Counting pixels

[lbuckley]

how do i count pixels? i am trying to calculate the projected area of a complex solid and it was recomended to count the number of pixels and compare it to an image of known size. any idea how to do this?

[Ostsol]

You could try occlusion queries. Take a look at ARB_occlusion_query.

[avocados]

If its 2d, look into glReadPixels for copying to mainboard ram where you can count the pixels.

[RigidBody]

you could use Shadow Volumes to render to the
stencil buffer.

with this technique you divide your solid into
polygons and render a shadow volume for each of
them to the stencil buffer. afterwards the stencil
buffer bits are 1 for each pixel, which is in the
shadow of the polygon.

i know it's a bit complicated, but you could
even use it to project an arbitrary solid onto
an arbitrarily curved surface.