Too Transformation Heavy?

OpenGL OpenGL: Basic Coding
1

In my game I am using the classic rocket-trail effect as seen in Half-Life or any number of games; basically a rocket has behind it a set of vertices and they are connected by (effectively) billboarded quads with a smokey texture to give the impression of a contrail behind the rocket.

However, in rendering this effect I use the procedure:

render the rocket
for each vertex in the contrail
push tranformation matrix
translate to that vertex
calculate and create the matrix for the billboard effect here (i.e. for a quad that goes from this vertex to the next)
multiply that matrix
draw the quad
pop the matrix

That too me seems to be a bit intensive for one rocket render, with (depending on contrail length and the amount of time between “vertices” being dropped along the path) probably ~10 vertices per rocket. I could just do the billboard transformation once, or another one of my plans was to have the contrail follow a helical pattern along an axis coaxial with its path giving a more interesting path, but I dunno.

But am I right, given a scenario of like 20-30 rockets being on the screen at the same time, plus the rest of the scene, I’m guessing this present scheme won’t scale very well?

2

Computing the matrix for each frame seems effectively a bottleneck. You should look more deeply at that.

Sorry but I can’t help you more.

3

Too much work.

investigate point sprites…this will save you having to use a tailored modelviewmatrix that doesn’t perform rotations - as point sprites are always screen aligned.
(The problem is that if the center of the sprite gets clipped, the whole thing gets clipped - because of this, I couldn’t use them)

You’re essentially creating a particle system that emits from the thruster of the rocket.

Don’t draw each quad seperately - use a data structure to store your particles, batch translate them all at once, and then use a vertex array to batch render them.

something like this:

struct particle
{
posX,Y,Z; //position
life; //die off rate
velX,Y,Z; //velocity
etc…
}
particle particles[100];

render rocket.
transform each particle.
glVertexArray(particles);
glDrawArrays();

this is all skim surface pseudocode, but hopefully you get the idea.
always try and batch your work as oft as possible.

hope some of that makes sense.