Google Choice: WebGL - Modes of Drawing

Friday, March 17, 2017

WebGL - Modes of Drawing

In the previous chapter (Chapter 12), we discussed how to draw a triangle using WebGL. In addition to triangles, WebGL supports various other drawing modes. This chapter explains the drawing modes supported by WebGL.

The mode Parameter

Let’s take a look at the syntax of the methods − drawElements() and draw Arrays().

void drawElements(enum mode, long count, enum type, long offset);

void drawArrays(enum mode, int first, long count);

If you clearly observe, both the methods accept a parameter mode. Using this parameter, the programmers can select the drawing mode in WebGL.
The drawing modes provided by WebGL are listed in the following table.

S.No.	mode	Description
1	gl.POINTS	To draw a series of points.
2	gl.LINES	To draw a series of unconnected line segments (individual lines).
3	gl.LINE_STRIP	To draw a series of connected line segments.
4	gl.LINE_LOOP	To draw a series of connected line segments. It also joins the first and last vertices to form a loop.
5	gl.TRIANGLES	To draw a series of separate triangles.
6	gl.TRIANGLE_STRIP	To draw a series of connected triangles in strip fashion.
7	gl.TRIANGLE_FAN	To draw a series of connected triangles sharing the first vertex in a fan-like fashion.

Example – Draw Three Parallel Lines

The following example shows how to draw three parallel lines using gl.LINES.

<!doctype html>
<html>
   <body>
      <canvas width = "300" height = "300" id = "my_Canvas"></canvas>

      <script>

         /*======= Creating a canvas =========*/

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


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

         var vertices = [
            -0.7,-0.1,0,
            -0.3,0.6,0,
            -0.3,-0.3,0,
            0.2,0.6,0,
            0.3,-0.3,0,
            0.7,0.6,0 
         ]
          
         // Create an empty buffer object
         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);

         /*=================== 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);

         // 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 and depth buffer
         gl.clear(gl.COLOR_BUFFER_BIT | gl.DEPTH_BUFFER_BIT);

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

         // Draw the triangle
         gl.drawArrays(gl.LINES, 0, 6);

         // POINTS, LINE_STRIP, LINE_LOOP, LINES,
         // TRIANGLE_STRIP,TRIANGLE_FAN, TRIANGLES
      
      </script>
  
   </body>
</html>

It will produce the following result −

Drawing Modes

In the above program, if you replace the mode of drawArrays() with one of the following drawing modes, it will produce different outputs each time.