// HelloTriangle.js
// 顶点着色器程序
var VSHADER_SOURCE =
    'attribute vec4 a_Position;'+
    'void main(){'+
        'gl_Position=a_Position;'+
        // 'gl_PointSize=10.0;'+   // 修改1 绘制单个点时起作用
    '}';

// 片元着色器程序
var FSHADER_SOURCE=
    '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");
    if(!canvas){
        console.log("Failed to retrieve the <canvas> element");
        return;
    }

    // 获取WebGL绘图上下文
    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_SOURCE,FSHADER_SOURCE)){
        console.log("Failed to initialize shaders.");
        return;
    }

    // 设置顶点位置
    var n = initVertexBuffers(gl);
    if (n < 0) {
        console.log('Failed to set the positions of the vertices');
        return;
    }


    // 指定清空<canvas>颜色
    gl.clearColor(0.0, 0.0, 0.0, 1.0);

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

    // 修改2 绘制三角形 
    gl.drawArrays(gl.TRIANGLES, 0, n);     // 三角形
    // gl.drawArrays(gl.LINE_LOOP, 0, n);  // 三角框
}

function initVertexBuffers(gl) {
    var vertices = new Float32Array([
        0.0, 0.5, -0.5, -0.5, 0.5, -0.5
    ]);
    var n=3; // 点的个数

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

    // 将缓冲区对象保存到目标上
    gl.bindBuffer(gl.ARRAY_BUFFER, vertexBuffer);

    // 向缓存对象写入数据
    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;
    }

    // 将缓冲区对象分配给a_Postion变量
    gl.vertexAttribPointer(a_Position, 2, gl.FLOAT, false, 0, 0);

    // 连接a_Postion变量与分配给它的缓冲区对象
    gl.enableVertexAttribArray(a_Position);

    return n;
}