Improve Particle system OpenGL

Question

I'm looking for a way to improve my particle system performance, since it is very costly in terms of FPS. This is because I call:

glDrawElements(GL_TRIANGLE_STRIP, mNumberOfIndices,
          GL_UNSIGNED_SHORT, 0);

I call this method for every particle in my application (which could be between 1000 and 5000 particles). Note that when increasing to over 1000 particles my application starting to drop down in FPS. I'm using VBO:s, but the performance when calling this method is too costly.

Any ideas how I can make the particle system more efficient?

Edit: This is how my particle system draw things:

glBindTexture(GL_TEXTURE_2D, textureObject);
glBindBuffer(GL_ARRAY_BUFFER, vboVertexBuffer[0]);
glVertexPointer(3, GL_FLOAT, 0, 0);
glBindBuffer(GL_ARRAY_BUFFER, vboTextureBuffer[0]);
glTexCoordPointer(2, GL_FLOAT, 0, 0); 
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, vboIndexBuffer[0]);

Vector3f partPos;

for (int i = 0; i < m_numParticles; i++) {
    partPos = m_particleList[i].m_pos;          
    glTranslatef(partPos.x, partPos.y, partPos.z);
    glDrawElements(GL_TRIANGLE_STRIP, mNumberOfIndices, 
        GL_UNSIGNED_SHORT, 0);
    gl.glTranslatef(-partPos.x, -partPos.y, -partPos.z);
}

what datatypes are you using for coordinates etc? if youre using other than GL_FLOAT, then you can see (huge) performance drop. — Rookie
– Rookie, Commented Jul 25, 2011 at 10:02
I know the post is a bit old but for anyone wondering, here is an amazing tutorial I found. (I'm not the author of this tutorial). opengl-tutorial.org/intermediate-tutorials/billboards-particles/… — Thibault Reuille
– Thibault Reuille, Commented Apr 15, 2014 at 0:13

datenwolf · Accepted Answer · 2011-07-25 09:58:21Z

7

The way you describe it, it sounds like you have a own VBO for each particle. This is not how it should be done. Put all particles into a single VBO and draw them all at once using a single glDrawElements or glDrawArrays call. Or even better, if available: Use instancing.

answered Jul 25, 2011 at 9:58

datenwolf

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6 Comments

Curtain Over a year ago

Thanks for the answer, but I reuse the same VBO for every particle. And then I just loop through every particle using glDrawElements(). The problem is that it are too many calls to glDrawElements(), and I'm not clear enough about how this should be solved.

kvark Over a year ago

The answer is clear: draw the whole particle system with a single draw call.

Curtain Over a year ago

@kvark: That wasn't a big wake up for me. The question is which is the best way to go since every particle needs their own position? Modifying the VBO during draw time?

datenwolf Over a year ago

Put them all at once into a single VBO. Make it a single (large) mesh of unconnected faces. Particle 1 and indices 0,1,2,3, particle 2 at indices 4,5,6,7 and so on.

kvark Over a year ago

Julian Assange. The point is: particle position data nevertheless goes from your system memory into VRAM for drawing. In your current implementation, it travels in form of internal uniforms, set via glTranslate. If you make particle position a part of the VBO, changing it per frame, it will still be the same amount of data travelling the bus. But the drawing operation will be way faster.

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Christian Rau · Accepted Answer · 2011-07-25 13:29:11Z

2

Expanding a bit on what datenwolf said, just pack all your particle indices into a single index buffer and draw all particles with a single glDrawElements call. This means you cannot use triangle strips anymore but a triangle set, but that shouldn't be too much of a problem.

Otherwise, if your hardware supports instanced rendering (or better, instanced arrays), you can do it by just rendering a single particle n times with the position and texCoord data taken from the respective arrays for each particle. You then still need to compute the four corner's position and texCoord data in the vertex shader (assuming you draw a quad for each particle), as with the instanced arrays you only get one attribute per instance (particle).

You might also use the geometry shader to create the particle's quad and just render a single point set, but I assume this might be slower than instancing, considering that SM4/GL3 hardware is quite likely to support instancing, too.

answered Jul 25, 2011 at 13:29

Christian Rau

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3 Comments

Curtain Over a year ago

Thanks for answering. Okay, then I need to put all particle's into a single VBO which holds the correct x, y and z coordinates (and I can just skip translate them) because they need to be rendered in different positions. However, is it efficient to upload a new VBO into OpenGL during draw time since the positions always change?

Christian Rau Over a year ago

@Julian Just make the position buffer GL_DYNAMIC_DRAW (or even GL_STREM_DRAW?) and update it every frame (using glMapBuffer or glBuffer(Sub)Data), It should at least be faster as your current solution.

Curtain Over a year ago

Thanks, will try this out when I'm at my working computer. :)

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