lab1.js

// Vertex shader program
const VSHADER_SOURCE =
  'attribute vec4 a_Position;\n' +
  'attribute vec4 a_Color;\n' + // Attribute variable color to bind the effective vertex color from the model data

  'varying vec4 v_Color;\n' +   // Varying variable color to pass the vertex color to the fragment shader

  'void main() {\n' +
  '  gl_Position = a_Position;\n' +
  '  v_Color = a_Color;\n' +    // Set the varying color value with the attribute color value
  '}\n';

// Fragment shader program
const FSHADER_SOURCE =
  'precision mediump float;\n' + // This determines how much precision the GPU uses when calculating floats
  'varying vec4 v_Color;\n' +   // Varying variable color to get the vertex color from
                                // The vertex shader (!!same name as in the vertex shafer)
  'void main() {\n' +
  '  gl_FragColor = v_Color;\n' + // Set the fragment color with the vertex shader
  '}\n';

function main() {
  // Retrieve <canvas> element
  const canvas = document.getElementById('my-canvas');

  // Get the rendering context for WebGL
  const gl = getWebGLContext(canvas);
  if (!gl) {
    console.log('Failed to get the rendering context for WebGL');
    return;
  }

  // Initialize shaders
  if (!initShaders(gl, VSHADER_SOURCE, FSHADER_SOURCE)) {
    console.log('Failed to intialize shaders.');
    return;
  }

  // Write the positions of vertices to a vertex shader
  const n = initVertexBuffers(gl);
  if (n < 0) {
    console.log('Failed to set the positions of the vertices');
    return;
  }

  // Specify the color for clearing <canvas>
  gl.clearColor(0, 0, 0, 1);

  // Clear <canvas>
  gl.clear(gl.COLOR_BUFFER_BIT);

  // Draw the vertext buffer using triangles
  gl.drawArrays(gl.TRIANGLES, 0, n);
}

function initVertexBuffers(gl) {
  // This is the model
  const vertices = new Float32Array([
    // x     y
    -3/5,  -5/7, 1.0, 0.0, 0.0,
    -1/5,  5/7, 1.0, 0.0, 0.0,
    -3/5,  5/7, 1.0, 0.0, 0.0,

    -3/5,  -5/7, 0.0, 1.0, 0.0,
    -1/5,  5/7, 0.0, 1.0, 0.0,
    -1/5,  -5/7, 0.0, 1.0, 0.0,
	
	-1/5,  5/7, 0.0, 0.0, 1.0,
    3/5,  5/7, 0.0, 0.0, 1.0,
    3/5,  3/7, 0.0, 0.0, 1.0,
	
	-1/5,  5/7, 1.0, 0.0, 1.0,
    -1/5,  3/7, 1.0, 0.0, 1.0,
    3/5,  3/7, 1.0, 0.0, 1.0,
	
	-1/5,  1/7, 1.0, 1.0, 0.0,
    1/5,  1/7, 1.0, 1.0, 0.0,
    1/5,  -1/7, 1.0, 1.0, 0.0,

    -1/5,  1/7, 0.0, 1.0, 1.0,
    -1/5,  -1/7, 0.0, 1.0, 1.0,
    1/5,  -1/7, 0.0, 1.0, 1.0,

    -1/5,  -3/7, 0.5, 0.7, 0.2,
    3/5,  -3/7, 0.5, 0.7, 0.2,
    3/5,  -5/7, 0.5, 0.7, 0.2,

    -1/5,  -3/7, 0.7, 0.2 , 0.5,
    -1/5,  -5/7, 0.7, 0.2 , 0.5,
    3/5,  -5/7, 0.7, 0.2 , 0.5,
	
	1/5,  3/7, 0.2, 0.3, 0.8,
    1/5,  -3/7, 0.2, 0.3, 0.8,
    3/5,  -3/7, 0.2, 0.3, 0.8,
	
	1/5,  3/7, 0.8, 0.1, 0.2,
    3/5,  -3/7, 0.8, 0.1, 0.2,
    3/5,  3/7, 0.8, 0.1, 0.2
  ]);
  const n = vertices.length / 5; // The number of vertices
  // Create a buffer object
  const vertexBuffer = gl.createBuffer();
  if (!vertexBuffer) {
    console.log('Failed to create the buffer object');
    return -1;
  }

  // Bind the buffer object to target
  gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);
  // Write date into the buffer object
  gl.bufferData(gl.ARRAY_BUFFER, vertices, gl.STATIC_DRAW);

  // Get the storage location of a_Position, assign buffer and enable
  const a_Position = gl.getAttribLocation(gl.program, 'a_Position');
  if (a_Position < 0) {
    console.log('Failed to get the storage location of a_Position');
    return -1;
  }
  // Assign the buffer object to a_Position variable
  // Read 2 elements of type gl.FLOAT, starting at the offset 0.
  // Next offset will be 20: after 5 elements (x, y, r, g, and b) of 4 bytes (32 bits) each
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 20, 0);
  // Enable the assignment to a_Position variable
  gl.enableVertexAttribArray(a_Position);

  // Get the storage location of a_Color, assign buffer and enable
  var a_Color = gl.getAttribLocation(gl.program, 'a_Color');
  if(a_Color < 0) {
    console.log('Failed to get the storage location of a_Color');
    return -1;
  }
  // Assign the buffer object to a_Color variable
  // Read 3 elements of type gl.FLOAT, starting at the offset 8 (after 2 elements (x and z) of 4 bytes each).
  // Next offset will be 20: after 5 elements (x, y, r, g, and b) of 4 bytes (32 bits) each
  gl.vertexAttribPointer(a_Color, 3, gl.FLOAT, false, 20, 8);
  // Enable the assignment to a_Color variable
  gl.enableVertexAttribArray(a_Color);

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

  return n;
}