webgl在顶点着色器中旋转三角形，不使用变换矩阵

// 顶点着色器代码
var vertexShaderSource = `
    attribute vec2 a_position;
    uniform float u_rotation;

    void main() {
        // 将顶点坐标绕原点旋转
        float x = a_position.x * cos(u_rotation) - a_position.y * sin(u_rotation);
        float y = a_position.x * sin(u_rotation) + a_position.y * cos(u_rotation);
        gl_Position = vec4(x, y, 0.0, 1.0);
    }
`;

// 片段着色器代码
var fragmentShaderSource = `
    void main() {
        gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0); // 设置片段颜色为红色
    }
`;

// 创建WebGL上下文
var canvas = document.getElementById("canvas");
var gl = canvas.getContext("webgl");

// 创建顶点着色器
var vertexShader = gl.createShader(gl.VERTEX_SHADER);
gl.shaderSource(vertexShader, vertexShaderSource);
gl.compileShader(vertexShader);

// 创建片段着色器
var fragmentShader = gl.createShader(gl.FRAGMENT_SHADER);
gl.shaderSource(fragmentShader, fragmentShaderSource);
gl.compileShader(fragmentShader);

// 创建着色器程序
var program = gl.createProgram();
gl.attachShader(program, vertexShader);
gl.attachShader(program, fragmentShader);
gl.linkProgram(program);
gl.useProgram(program);

// 获取顶点着色器中的属性和统一变量的位置
var positionAttributeLocation = gl.getAttribLocation(program, "a_position");
var rotationUniformLocation = gl.getUniformLocation(program, "u_rotation");

// 创建缓冲区并绑定顶点数据
var positionBuffer = gl.createBuffer();
gl.bindBuffer(gl.ARRAY_BUFFER, positionBuffer);
var positions = [
    -0.5, -0.5,
    0.5, -0.5,
    0.0, 0.5
];
gl.bufferData(gl.ARRAY_BUFFER, new Float32Array(positions), gl.STATIC_DRAW);

// 设置顶点属性指针
gl.enableVertexAttribArray(positionAttributeLocation);
gl.vertexAttribPointer(positionAttributeLocation, 2, gl.FLOAT, false, 0, 0);

// 设置旋转角度并传递给顶点着色器
var rotation = Math.PI / 4; // 45度
gl.uniform1f(rotationUniformLocation, rotation);

// 清空画布并绘制三角形
gl.clearColor(0.0, 0.0, 0.0, 1.0);
gl.clear(gl.COLOR_BUFFER_BIT);
gl.drawArrays(gl.TRIANGLES, 0, 3);