Question

I have a simple task: I have 10,000 3D boxes, each with a x,y,z, width, height, depth, rotation, and color. I want to throw them into a 3D space, visualize it, and let the user fly through it using the mouse. Is there an easy way to put this together?

Answer 1

One easy way of doing this using recent (v 3.2) OpenGL would be:

• make an array with 8 vertices (the corners of a cube), give them coordinates on the unit cube, that is from (-1,-1, -1) to (1, 1, 1)
• create a vertex buffer object
• use glBufferData to get your array into the vertex buffer
• bind the vertex buffer
• create, set up, and bind any textures that you may want to use (skip this if you don’t use textures)
• create a vertex shader which applies a transform matrix that is read from “some source” (see below) according to the value of gl_InstanceID
• compile the shader, link the program, bind the program
• set up the instance transform data (see below) for all cube instances
• depending on what method you use to communicate the transform data, you may draw everything in one batch, or use several batches
• call glDrawElementsInstanced N number of times with count set to as many elements as will fit into one batch
• if you use several batches, update the transform data in between
• the vertex shader applies the transform in addition to the normal MVP stuff

To communicate the per-cube transform data, you have several alternatives, among them are:

• uniform buffer objects, you have a guaranteed minimum of 4096 values, respectively 256 4×4 matrices, but you can query the actual value
• texture buffer objects, again you have a guaranteed minimum of 65536 values, respectively 4096 4×4 matrices (but usually something much larger, my elderly card can do 128,000,000 values, you should query the actual value)
• manually set uniforms for each batch, this does not need any “buffer” stuff, but is most probably somewhat slower

Alternatively: Use pseudo-instancing which will work even on hardware that does not support instancing directly. It is not as elegant and very slightly slower, but it does the job.