WebGL – Sample Application


WebGL – Sample Application

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We have discussed the basics of WebGL and the WebGL pipeline (a procedure followed to render Graphics applications). In this chapter, we are going to take a sample application to create a triangle using WebGL and observe the steps followed in the application.

Structure of WebGL Application

WebGL application code is a combination of JavaScript and OpenGL Shader Language.

  • JavaScript is required to communicate with the CPU
  • OpenGL Shader Language is required to communicate with the GPU.

WebGL Application Structure

Sample Application

Let us now take a simple example to learn how to use WebGL to draw a simple triangle with 2D coordinates.

Live Demo 
<!doctype html>
<html>
   <body>
      <canvas width = "300" height = "300" id = "my_Canvas"></canvas>
		
      <script>
         /* Step1: Prepare the canvas and get WebGL context */

         var canvas = document.getElementById(''my_Canvas'');
         var gl = canvas.getContext(''experimental-webgl'');

         /* Step2: Define the geometry and store it in buffer objects */

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

         // Create a new buffer object
         var vertex_buffer = gl.createBuffer();

         // Bind an empty array buffer to it
         gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);
         
         // Pass the vertices data to the buffer
         gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);

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

         /* Step3: Create and compile Shader programs */

         // Vertex shader source code
         var vertCode =
            ''attribute vec2 coordinates;'' + 
            ''void main(void) {'' + '' gl_Position = vec4(coordinates,0.0, 1.0);'' + ''}'';

         //Create a vertex shader object
         var vertShader = gl.createShader(gl.VERTEX_SHADER);

         //Attach vertex shader source code
         gl.shaderSource(vertShader, vertCode);

         //Compile the vertex shader
         gl.compileShader(vertShader);

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

         // Create fragment shader object
         var fragShader = gl.createShader(gl.FRAGMENT_SHADER);

         // Attach fragment shader source code
         gl.shaderSource(fragShader, fragCode);

         // Compile the fragment shader
         gl.compileShader(fragShader);

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

         // Attach a vertex shader
         gl.attachShader(shaderProgram, vertShader); 
         
         // Attach a fragment shader
         gl.attachShader(shaderProgram, fragShader);

         // Link both programs
         gl.linkProgram(shaderProgram);

         // Use the combined shader program object
         gl.useProgram(shaderProgram);

         /* Step 4: Associate the shader programs to buffer objects */

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

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

         //point an attribute to the currently bound VBO
         gl.vertexAttribPointer(coord, 2, gl.FLOAT, false, 0, 0);

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

         /* Step5: Drawing the required object (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.drawArrays(gl.TRIANGLES, 0, 3);
      </script>
   </body>
</html>

It will produce the following result −