[pdftex] Bitmaps with transparent background?

Hans Hagen pragma at wxs.nl
Thu Feb 22 13:13:29 CET 2001

At 12:15 PM 2/22/01 +0100, Dok, D. van wrote:
>> -----Original Message-----
>> From: Sebastian Tannert [mailto:tannert at physik.hu-berlin.de]
>> Sent: Thursday, February 22, 2001 12:06
>> To: pdftex at tug.org
>> Subject: [pdftex] Bitmaps with transparent background?
>> Is it possible to include a bitmap graphic with transparent 
>> background?
>> And when it is possible, how it is done, I tried with PNG but
>> this result in black background.
>> May be PDF does not allow transparent background in bitmaps?
>I have a similar question: does pdftex have a concept of `layers' such as
>would be found in graphical design tools? I've been successful so far
>putting one thing over another by working bottom-to-top. I have the feeling
>this works more by accident then by design.
>In other words, does the PDF specification have a concept of `transparency'
>and `layers' and, if so, how can it be addressed from pdftex?
In the pdf spec you can find more about that. There are several layers, for
instance one for text and one for annotations. If you stack things as you
say, they end up like that in the text/graphic layer (indeed bottom up,
like in postscript). 

It is possible to compose the text layer of multiple streams (which is what
acrobat does when it add bitmaps below ocr text) but the functionality
depends on viewers (like searching in the pure recognized text and showing
the bitmaps). 

The annotation layer is independant and has its own coordinates, and
(unfortunately) does not scale. PDF would have been more powerful when
annotation refs would have been part of the stream. 

Transparancy is more a graphic attribute and as soon as the png libs
support it, it should work ok in pdftex. One way of manipulating graphics
is to play with their attributes, 

                                  Hans Hagen | PRAGMA ADE | pragma at wxs.nl
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