Re: [Mesa3d-users] question about anti-aliasing in OSMesa

[Brian P. <br...@vm...>]

Martijn Sanderse wrote:
> Hi,
> 
> I'm pretty new to OpenGL and Mesa (I am reading my redbook...). If  I
> picked the wrong place for my question, please let me know.
> 
> I'm trying to find out whether I can use mesa for rendering polygons
> to offscreen images. In particular I'm interested in the accuracy of
> the anti-aliasing routines.
> 
> To find out I followed the example in
> MesaDemos/progs/osdemos/osdemo32.c. I changed the function
> render_image() to the one described below. I basically draw a simple
> polygon with three vertices.
> 
> Because the top vertex of my polygon is almost flat, but not entirely,
> I expect (theoretically) to find a  row with increasing pixel values
> in the buffer. However, the total number of unique pixel values is 23
> at maximum. I inspect only the "R" color component, as I'm interested
> only in greyscale images, and assume that anti-aliasing is equal for
> all color components.
> 
> I suspect that the anti-aliasing routine is written for speed,
> producing a result that is visually acceptable. Is there a way that I
> can increase the accuracy of the anti-aliasing routine? Or, do I have
> to turn to more GPGPU/shader kind of solutions, when I'm more
> interested in accuracy then in speed?
> 
> I use Mesa in a VirtualBox, so I guess it's all software rendering.

The mesa swrast AA triangle code is in src/mesa/swrast/s_aatriangle.c 
and s_aatritemp.h

The compute_coverage() functions (different versions for RGB vs. CI 
mode) basically count how many sub-pixel samples lie inside the 
triangle for each pixel.  A jittered 4x4 sample pattern is used.  You 
could probably increase the number of samples to improve AA quality a 
little.

-Brian