It finally works

Pendulum/Pendulum.js

@@ -18,9 +18,11 @@ const vertex_shader = `
 
 
 const fragment_shader = `
+  precision mediump float;
+  uniform vec4 u_Color;
   void main()
   {
-    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
+    gl_FragColor = u_Color;
   }	`;
 
 // Point size of the anchor
@@ -53,7 +55,7 @@ function main() {
 
   // create a pendulum object
   var pendulum = new Pendulum({ angle: 0, length: PEN_LENGTH, radius: BOB_RADIUS });
-  init(gl, pendulum);
+  pendulum.init(gl);
 
   // Specify the color for clearing <canvas>
   gl.clearColor(0, 0, 0, 1);
@@ -82,10 +84,6 @@ function main() {
   tick();
 }
 
-function init(gl, pendulum) {
-  pendulum.init(gl);
-}
-
 function draw(gl, pendulum, modelMatrix, u_ModelMatrix) {
   // Clear <canvas>
   gl.clear(gl.COLOR_BUFFER_BIT);

Pendulum/PendulumComponents.js

@@ -16,7 +16,7 @@ function acquire_buffer(gl) {
    return vertexBuffer;
 }
 
-function acquire_VAO(gl, name, size, type) {
+function acquire_attribute(gl, name, size, type) {
    // Acquire a reference to named attr
    var ret = gl.getAttribLocation(gl.program, name);
    if (ret < 0) {
@@ -26,10 +26,19 @@ function acquire_VAO(gl, name, size, type) {
    gl.vertexAttribPointer(ret, size, type, false, 0, 0);
    return ret;
 }
+function acquire_uniform(gl, name) {
+   // Acquire a reference to named attr
+   var ret = gl.getUniformLocation(gl.program, name);
+   if (!ret) {
+      console.log(`Failed to get the storage location of ${name}`);
+      return -1;
+   }
+   return ret;
+}
 
 // Classes
 //   homo2: Stores a 2d vector or point in homog. coords
-class homog2 {
+class Homogeneous2D {
    constructor(x = 0, y = 0, w = 0) {
       this.x = x;
       this.y = y;
@@ -57,7 +66,7 @@ class homog2 {
    }
    // copy
    copy() {
-      return new homog2(this.x, this.y, this.w);
+      return new Homogeneous2D(this.x, this.y, this.w);
    }
    // create vector/point from 2d polar coordinates
    from_polar(r = 0, theta = 0, w = 0) {
@@ -68,12 +77,78 @@ class homog2 {
    }
 }
 
+class Rod {
+   constructor({ anchor = { x: 0, y: 0 }, bob = { x: 0, y: 0 }, color = { r: 1.0, g: 0.0, b: 0.0, a: 1.0 } } = {}) {
+      // Set the object's vertices
+      this.vertices = [ new Homogeneous2D(anchor.x, anchor.y, 1),
+      new Homogeneous2D(bob.x, bob.y, 1)]
+      // Set object's color
+      this.color = [color.r, color.g, color.b, color.a];
+   }
+   // Convert to Float32Array [x, y]
+   vertex_array() {
+      // allocate space for center.xy + vertices.xy
+      var ret = new Float32Array((this.vertices.length) * 2);
+      // save the vertices
+      for (var i = 0; i < this.vertices.length; i++) {
+         var j = 2 * i;
+         ret[j] = this.vertices[i].x;
+         ret[j + 1] = this.vertices[i].y;
+      }
+      return ret;
+   }
+   // Generate the index_array for the shape
+   index_array() {
+      var ret = new Uint8Array(this.vertices.length + 1);
+      for (var i = 0; i < this.vertices.length; i++) {
+         ret[i] = i
+      }
+      ret[this.vertices.length] = 0;
+      return ret;
+   }
+   // Initialize the object's drawing parameters
+   init(gl) {
+      // Make the buffers
+      this.vertexBuffer = acquire_buffer(gl);
+      this.indexBuffer = acquire_buffer(gl);
+
+      // Bind the buffers
+      gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);
+      gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, this.indexBuffer);
+
+      // Write date into the buffers
+      gl.bufferData(gl.ARRAY_BUFFER, this.vertex_array(), gl.STATIC_DRAW);
+      gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, this.index_array(), gl.STATIC_DRAW);
+   }
+   // Draw the object
+   draw(gl, modelMatrix, u_ModelMatrix) {
+      // Bind the buffers
+      gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);
+      gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, this.indexBuffer);
+
+      // Set the object's color
+      var u_Color = acquire_uniform(gl, 'u_Color');
+      gl.uniform4fv(u_Color, this.color);
+
+      // get a_Position
+      var a_Position = acquire_attribute(gl, 'a_Position', 2, gl.FLOAT);
+      // Enable the assignment to a_Position variable
+      gl.enableVertexAttribArray(a_Position);
+
+      // Pass the rotation matrix to the vertex shader
+      gl.uniformMatrix4fv(u_ModelMatrix, false, modelMatrix.elements);
+      // Draw the Anchor
+      gl.drawElements(gl.LINES, 2, gl.UNSIGNED_BYTE, 0);
+   }
+}
 
