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Friday, March 17, 2017

WebGL - Drawing a Quad

In the previous chapter, we discussed the different drawing modes provided by WebGL. We can also use indices to draw primitives using one of these modes. To draw models in WebGL, we have to choose one of these primitives and draw the required mesh (i.e., a model formed using one or more primitives).
In this chapter, we will take an example to demonstrate how to draw a quadrilateral using WebGL.

Steps to Draw a Quadrilateral

The following steps are required to create a WebGL application to draw a quadrilateral.
Step 1 − Prepare the Canvas and Get the WebGL Rendering Context
In this step, we obtain the WebGL Rendering context object using getContext().
Step 2 − Define the Geometry and Store it in the Buffer Objects
A square can be drawn using two triangles. In this example, we provide the vertices for two triangles (with one common edge) and indices.
var vertices = [
   -0.5,0.5,0.0,
   -0.5,-0.5,0.0,
   0.5,-0.5,0.0,
   0.5,0.5,0.0 
];

indices = [3,2,1,3,1,0];
Step 3 − Create and Compile the Shader Programs
In this step, you need to write the 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(0.5, 0.3, 0.0, 7.5);' +'}';
Step 4 − Associate the Shader Programs to Buffer Objects
In this step, we associate the buffer objects with the shader program.
Step 5 − Drawing the Required Object
Since we are drawing two triangles to form a quad, using indices, we will use the method drawElements(). To this method, we have to pass the number of indices. The value of indices.length gives the number of indices.
gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);

Example – Draw a Quadrilateral

The following program shows how to create a WebGL application to draw a quadrilateral.
<!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,
            0.5,0.5,0.0 
         ];
          
         indices = [3,2,1,3,1,0];
      
         // 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);

         /*====================== Shaders =======================*/
 
         // Vertex shader source code
         var vertCode =
            'attribute vec3 coordinates;' +
            'void main(void) {' +
               ' gl_Position = vec4(coordinates, 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 fragmentt shader
         gl.compileShader(fragShader);

         // Create a shader program object to
         // store the 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 the programs
         gl.linkProgram(shaderProgram);

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

         /* ======= 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 Quad ================*/

         // 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>
It will produce the following result −
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