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Bullet Billboards

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I need to draw large numbers of non spherical projectiles, to create a tracer effect. I was thinking that perhaps one way to do this is via billboards. I would need billboards that are always pointing toward the screen, but still aligned by their longest axis with the direction of travel.

Right now I've implemented billboards using point sprites, using XNA's standard VertexPositionNormalTexture buffer, and an effect that uses the normal to expand the 4 points into a quad in the vertex shader:

output.Position = mul( mul( mul(input.Position, matWorld) , matView), matProjection) + float4(input.Normal.xy * input.Normal.z, 0, 0);

I encode the X and Y position of each point on the screen, and the age of the particle, into the X Y and Z components of the normal.

Is there a way I could do something similar with my projectiles? Currently, I'm drawing them all as a buffer of line strips, in a single draw call, and I don't want to lose this ability, because I want the sky to be lit up with bullets.

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