Creating 3D objects using WebGL
Let's take our square plane into three dimensions by adding five more faces to create a cube. To do this efficiently, we're going to switch from drawing using the vertices directly by calling the gl.drawArrays()
method to using the vertex array as a table, and referencing individual vertices in that table to define the positions of each face's vertices, by calling gl.drawElements()
.
Consider: each face requires four vertices to define it, but each vertex is shared by three faces. We can pass a lot fewer data around by building an array of all 24 vertices, then referring to each vertex by its index into that array instead of moving entire sets of coordinates around. If you wonder why we need 24 vertices, and not just 8, it is because each corner belongs to three faces of different colors, and a single vertex needs to have a single specific color; therefore we will create three copies of each vertex in three different colors, one for each face.
Define the positions of the cube's vertices
First, let's build the cube's vertex position buffer by updating the code in initBuffers()
. This is pretty much the same as it was for the square plane, but somewhat longer since there are 24 vertices (4 per side).
Note: In the initPositionBuffer()
function of your "init-buffers.js" module, replace the positions
declaration with this code:
const positions = [
// Front face
-1.0, -1.0, 1.0, 1.0, -1.0, 1.0, 1.0, 1.0, 1.0, -1.0, 1.0, 1.0,
// Back face
-1.0, -1.0, -1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0, 1.0, -1.0, -1.0,
// Top face
-1.0, 1.0, -1.0, -1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, 1.0, -1.0,
// Bottom face
-1.0, -1.0, -1.0, 1.0, -1.0, -1.0, 1.0, -1.0, 1.0, -1.0, -1.0, 1.0,
// Right face
1.0, -1.0, -1.0, 1.0, 1.0, -1.0, 1.0, 1.0, 1.0, 1.0, -1.0, 1.0,
// Left face
-1.0, -1.0, -1.0, -1.0, -1.0, 1.0, -1.0, 1.0, 1.0, -1.0, 1.0, -1.0,
];
Since we've added a z-component to our vertices, we need to update the numComponents
of our vertexPosition
attribute to 3.
Note: In the setPositionAttribute()
function of your "draw-scene.js" module, change the numComponents
constant from 2
to 3
:
const numComponents = 3;
Define the vertices' colors
We also need to build an array of colors for each of the 24 vertices. This code starts by defining a color for each face, then uses a loop to assemble an array of all the colors for each of the vertices.
Note: In the initColorBuffer()
function of your "init-buffers.js" module, replace the colors
declaration with this code:
const faceColors = [
[1.0, 1.0, 1.0, 1.0], // Front face: white
[1.0, 0.0, 0.0, 1.0], // Back face: red
[0.0, 1.0, 0.0, 1.0], // Top face: green
[0.0, 0.0, 1.0, 1.0], // Bottom face: blue
[1.0, 1.0, 0.0, 1.0], // Right face: yellow
[1.0, 0.0, 1.0, 1.0], // Left face: purple
];
// Convert the array of colors into a table for all the vertices.
var colors = [];
for (var j = 0; j < faceColors.length; ++j) {
const c = faceColors[j];
// Repeat each color four times for the four vertices of the face
colors = colors.concat(c, c, c, c);
}
Define the element array
Once the vertex arrays are generated, we need to build the element array.
Note: In your "init-buffer.js" module, add the following function:
function initIndexBuffer(gl) {
const indexBuffer = gl.createBuffer();
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, indexBuffer);
// This array defines each face as two triangles, using the
// indices into the vertex array to specify each triangle's
// position.
const indices = [
0,
1,
2,
0,
2,
3, // front
4,
5,
6,
4,
6,
7, // back
8,
9,
10,
8,
10,
11, // top
12,
13,
14,
12,
14,
15, // bottom
16,
17,
18,
16,
18,
19, // right
20,
21,
22,
20,
22,
23, // left
];
// Now send the element array to GL
gl.bufferData(
gl.ELEMENT_ARRAY_BUFFER,
new Uint16Array(indices),
gl.STATIC_DRAW,
);
return indexBuffer;
}
The indices
array defines each face like a pair of triangles, specifying each triangle's vertices as an index into the cube's vertex arrays. Thus the cube is described as a collection of 12 triangles.
Next, you need to call this new function from initBuffers()
, and return the buffer it creates.
Note: At the end of the initBuffers()
function of your "init-buffers.js" module, add the following code, replacing the existing return
statement:
const indexBuffer = initIndexBuffer(gl);
return {
position: positionBuffer,
color: colorBuffer,
indices: indexBuffer,
};
Drawing the cube
Next we need to add code to our drawScene()
function to draw using the cube's index buffer, adding new gl.bindBuffer()
and gl.drawElements()
calls.
Note: In your drawScene()
function, add the following code just before the gl.useProgram
line:
// Tell WebGL which indices to use to index the vertices
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, buffers.indices);
Note: In the drawScene()
function of your "draw-scene.js" module, replace the block just after the two gl.uniformMatrix4fv
calls, that contains the gl.drawArrays()
line, with the following block:
{
const vertexCount = 36;
const type = gl.UNSIGNED_SHORT;
const offset = 0;
gl.drawElements(gl.TRIANGLES, vertexCount, type, offset);
}
Since each face of our cube is comprised of two triangles, there are 6 vertices per side, or 36 total vertices in the cube, even though many of them are duplicates.
Finally, let's replace our variable squareRotation
by cubeRotation
and add a second rotation around the x axis.
Note: At the start of your "webgl-demo.js" file, replace the squareRotation
declaration with this line:
let cubeRotation = 0.0;
Note: In your drawScene()
function declaration, replace the squareRotation
with cubeRotation
:
function drawScene(gl, programInfo, buffers, cubeRotation) {
Note: In your drawScene()
function, replace the mat4.rotate
call with the following code:
mat4.rotate(
modelViewMatrix, // destination matrix
modelViewMatrix, // matrix to rotate
cubeRotation, // amount to rotate in radians
[0, 0, 1],
); // axis to rotate around (Z)
mat4.rotate(
modelViewMatrix, // destination matrix
modelViewMatrix, // matrix to rotate
cubeRotation * 0.7, // amount to rotate in radians
[0, 1, 0],
); // axis to rotate around (Y)
mat4.rotate(
modelViewMatrix, // destination matrix
modelViewMatrix, // matrix to rotate
cubeRotation * 0.3, // amount to rotate in radians
[1, 0, 0],
); // axis to rotate around (X)
Note: In your main()
function, replace the code that calls drawScene()
and updates squareRotation
to pass in and update cubeRotation
instead:
drawScene(gl, programInfo, buffers, cubeRotation);
cubeRotation += deltaTime;
At this point, we now have an animated cube rotating, its six faces rather vividly colored.