We've covered all the methods of drawing arrays in WebGL. You should be able to make some pretty cool stuff at this point. But if you tried making something a bit more complex, you might see that there's something lacking. Even if we tried making something as simple as a cube, we'll run into problems.
A cube has eight corners. I've labeled them here as zero to seven. What we'd like to do is draw two triangles for each of the six faces of the cube. Let's jump over to the code and try it.
I'll get rid of the blue background here and all this C Shell code and paste in a vertex array that contains eight points of a cube. Each vertex also contains a separate color. In the translate line, I'm going to push this out a bit away from us so we can see the whole cube. Down in the draw function, I'll make sure we're using triangle strip and then run this.
We have a strip of triangles from the first three vertices in the array down to the last three. But this only forms three sides of a cube. There's no way to finish the cube without creating more vertices which exactly duplicate the existing vertices.
For example, I can copy the first two vertices and add them to the end of the array. This will give us our fourth face. But the last two faces require even more duplicated points. You can get tricky with it, but you'll always need a number of new duplicates. Ideally we'd be able to draw this cube using just the eight existing vertices.
That's where DrawElements comes in, glDrawArrays runs through the vertices that are in the vertex buffer from beginning to end. You can choose how many vertices you want to draw and where to start, but you can't rearrange them or use the same vertex more than one time in a single call.
But glDrawElements allows you to pick and choose and assemble a custom list of vertices to draw just by providing a list of indices that point to specific vertices. Sound confusing? Let's go back to the drawing of the cube.
All the eight points shown here, zero to seven, will be in a vertex buffer. If I want to draw a triangle with points zero, one, and two, I can create an indices array with those three values in it, just the numbers zero, one, and two. Then I can add the numbers one, two, and three to that indices array to draw the second triangle. There's our first face.
Notice that I reused points one and two. Here are the indices that make up the two triangles for the second face, and the third, fourth, fifth, and sixth. Now we've individually defined every triangle that makes up the entire cube just by specifying which vertices to draw with.
Back in the code, I'll make a new function called, "Create indices," just after, "Create vertices," and then create that function in the same location. What we need to do is make an element array buffer that will hold our index list.
This is process very similar to how we created the vertex buffer. First I'm going to paste the array of indices that we just came up with. Now I'll need to know how many indices we have. I'll create a variable up at the top here called, "Index count." Back in the function I'll say, "Index count equals indices length."
Now we can create a buffer. I'll say, "Index buffer equals GL create buffer." Then we need a way to bind the buffer to a target. When binding the vertex buffer, the target was, "GL array buffer." But for indices, we want to use the target, "GL element array buffer."
Here we're using integers though, so we'll wrap indices in a Uint8Array. This is an unsigned 8-bit integer, fine for what we're doing here. If you wind up with a lot more vertices you need to index, you should use a Uint16Array, which would allow for larger numbers.
Finally, the mode. Like before, it's GL static draw. I'm not going to unbind this buffer now because it needs to be an element buffer bound when we call to our elements in the draw function.
In a more complex application, you might have multiple objects composed of multiple element buffers. You could switch out which elements you want to draw by binding the appropriate buffer. Since we just have the one here, I'll leave it permanently bound.
Now down to the draw function. I'll get rid of this first call to draw arrays and instead call draw elements. This again takes the drawing mode first. We've set up individual triangles so the mode is GL triangles.
Next is the number of indices in the element buffer. This is index count. Then the data type, a Uint8 corresponds to GL unsigned byte. If you happen to use a Uint16Array, you should use GL unsigned short here to correspond to the 16 bit data type.
Final parameter is the offset in case you wanted to start some number of indices into the array. Oddly while the second parameter's a simple integer for the number of indices to draw, the offset parameter requires a number of bytes.
If you wanted to start with an offset of say three, you'd have to multiply three by Uint8Array.bytes per element or Uint16Array.bytes per element if that's what you used to get the proper value. We'll just use zero here though as we'll be drawing everything.
We're ready to roll. Let's see what we have. Excellent, a nice multi-colored cube with only eight vertices used.
You could different drawing modes here, points or any of the line modes or triangle strip or fan. You can make multiple calls to draw elements with different offsets and counts to draw different portions of the data. As I just said, you could even have multiple element buffers and draw each one separately. You now have all the tools you need to model just about any type of two or three-D object. Have fun with it.