[poppler] Proper Transparency Support in pdftops

[Adrian Johnson]

On 24/03/12 03:47, James Cloos wrote:
>>>>>> "AJ" == Adrian Johnson <ajohnson at redneon.com> writes:
> 
> AJ> Cairo PS output will only rasterize the regions that use transparency.
> AJ> You can try "pdftocairo -ps" to see if it solves your problem.
> 
> Cairo won't be truly suitable for a print workflow until it has proper
> colourspace support.  Squeezing, eg, a cmyk pdf to srgb to print on a
> colour-managed ps printer is harmful.

It is on my TODO list. When I'm not busy fixing bugs I plan on working
on this.

> In addition to minimizing rasterization a la cairo, there are a couple
> of other tricks which would help.  In many cases it would be good enough
> to approximate transparency by just multiplying the colourants by alpha.
> That isn't true transparency, of course, but is enough for many files
> which use argb or acmyk purely for convenience.

Cairo PS already blends transparent colors into white when there is
nothing underneath.

> Longer term, both poppler and cairo could benefit from rasterizing to a
> shading rather than to a pixmap.  OpenGL apps likely have suitable
> algorithms for rasterizing to triangles which p & c could borrow.
> Rasterizing to a tensor product shading, though, should be able to
> replicate anything pdf can do without loss of precision.

I'm not sure this is worth the effort. Printers are now starting to
support PDF. By the time such a feature is completed everyone will
either be printing with PDF or using a cheap printer that doesn't
support PS or PDF.

> Ghostscript's ps2writer also has unfortunate limitations.  Notably,
> since it only targets level2 ps, it converts CIDFonts to bitmap
> type 3 fonts¹.
> 
> 1] from a comment posted today to gs-devel or to their bugzilla 
> 
> -JimC