CODEWITHSUNDEEP
performanceopenglgraphics
Gravity
Gravity

Reputation: 2744

Performance of using a large number of small VBOs

I wanted to know whether OpenGL VBOs are meant to be used only for large geometry arrays, or whether it makes sense to use them even for small arrays. I have code that positions a variety of geometry relative to other geometry, but some of the "leaf" geometry objects are quite small, 10x10 quad spheres and the like (200 triangles apiece). I may have a very large number of these small leaf objects. I want to be able to have a different transform matrix for each of these small leaf objects. It seems I have 2 options:

  1. Use a separate VBO for each leaf object. I may end up with a large number of VBOs.

  2. I could store data for multiple objects in one VBO, and apply the appropriate transforms myself when changing the data for a given leaf object. This seems strange because part of the point of OpenGL is to efficiently do large numbers of matrix ops in hardware. I would be doing in software something OpenGL is designed to do very well in hardware.

Is having a large number of VBOs inefficient, or should I just go ahead and choose option 1? Are there any better ways to deal with the situation of having lots of small objects? Should I instead be using vertex arrays or anything like that?

Upvotes: 1

Views: 2288

Answers (2)

Robert Rouhani
Robert Rouhani

Reputation: 14688

The most optimal solution if your data is static and consists of one object would be to use display lists and one VBO for only one mesh. Otherwise the general rule is that you want to avoid doing anything other than rendering in the main draw loop.

  • If you'll never need to add or remove an object after initialization (or morph any object) then it's probably more efficient to bind a single buffer permanently and change the stride/offset values to render different objects.

  • If you've only got a base set of geometry that will remain static, use a single VBO for that and separate VBOs for the geometry that can be added/removed/morphed.

  • If you can drop objects in or remove objects at will, each object should have it's own VBO to make memory management much simpler.

Upvotes: 2

fen
fen

Reputation: 10125

some good info is located at: http://www.opengl.org/wiki/Vertex_Specification_Best_Practices

I think that 200 triangles per one mesh is not so small number and maybe the performance with VBO for each of that meshes will not decrease so much. Unfortunately it is depended on hardware spec.

One huge buffer will no gain huge performance difference... I think that the best option would be to store several (but not all) objects per one VBO.

renderin using one buffer: there is no problem with that... you simply have one buffer that is bound and then you can use glDrawArrays with different parameters. For instance if one mesh consists of 100 verts, and in th buffer you have 10 of those meshes you can use

glDrawArrays(triangles, 0, 100);
glDrawArrays(triangles, 100, 100);
glDrawArrays(triangles, ..., 100);
glDrawArrays(triangles, 900, 100);

in that way you minimize changing buffers and still you are able to render it quite efficient.

Are those "small" objects the same? do they have the same geometry but different transformations/materials? Because maybe it is worth using "instancing"?

Upvotes: 1

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