# WebGL - Drawing a Triangle

#### Three.js & WebGL 3D Programming Crash Course (VR, OpenGL)

10 Lectures 1 hours

#### WebGL 2D/3D Programming and Graphics Rendering For The Web

28 Lectures 4 hours

In the previous chapter (Chapter 11), we discussed how to draw three points using WebGL. In Chapter 5, we took sample application to demonstrate how to draw a triangle. In both the examples, we have drawn the primitives using only vertices.

To draw more complex shapes/meshes, we pass the indices of a geometry too, along with the vertices, to the shaders. In this chapter, we will see how to draw a triangle using indices.

## Steps Required to Draw a Triangle

The following steps are required to create a WebGL application to draw a triangle.

Step 1 − Prepare the Canvas and Get WebGL Rendering Context

In this step, we obtain the WebGL Rendering context object using getContext().

Step 2 − Define the Geometry and Store it in Buffer Objects

Since we are drawing a triangle using indices, we have to pass the three vertices of the triangle, including the indices, and store them in the buffers.

```var vertices = [
-0.5,0.5,0.0,
-0.5,-0.5,0.0,
0.5,-0.5,0.0,
];

indices = [0,1,2];
```

Step 3 − Create and Compile the Shader Programs

In this step, you need to write vertex shader and fragment shader programs, compile them, and create a combined program by linking these two programs.

• Vertex Shader − In the vertex shader of the program, we define the vector attribute to store 3D coordinates and assign it to gl_position.

```var vertCode =
'attribute vec3 coordinates;' +

'void main(void) {' +
' gl_Position = vec4(coordinates, 1.0);' +
'}';
```
• Fragment Shader − In the fragment shader, we simply assign the fragment color to the gl_FragColor variable.

```var fragCode = 'void main(void) {' +
' gl_FragColor = vec4(1, 0.5, 0.0, 1);' +
'}';
```

Step 4 − Associate the Shader Programs to the Buffer Objects

In this step, we associate the buffer objects and the shader program.

Step 5 − Drawing the Required Object

Since we are drawing a triangle using indices, we will use `drawElements()`. To this method, we have to pass the number of indices. The value of the indices.length signifies the number of indices.

```gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);
```

## Example – Drawing a Triangle

The following program code shows how to draw a triangle in WebGL using indices −

```<!doctype html>
<html>
<body>
<canvas width = "570" height = "570" id = "my_Canvas"></canvas>

<script>
/*============== Creating a canvas ====================*/
var canvas = document.getElementById('my_Canvas');
gl = canvas.getContext('experimental-webgl');

/*======== Defining and storing the geometry ===========*/

var vertices = [
-0.5,0.5,0.0,
-0.5,-0.5,0.0,
0.5,-0.5,0.0,
];

indices = [0,1,2];

// Create an empty buffer object to store vertex buffer
var vertex_buffer = gl.createBuffer();

// Bind appropriate array buffer to it
gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);

// Pass the vertex data to the buffer
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);

// Unbind the buffer
gl.bindBuffer(gl.ARRAY_BUFFER, null);

// Create an empty buffer object to store Index buffer
var Index_Buffer = gl.createBuffer();

// Bind appropriate array buffer to it
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);

// Pass the vertex data to the buffer
gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(indices), gl.STATIC_DRAW);

// Unbind the buffer
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);

// Vertex shader source code
var vertCode =
'attribute vec3 coordinates;' +

'void main(void) {' +
' gl_Position = vec4(coordinates, 1.0);' +
'}';

// Create a vertex shader object

// Attach vertex shader source code

// Compile the vertex shader

//fragment shader source code
var fragCode =
'void main(void) {' +
' gl_FragColor = vec4(0.0, 0.0, 0.0, 0.1);' +
'}';

// Create fragment shader object

// Attach fragment shader source code

// Compile the fragmentt shader

// Create a shader program object to store
// the combined shader program
var shaderProgram = gl.createProgram();

// Attach a vertex shader

// Attach a fragment shader

// Link both the programs

// Use the combined shader program object

/*======= Associating shaders to buffer objects =======*/

// Bind vertex buffer object
gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);

// Bind index buffer object
gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);

// Get the attribute location
var coord = gl.getAttribLocation(shaderProgram, "coordinates");

// Point an attribute to the currently bound VBO
gl.vertexAttribPointer(coord, 3, gl.FLOAT, false, 0, 0);

// Enable the attribute
gl.enableVertexAttribArray(coord);

/*=========Drawing the triangle===========*/

// Clear the canvas
gl.clearColor(0.5, 0.5, 0.5, 0.9);

// Enable the depth test
gl.enable(gl.DEPTH_TEST);

// Clear the color buffer bit
gl.clear(gl.COLOR_BUFFER_BIT);

// Set the view port
gl.viewport(0,0,canvas.width,canvas.height);

// Draw the triangle
gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);
</script>
</body>
</html>
```

If you run this example, it will produce the following output −