Efficient way of drawing in OpenGL ES

Question

In my application I draw a lot of cubes through OpenGL ES Api. All the cubes are of same dimensions, only they are located at different coordinates in space. I can think of two ways of drawing them, but I am not sure which is the most efficient one. I am no OpenGL expert, so I decided to ask here.

Method 1, which is what I use now: Since all the cubes are of identical dimensions, I calculate vertex buffer, index buffer, normal buffer and color buffer only once. During a refresh of the scene, I go over all cubes, do bufferData() for same set of buffers and then draw the triangle mesh of the cube using drawElements() call. Since each cube is at different position, I translate the mvMatrix before I draw. bufferData() and drawElements() is executed for each cube. In this method, I probably save a lot of memory, by not calculating the buffers every time. But I am making lot of drawElements() calls.

Method 2 would be: Treat all cubes as set of polygons spread all over the scene. Calculate vertex, index, color, normal buffers for each polygon (actually triangles within the polygons) and push them to graphics card memory in single call to bufferData(). Then draw them with single call to drawElements(). The advantage of this approach is, I do only one bindBuffer and drawElements call. The downside is, I use lot of memory to create the buffers.

My experience with OpenGL is limited enough, to not know which one of the above methods is better from performance point of view.

I am using this in a WebGL app, but it's a generic OpenGL ES question.

Answer 1

I implemented method 2 and it wins by a landslide. The supposed downside of high amount of memory seemed to be only my imagination. In fact the garbage collector got invoked in method 2 only once, while it was invoked for 4-5 times in method 1.

Your OpenGL scenario might be different from mine, but if you reached here in search of performance tips, the lesson from this question is: Identify the parts in your scene that don't change frequently. No matter how big they are, put them in single buffer set (VBOs) and upload to graphics memory minimum number of times. That's how VBOs are meant to be used. The memory bandwidth between client (i.e. your app) and graphics card is precious and you don't want to consume it often without reason.

Read the section "Vertex Buffer Objects" in Ch. 6 of "OpenGL ES 2.0 Programming Guide" to understand how they are supposed to be used. http://opengles-book.com/

Answer 2

I propose following approach:

On load:

1. Generate coordinates buffer (for one cube) and load it into VBO (gl.glGenBuffers, gl.glBindBuffer)

On draw:

1. Bind buffer (gl.glBindBuffer)

2. Draw each cell (loop)

   2.1. Move current position to center of current cube (gl.glTranslatef(position.x, position.y, position.z)

   2.2. Draw current cube (gl.glDrawArrays)

   2.3. Move position back (gl.glTranslatef(-position.x, -position.y, -position.z))