[cairo] Best strategy for complex screen drawing

[Bill Spitzak]

Timothée Lecomte wrote:

> http://tipote.free.fr/wxt16.png
> 
> (to compare with desired output : http://tipote.free.fr/wxt15.png )
> 
> Any idea to workaround it ?

I'm beginning to think that, despite it's appeal, the current approach 
to antialiasing is not going to work for real-world applications, such 
as the one being shown in these sample images.

The solution with current Cairo is: SATURATE will kind of work, but 
requires a seperate buffer. If the figure overlaps itself you will have 
to draw each overlapping layer seperately, you need to split it whenever 
the visible side of the polygon changes, splitting "bowtie" ones into 
two triangles. I think there may be problems if you try to texture-map 
your surface with any kind of transparency, but I am not sure.

Because of the difficulty in figuring out how to use SATURATE and set up 
a second buffer, and the fact that it cannot be used in many cases, I am 
afraid that most programs will give up and turn off antialiasing. This 
means that cairo output is not going to look very good.

I really feel that the solution is to add "aliased_lineto" to the 
path-construction operators. This acts exactly like lineto except if the 
path is filled (or used in a filled way, such as a clipping path) then 
no antialiasing is done to that straight edge.

If the example program then used aliased_lineto for each internal edge 
of the polygon, and regular lineto for the outer 1 or 2 edges of the 
polygons at the edge, and drew from back to front, using OVER in the 
main image buffer, would draw mostly correctly, perhaps good enough for 
what you want.

To get it perfect, the edges where it curls over so you face the other 
side must be detected and draw antialiased. This is not as hard as the 
work needed for SATURATE, as you can draw the polygons on each side in 
any order, rather than having to defer all the ones that face one 
direction until the next layer. You also have to split "bowtie" polygons 
into two triangles.