using RGB EPS and distiller.

It is easy to generate PNG images with the same color LUT and pixel values
as the GIF, but the colors using pdftex are not at all the same as those
from the above process.  In addition to the gamma issue, these images use
a screen palette with colors that are far outside the printer's gammut.
The above process yields PDF images that look good on PC screens and on
the typesetter.  We tried using CMYK colors and they did not work as well,
in part because the typesetter has tuned their system for RGB images
created on PC's.

Rather than trying to improve support for images in pdftex I would
prefer to see better tools to generate PDF files with images that
can be placed into pdftex documents.  It is useful to have the
current basic PNG support for less demanding applications, but
it would be difficult to provide access to the full image capabilities
of PDF from pdeftex, and these capabilities are likely to change in
the future to support things like JPEG 2000.

-- 
George N. White III <gnw3 at acm.org> Bedford Institute of Oceanography