WebGL编程指南案例解析之绘制四边形

//案例4，绘制矩形，和三角形类似，但是注意因为一个矩形有4个顶点，按照两个三角形绘制矩形的话，顶点顺序要注意



var vShader = `
  attribute vec4 a_Position;
  void main(){
    gl_Position = a_Position;
  }
`;
var fShader = `
  void main(){
    gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
  }
`;



function main(){
  //获取canvas元素
  var canvas = document.getElementById(‘webgl‘);

  //获取webgl上下文
  var gl = getWebGLContext(canvas);

  if(!gl){
    console.log(‘Failed to get the rendering context for WebGL!‘);
    return;
  }

  

  //初始化着色器
  if(!initShaders(gl,vShader,fShader)){
    console.log(‘Failed to initialize shaders.‘);
    return;
  }


  var n = initVertexBuffers(gl);
  if(n < 0){
    console.log(‘Failed to set the positions of the vertices!‘);
    return;
  }

  //用指定颜色填充webgl容器，就是设置背景
  gl.clearColor(0.4, 0.5, 0.0, 1.0);
  gl.clear(gl.COLOR_BUFFER_BIT);



  gl.drawArrays(gl.TRIANGLE_STRIP,0,n);



  function initVertexBuffers(gl){
　　 //四个顶点
    var vertices =  new Float32Array([
      -0.5,0.5,-0.5,-0.5,0.5,0.5,0.5,-0.5
    ]);
    var n = 4;//点的个数

    //创建缓冲区对象
    var vertexBuffer = gl.createBuffer();
    if(!vertexBuffer){
      console.log(‘Failed to create the buffer object!‘);
      return -1;
    }

    //将缓冲区对象绑定到目标ARRAY_BUFFER
    gl.bindBuffer(gl.ARRAY_BUFFER,vertexBuffer);
    //往ARRAY_BUFFER
    gl.bufferData(gl.ARRAY_BUFFER,vertices,gl.STATIC_DRAW);

    //获取shaderProgram中attribute变量‘a_Position’的地址
    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;
    }
    //将缓冲区对象分配给a_Position变量
    gl.vertexAttribPointer(a_Position,2,gl.FLOAT,false,0,0);
    //开启着色器对缓冲区数据的访问
    gl.enableVertexAttribArray(a_Position);
    return n;
  }
}


main();