CODEWITHSUNDEEP
c++openglglslopengl-3
Boumbles
Boumbles

Reputation: 2523

OpenGL avoid calling glDrawElements multiple times

I'm migrating our graphics ending from using the old fixed pipeline functions to making use of the programmable pipeline. Our simplest model is just a collection of points in space where each point can be represented by different shapes. One of these being a cube.

I'm basing my code off the cube example from the OpenGL superbible.

In this example the cubes are placed at somewhat random places whereas I will have a fixed lit of points in space. I'm wondering if there is a way to pass that list to my shader so that a cube is drawn at each point vs looping through the list and calling glDrawElements each time. Is that even worth the trouble (performance wise)?

PS we are limited to OpenGL 3.3 functionality.

Upvotes: 1

Views: 1890

Answers (2)

sbabbi
sbabbi

Reputation: 11181

Is that even worth the trouble (performance wise)?

Probably yes, but try to profile nonetheless.

What you are looking for is instanced rendering, take a look at glDrawElementsInstanced and glVertexAttribDivisor.

What you want to do is store the 8 vertices of a generic cube (centered on the origin) in one buffer, and also store the coordinates of the center of each cube in another vertex attribute buffer.

Then you can use glDrawElementsInstanced to draw N cubes taking the vertices from the first buffer, and translating them in the shader using the specific position stored in the second buffer.

Something like this:

glVertexAttribPointer( vertexPositionIndex, /** Blah .. */ );
glVertexAttribPointer( cubePositionIndex, /** Blah .. */ );
glVertexAttribDivisor( cubePositionIndex, 1 ); // Advance one vertex attribute per instance

glDrawElementsInstanced( GL_TRIANGLES, 36, GL_UNSIGNED_BYTE, indices, NumberOfCubes );

In your vertex shader you need two attributes:

 vec3 vertexPosition; // The coordinates of a vertex of the generic cube
 vec3 cubePosition; // The coordinates of the center the specific cube being rendered

 // .... 
 vec3 vertex = vertexPosition + cubePosition;

Obviously you can have also a buffer to store the size of each cube, or another one for the orientation, the idea remains the same.

Upvotes: 5

dari
dari

Reputation: 2455

In your example every cube uses its own model matrix per frame. If you want to keep that you need multiple drawElements calls. If some cubes don't move (don't need a per frame model matrix) you should combine these cubes into one VBO.

Upvotes: 0

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