[Mesa-dev] opengl fill modes

[Brian Paul]

On 12/05/2014 08:30 PM, Roland Scheidegger wrote:
> Hi,
>
> this might be a stupid question, but if you use opengl fill mode line /
> point are the shared edges/vertices in strip (or fan) primitives drawn
> multiple times? My pedantic reading of the spec would seem to say yes
> (because it essentially says tri strips form individual triangles for
> rasterization, and only there are they converted to points/lines), and
> this is what we actually do in the draw module, but I have some doubts
> that it's actually true or at least that modern hw operates like that?

Coincidentally, I was wondering the same thing a few weeks ago and 
hacked a Mesa trivial test to check it out.  nvidia's driver and Mesa do 
as you suspect (shared edge lines are drawn twice, shared vertex points 
are drawn twice or three times).  I'm attaching my test program.  It 
uses ADD blending to it's easy to see the effect.


> Oh and a followup question if it's wrong, what would be the primitive
> ids associated with such lines / points when resulting from such
> decomposed tri strips?

I suspect they're inherited from the original triangle, but I'd have to 
write a test program to see what a few drivers do.  Your turn. :-)

-Brian

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