 class Anchor {
-   constructor({ center = { x: 0, y: 0 } } = {}) {
+   constructor({ center = { x: 0, y: 0 }, color = { r: 0.0, g: 1.0, b: 0.0, a: 1.0 } } = {}) {
-      this.position = new homog2(center.x, center.y, 1);
+      this.position = new Homogeneous2D(center.x, center.y, 1);
+      // Set object color
+      this.color = [color.r, color.g, color.b, color.a];
    }
-   // Convert to Float32Array [center, v0, v1, ...]
+   // Convert to Float32Array [x, y]
    vertex_array() {
       // allocate space for center.xy + vertices.xy
       var ret = new Float32Array(2);
@@ -89,7 +164,7 @@ class Anchor {
       console.log(ret);
       return ret;
    }
-
+   // Initialize the object's drawing parameters
    init(gl) {
       // Make the buffers
       this.vertexBuffer = acquire_buffer(gl);
@@ -103,14 +178,18 @@ class Anchor {
       gl.bufferData(gl.ARRAY_BUFFER, this.vertex_array(), gl.STATIC_DRAW);
       gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, this.index_array(), gl.STATIC_DRAW);
    }
-
+   // Draw the object
    draw(gl, modelMatrix, u_ModelMatrix) {
       // Bind the buffers
       gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);
       gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, this.indexBuffer);
 
+      // Set the object's color
+      var u_Color = acquire_uniform(gl, 'u_Color');
+      gl.uniform4fv(u_Color, this.color);
+
       // get a_Position
-      var a_Position = acquire_VAO(gl, 'a_Position', 2, gl.FLOAT);
+      var a_Position = acquire_attribute(gl, 'a_Position', 2, gl.FLOAT);
       // Enable the assignment to a_Position variable
       gl.enableVertexAttribArray(a_Position);
 
@@ -121,18 +200,21 @@ class Anchor {
    }
 }
 
-// hexagon2 class: Holds vertices and a center point. Only constructs regular polygons
+// Polygon class: Holds vertices and a center point. Only constructs regular polygons
 class Polygon {
    // constructor: Make a new regular polygon
-   constructor({ sides = 6, radius = 1, rotation = 0, center = { x: 0, y: 0 }, color = { r: 0, g: 0, b: 1 } } = {}) {
+   constructor({ sides = 6, radius = 1, rotation = 0, center = { x: 0, y: 0 }, color = { r: 0, g: 0, b: 1, a: 1 } } = {}) {
       this.length = 0;
-      // vertices
+      // Mark center point
+      this.center = new Homogeneous2D(center.x, center.y, 1);
+      // Create vertices
       this.vertices = [];
-      this.center = new homog2(center.x, center.y, 1);
       for (var i = 0; i <= sides; i++) {
-         this.vertices[i] = new homog2().from_polar(radius, rotation + (i * 2 * Math.PI / sides)).add_m(this.center);
+         this.vertices[i] = new Homogeneous2D().from_polar(radius, rotation + (i * 2 * Math.PI / sides)).add_m(this.center);
          this.length++;
       }
+      // Set object color
+      this.color = [color.r, color.g, color.b, color.a];
    }
    // Convert to Float32Array [center, v0, v1, ...]
    vertex_array() {
@@ -155,7 +237,7 @@ class Polygon {
       ret[this.vertices.length] = 0;
       return ret;
    }
-
+   // Initialize the object's drawing parameters
    init(gl) {
       // Make the buffers
       this.vertexBuffer = acquire_buffer(gl);
@@ -169,14 +251,18 @@ class Polygon {
       gl.bufferData(gl.ARRAY_BUFFER, this.vertex_array(), gl.STATIC_DRAW);
       gl.bufferData(gl.ELEMENT_ARRAY_BUFFER, this.index_array(), gl.STATIC_DRAW);
    }
-
+   // Draw the object
    draw(gl, modelMatrix, u_ModelMatrix) {
       // Bind the buffers
       gl.bindBuffer(gl.ARRAY_BUFFER, this.vertexBuffer);
       gl.bindBuffer(gl.ELEMENT_ARRAY_BUFFER, this.indexBuffer);
 
+      // Set the object's color
+      var u_Color = acquire_uniform(gl, 'u_Color');
+      gl.uniform4fv(u_Color, this.color);
+
       // get a_Position
-      var a_Position = acquire_VAO(gl, 'a_Position', 2, gl.FLOAT);
+      var a_Position = acquire_attribute(gl, 'a_Position', 2, gl.FLOAT);
       // Enable the assignment to a_Position variable
       gl.enableVertexAttribArray(a_Position);
 
@@ -190,7 +276,9 @@ class Polygon {
 class Pendulum {
    constructor({ angle = 0, length = PEN_LENGTH, radius = BOB_RADIUS, anchor = { x: 0, y: 0 } } = {}) {
       this.angle = angle;
+      var bob = {x: 0, y: -length};
       this.components = [
+         new Rod({ anchor: anchor, bob: bob}),
          new Anchor({ center: anchor }),
          new Polygon({ radius: radius, center: { x: 0, y: -length } })
       ];