webgl动画小测试

// MultiPoint.js (c) 2012 matsuda
// Vertex shader program
var VSHADER_SOURCE =
  'attribute vec4 a_Position;
' +
  'void main() {
' +
  '  gl_Position = a_Position;
' +
  '  gl_PointSize = 10.0;
' +
  '}
';

// Fragment shader program
var FSHADER_SOURCE =
  'precision mediump float;
' +
  'void main() {
' +    // Center coordinate is (0.5, 0.5)
  '  float d = distance(gl_PointCoord, vec2(0.5, 0.5));
' +
  '  if(d < 0.6) {
' +  // Radius is 0.5
  '    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
' +
  '  } else {discard;}
' +
  '}
';

function main() {
  // Retrieve <canvas> element
  var canvas = document.getElementById('webgl');

  // Get the rendering context for WebGL
  var 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
  var 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 three points

var g_init = Date.now();
var speed = 20;
function pointCount() {
    var now = Date.now();
    var count = (now - g_init) * (speed * 0.001 )
    if (count > n) {
      return n
    }
    else if (count<1){
      return 1;
    }
    else {
      return count;
    }
  }

var tick = function() {
  gl.clear(gl.COLOR_BUFFER_BIT);
  gl.drawArrays(gl.POINTS, 0, pointCount());
  requestAnimationFrame(tick);
};
tick();
}



function initVertexBuffers(gl) {
  var vertices = new Float32Array([
    -0.9, 0.0, -0.8, 0.0, -0.7, 0.0,
    -0.6, 0.0, -0.5, 0.0, -0.4, 0.0,
    -0.3, 0.0, -0.2, 0.0, -0.1, 0.0,
    0.0, 0.0,
    0.0, -0.1, 0.0, -0.2, 0.0, -0.3,
    0.0, -0.4, 0.0, -0.5, 0.0, -0.6,
    0.0, -0.7, 0.0, -0.8, 0.0, -0.9
  ]);
  var n = 19; // The number of vertices

  // Create a buffer object
  var 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);

  var 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
  gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);

  // Enable the assignment to a_Position variable
  gl.enableVertexAttribArray(a_Position);

  return n;
}