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

[Martijn S. <mar...@gm...>]

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.

Thanks a lot,
Martijn Sanderse

#define WIDTH 2500
#define HEIGHT 20

static void render_image( void )
{
   glViewport(0, 0, WIDTH, HEIGHT);
   glMatrixMode(GL_PROJECTION);
   glLoadIdentity();
   gluOrtho2D(0, WIDTH, 0, HEIGHT);

   glEnable (GL_POLYGON_SMOOTH);
   glEnable (GL_BLEND);
   glBlendFunc (GL_SRC_ALPHA, GL_ONE_MINUS_SRC_ALPHA);
   glHint (GL_POLYGON_SMOOTH_HINT, GL_NICEST);

   glClearColor(0.0, 0.0, 0.0, 0.0); //black
   glClear(GL_COLOR_BUFFER_BIT);

   glColor3f (1.0, 1.0, 1.0); // white
   glPushMatrix();
   glBegin (GL_POLYGON);
      glVertex2d (0.0, 10.0);
      glVertex2d (2500.0, 11.0); // slope = 1/2500
      glVertex2d (2500.0, 10.0);
   glEnd ();
   glPopMatrix();

   glFinish();
}