CODEWITHSUNDEEP
performanceopengl
Jeff Storey
Jeff Storey

Reputation: 57192

OpenGL drawing the same polygon many times in multiple places

In my opengl app, I am drawing the same polygon approximately 50k times but at different points on the screen. In my current approach, I do the following:

However, with 50k polygons, this is 50k push and pops and computations of the correct matrix translations to move to the correct point.

A coworker of mine also suggested drawing the entire scene into a buffer and then just drawing the whole buffer with a single translation. The tradeoff here is that we need to keep all of the polygon vertices in memory rather than just the display list, but we wouldn't need to do a push/translate/scale/rotate/pop for each vertex.

The first approach is the one we currently have implemented, and I would prefer to see if we can improve that since it would require major changes to do it the second way (however, if the second way is much faster, we can always do the rewrite).

Are all of these push/pops necessary? Is there a faster way to do this? And should I be concerned that this many push/pops will degrade performance?

Upvotes: 3

Views: 2821

Answers (2)

TheBuzzSaw
TheBuzzSaw

Reputation: 8826

It depends on your ultimate goal. More recent OpenGL specs enable features for "geometry instancing". You can load all the matrices into a buffer and then draw all 50k with a single "draw instances" call (OpenGL 3+). If you are looking for a temporary fix, at the very least, load the polygon into a Vertex Buffer Object. Display Lists are very old and deprecated.

Are these 50k polygons going to move independently? You'll have to put up with some form of "pushing/popping" (even though modern scene graphs do not necessarily use an explicit matrix stack). If the 50k polygons are static, you could pre-compile the entire scene into one VBO. That would make it render very fast.

Upvotes: 4

Jerry Coffin
Jerry Coffin

Reputation: 490108

If you can assume a recent version of OpenGL (>=3.1, IIRC) you might want to look at glDrawArraysInstanced and/or glDrawElementsInstanced. For older versions, you can probably use glDrawArraysInstancedEXT/`glDrawElementsInstancedEXT, but they're extensions, so you'll have to access them as such.

Either way, the general idea is fairly simple: you have one mesh, and multiple transforms specifying where to draw the mesh, then you step through and draw the mesh with the different transforms. Note, however, that this doesn't necessarily give a major improvement -- it depends on the implementation (even more than most things do).

Upvotes: 3

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