[cairo] Implementing PCL's display model using cairo.

[Bill Spitzak]

Wolfgang Glas wrote:

>   Basically, I figured out to boil down ROP3 operations into one or two 
> consecutive operations in the form
> 
> 1) dst = dst OR src
> 2) dst = dst AND src
> 3) dst = dst XOR src
> 4) dst = src
> 
>   Moreover, I played a bit with the PDF display model, which is claimed to be 
> represented by cairo's display model. In PDF, I could map my 4 operations in 
> a suitable way by choosing the following mapping:
> 
> 1) Lighten
> 2) Darken
> 3) Exclusion
> 4) Normal

I believe the intention is to add the PDF/SVG set of operators to Cairo. 
However, implementation wise, they are all very similar to the Saturate 
one that is there already, so I'm thinking this is low-priority. There 
is also the need to track down exact math for these so there is no 
disagreement with PDF.

Also beware that it is very unlikely that PDF or Cairo is going to 
exactly reproduce your ROP operations. Those sound like bitwise 
operations on the numbers representing the pixels. There is no intention 
whatsoever of having Cairo support that.