Re: evince, beamer, gradient problem

[Kristian Høgsberg <krh redhat com>]

On Mon, 2007-11-12 at 12:41 -0500, Pat Suwalski wrote:
> Cześć Marek,
> 
> Marek Kozlowski wrote:
> > I use the beamer LaTeX package (teTeX) for generating pdf slides. Those 
> > can be perfectly displayed with the Acrobat 6/7/8 under Windows or 
> > Linux. Unfortunately, the Evice incorrectly displays slide titles. Looks 
> > like some problem with the gradient? See the file beamer.png attached to 
> > this e-mail.
> 
> This is a common problem with gradients in PDF files. The ones exported 
> from OpenOffice do that as well.
> 
> What happens is that every rectangle "step" of the gradient has an 
> antialiased edge, and so you end up with a two anti-aliased edges 
> forming a lighter-coloured gap between the rectangles.
> 
> Programs that export gradients in a more overlapped fashion (first step 
> full width of of object, next width-1, next width-2) avoid this problem 
> altogether.
> 
> As an aside, does anyone know of any software that actually uses 
> gradients as specified fill methods from the PDF format?

It's actually a problem with poppler, not the PDF generator - I don't
think any PDF generator splits the gradients.  However, poppler inherits
xpdfs gradient support, which is implemented by splitting the PDF
gradients and feeding solid color polygons to the rasterizer.  xpdf has
a built-in rasterizer (called splash) that doesn't support
anti-aliasing, and for that setup this scheme works.  When using cairo
we get the seams shown in the screenshot.

Kristian