[NTG-pdftex] PNG handling, transparency

Hans Hagen pragma at wxs.nl
Mon Nov 1 18:27:57 CET 2004

Hartmut Henkel wrote:

> Now should one just flatten a PNG with alpha so that it blends to an
> opaque white background (or the one provided with the image), in the way
> the above external processing does? Then there is no way to supply the
> background color if it's missing in the image.

that is no real problem since one can add backgrounds using tex; keeping 
the background separate has the advantage that it is less tricky with 
regards to image reuse (on different backgrounds); it also makes it 
possible to extract the image as it is afterwards

> Apart from this PNG opaque blending, there is also the "Transparency"
> feature in PDF (PDF Ref. chapter 7). Looks complicated. Has anybody ever
> played with transparency, e. g. by some \pdfliteral tricks?

sure, it's part of context for years already; one should build some 
additional resource dicts; i wonder if we have to go that way, since i 
always thought that transparency of bitmaps is a characteristic of the 
bitmaps image itself (so, i must look into it)

> Or should one stay away from all this alpha and transparency business,
> flag an error on PNG's with alpha (they likely look wrong anyway), and
> recommend that alpha needs the shown preprocessing?

preprocessing is ok, since doing that each run takes time as well; but 
then we must add an howto section to the manual

how about transparent images on top of text

> Maybe it also helps just to do nothing and wait for libpng 1.2.8..

(btw, having access to the color dict of an image would be a nice 
feature as well, see some post long ago)

and then we don't even talk about image masks


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