Friday, March 17, 2017

WebGL - Colors

In all our previous examples, we applied color to the object by assigning a desired color value to the gl_FragColor variable. In addition to that, we can define colors for each vertex − just like vertex coordinates and indices. This chapter takes an example to demonstrate how to apply colors to a quadrilateral using WebGL.

Applying Colors

To apply colors, you have to define the colors for each vertex using the RGB values, in JavaScript array. You can assign the same values to all the vertices to have a unique color to the object. After defining the colors, you have to create a color buffer and store these values in it, and associate it to the vertex shader attributes.
In the vertex shader, along with the coordinates attribute (that holds the position of the vertices), we define an attribute and a varying to handle colors.
The color attribute holds the color value per vertex, and varying is the variable that is passed as an input to the fragment shader. Therefore, we have to assign the color value to varying.
In the fragment shader, the varying that holds the color value is assigned to gl_FragColor, which holds the final color of the object.

Steps to Apply Colors

The following steps are required to create a WebGL application to draw a Quad and apply colors to it.
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. Therefore, in this example, we provide the vertices for two triangles (with one common edge) and indices. Since we want to apply colors to it, a variable holding the color values is also defined and the color values for each (Red, Blue, Green, and Pink) are assigned to it.
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 
];

var colors = [ 0,0,1, 1,0,0, 0,1,0, 1,0,1,];

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 vector attributes to store 3D coordinates (position), and the color of each vertex. A varing variable is declared to pass the color values from the vertex shader to the fragment shader. And finally, the value stored in the color attribute is assigned to varying.
var vertCode = 'attribute vec3 coordinates;'+
   'attribute vec3 color;'+
   'varying vec3 vColor;'+
 
   'void main(void) {' +
      ' gl_Position = vec4(coordinates, 1.0);' +
      'vColor = color;'+
   '}';
  • Fragment Shader − In the fragment shader, we assign the varying to the gl_FragColor variable.
var fragCode = 'precision mediump float;'+
   'varying vec3 vColor;'+
   'void main(void) {'+
      'gl_FragColor = vec4(vColor, 1.);'+
   '}';
Step 4 − Associate the Shader Programs with 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 two triangles that will 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 indicates the number of indices.
gl.drawElements(gl.TRIANGLES, indices.length, gl.UNSIGNED_SHORT,0);

Example – Applying Color

The following program demonstrates how to draw a quad using WebGL application and apply colors to it.
<!doctype html>
<html>
   <body>
    <canvas width = "300" height = "300" 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
         ];

         var colors = [0,0,1, 1,0,0, 0,1,0, 1,0,1,];
         
         indices = [3,2,1,3,1,0];
         
         // Create an empty buffer object and store vertex data
         var vertex_buffer = gl.createBuffer();
         gl.bindBuffer(gl.ARRAY_BUFFER, vertex_buffer);
         gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(vertices), gl.STATIC_DRAW);
         gl.bindBuffer(gl.ARRAY_BUFFER, null);

         // Create an empty buffer object and store Index data
         var Index_Buffer = gl.createBuffer();
         gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, Index_Buffer);
         gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, new Uint16Array(indices), gl.STATIC_DRAW);
         gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, null);

         // Create an empty buffer object and store color data
         var color_buffer = gl.createBuffer ();
         gl.bindBuffer(gl.ARRAY_BUFFER, color_buffer);
         gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(colors), gl.STATIC_DRAW);

         /*======================= Shaders =======================*/
         
         // vertex shader source code
         var vertCode = 'attribute vec3 coordinates;'+
            'attribute vec3 color;'+
            'varying vec3 vColor;'+
            'void main(void) {' +
               ' gl_Position = vec4(coordinates, 1.0);' +
               'vColor = color;'+
            '}';
            
         // 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 = 'precision mediump float;'+
            'varying vec3 vColor;'+
            'void main(void) {'+
               'gl_FragColor = vec4(vColor, 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);

         // bind the color buffer
         gl.bindBuffer(gl.ARRAY_BUFFER, color_buffer);
         
         // get the attribute location
         var color = gl.getAttribLocation(shaderProgram, "color");
 
         // point attribute to the volor buffer object
         gl.vertexAttribPointer(color, 3, gl.FLOAT, false,0,0) ;
 
         // enable the color attribute
         gl.enableVertexAttribArray(color);

         /*============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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