Question

I am working on a small game using webgl. Within this game I have some kind of forest which consists out of many (100+) tree objects. Because I only have a few different tree models, I rotate and scale these models in a different way before I display them.

At the moment I loop over all trees to display them:

for (var tree in trees) {
  tree.display();
}

While the display() method of tree looks like:

display : function() { // tree
  this.treeModel.setRotation(this.rotation);
  this.treeModel.setScale(this.scale);
  this.treeModel.setPosition(this.position);
  this.treeModel.display();
}

Many tree objects share the same treeModel object, so I have to set rotation/scale/position of the model everytime before I display it. The rotation/scale/position values are different for every tree.

The display method of treeModel does all the gl stuff:

display : function() { // treeModel
  // bind texture
  // set uniforms for projection/modelview matrix based on rotation/scale/position
  // bind buffers
  // drawArrays
}

All tree models use the same shader but can use different textures.

Because a single tree model consists only out of a few triangles I want to combine all trees into one VBO and display the whole forest with one drawArrays() call.

Some assumptions to make talking about numbers easier:

• There are 250 trees to display
• There are 5 different tree models
• Every tree model has 50 triangles

Questions I have:

• At the moment I have 5 buffers that are 50 * 3 * 8 (position + normal + texCoord) * floatSize bytes large. When i want to display all trees with one vbo i would have a buffer with 250 * 50 * 3 * 8 * floatSize byte size. I think I can’t use an index buffer because I have different position values for every tree (computed out of the position value of the tree model and the tree position/scale/rotation). Is this correct or is there still a way I can use index buffers to reduce the buffer size at least a bit? Maybe there are other ways to optimize this?

• How to handle different textures of the tree models? I can bind all textures to different texture units but how can I decide within the shader which texture should be used for the fragment that is currently displayed?

• When I want to add a new tree (or any other kind of object) to this buffer at runtime: Do I have to create a new buffer and copy the content? I think new values can’t be added by using glMapBuffer. Is this correct?

Answer 1

Index element buffers can only reach over attributes that are equal to or below 65535 in length, so you need to use drawArrays instead. It’s usually not a big loss.

You can add trees to the end of the buffers using GL.bufferSubData.

If your textures are in reasonable sized (like 128×128 or 256×256), you can probably merge them into one big texture and handle the whole thing with the UV-coords. If not, you can add another attribute saying what texture the vertex belongs to and have a condition in the vertex shader, alternatively an array of sampler2Ds (not sure it works, never tried it). Remember that conditions in shaders are pretty slow.

If you decide to stick to your current solution, make sure to sort the trees so the once using the same textures are rendered after each other – keeping state switching down is essential, always.