So in the spirit of writing down about the things I'm working on on a daily basis today comes hardware accelerated rendering of complex polygons. Ignacio Castaño finally convinced me to this ingenious method so all credit for it should go to him. I've spent last few moments at the office today looking into this method and it's just gorgeous so I'll give a brief overview of it.
In general all (at least Open Source) hardware accelerated vector graphics frameworks break up complex polygons in the process of tessellation. Like I mentioned in a few blogs before this process turns out to be quite complicated when trying to make it robust and fast. Ignacio's method beautifully avoids both problems.
So now to the method - the core idea is that we render to the stencil buffer and use it to correctly render the final polygon. So to use the Odd-Even fill rule we enable stencil writes and set up the stencil mask with glStencilMask(0x01) call. Next we make sure that the on passing the pixels are inverted with glStencilOp(GL_KEEP, GL_KEEP, GL_INVERT);. Then we can set the function and reference value for stencil testing by calling glStencilFunc(GL_ALWAYS, 0, ~0). Now we just render the vertices of our complex polygons as, for example triangles (fans would be just as suitable). Finally, we can enable color writes, disable stencil writes and enable stencil test and render a quad with the min/max coordinates of our polygon.
The method is awesome, as it (in theory) doesn't involve any kind of client side computations (besides a trivial min/max test) and (again in theory) operates in whole on the hardware. A wonderful sideeffect of all of this is that we avoid robustness issues that tessellation introduces.
To test this method I wrote today an application to compare different methods of rendering complex polygons (full client side rasterization, trapezoidation on X11 with Xrender, triangulation with OpenGL and finally stencil tests with OpenGL). So far Ignacios stencil method is by far the best. A sample result showing rendering of a rather complex (1000+ vertices) polygon follows, first client side rasterization:
And now Ignacio's method:
Finally an insane polygon I created for testing robustness (intersections at distances smaller than the resolution of doubles) and speed (it has segments with 100+ vertical vertices falling on a scanline). The new method can actually handle it correctly and with 100000+ vertices we still get usable performance. Pretty amazing, so thanks Ignacio for letting me know about this!