WebGL API: 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 less data around by building an array of all 24 vertices, then referring to each vertex by its index into that array instead of passing 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 3 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 rather longer since there are 24 vertices (4 per side):

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.

// Tell WebGL how to pull out the positions from the position
// buffer into the vertexPosition attribute
{
  const numComponents = 3;
  ...
  gl.vertexAttribPointer(
      programInfo.attribLocations.vertexPosition,
      numComponents,
      type,
      normalize,
      stride,
      offset);
  gl.enableVertexAttribArray(
      programInfo.attribLocations.vertexPosition);
}

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.

  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);
  }

  const colorBuffer = gl.createBuffer();
  gl.bindBuffer(gl.ARRAY_BUFFER, colorBuffer);
  gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(colors), gl.STATIC_DRAW);

Define the element array

Once the vertex arrays are generated, we need to build the element array.

  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 {
    position: positionBuffer,
    color: colorBuffer,
    indices: indexBuffer,
  };
}

The indices array defines each face as 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.

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:

  // Tell WebGL which indices to use to index the vertices
  gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, buffers.indices);

...

  {
    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:

mat4.rotate(modelViewMatrix, modelViewMatrix, cubeRotation * .7, [0, 1, 0]);

At this point, we now have an animated cube rotating, its six faces rather vividly colored.

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