3.2第一帧的三部曲

object ShaderUtil {

    /** 编译单个 Shader。type 取 GL_VERTEX_SHADER 或 GL_FRAGMENT_SHADER */
    fun compileShader(type: Int, source: String): Int {
        val shader = GLES30.glCreateShader(type)
        GLES30.glShaderSource(shader, source)     // 塞入源码
        GLES30.glCompileShader(shader)            // 现场编译

        val status = IntArray(1)
        GLES30.glGetShaderiv(shader, GLES30.GL_COMPILE_STATUS, status, 0)
        if (status[0] == 0) {
            // 编译失败:日志里有行号和原因,GLSL 的"编译器报错"全靠它
            Log.e("Shader", "编译失败:\n${GLES30.glGetShaderInfoLog(shader)}")
            GLES30.glDeleteShader(shader)
            return 0
        }
        return shader
    }

    /** 链接 VS + FS 为一个可用的 Program */
    fun createProgram(vertSrc: String, fragSrc: String): Int {
        val vs = compileShader(GLES30.GL_VERTEX_SHADER, vertSrc)
        val fs = compileShader(GLES30.GL_FRAGMENT_SHADER, fragSrc)
        if (vs == 0 || fs == 0) return 0

        val program = GLES30.glCreateProgram()
        GLES30.glAttachShader(program, vs)
        GLES30.glAttachShader(program, fs)
        GLES30.glLinkProgram(program)

        val status = IntArray(1)
        GLES30.glGetProgramiv(program, GLES30.GL_LINK_STATUS, status, 0)
        if (status[0] == 0) {
            Log.e("Shader", "链接失败:\n${GLES30.glGetProgramInfoLog(program)}")
            GLES30.glDeleteProgram(program)
            return 0
        }
        // 链接完成后,shader 对象已被吸收进 program,可以释放
        GLES30.glDeleteShader(vs)
        GLES30.glDeleteShader(fs)
        return program
    }
}

#version 300 es
layout(location = 0) in vec2 aPos;   // 顶点坐标,由 CPU 提供

void main() {
    // NDC(标准化设备坐标):x、y 都是 -1~1,恰好铺满视口
    // 顶点本来就给的 NDC,所以——原样输出,不做任何变换
    gl_Position = vec4(aPos, 0.0, 1.0);
}

private var program = 0
private var vao = 0

// 全屏 Quad:TRIANGLE_STRIP 顺序(左下、右下、左上、右上)
private val quad = floatArrayOf(
    -1f, -1f,   1f, -1f,   -1f, 1f,   1f, 1f,
)

override fun onSurfaceCreated(gl: GL10?, config: EGLConfig?) {
    program = ShaderUtil.createProgram(VERT_SRC, FRAG_SRC)

    // VAO:先建档,后面的顶点配置都记在它名下
    val ids = IntArray(1)
    GLES30.glGenVertexArrays(1, ids, 0)
    vao = ids[0]
    GLES30.glBindVertexArray(vao)

    // VBO:开显存,把顶点数组搬进去
    GLES30.glGenBuffers(1, ids, 0)
    GLES30.glBindBuffer(GLES30.GL_ARRAY_BUFFER, ids[0])
    val buf = ByteBuffer.allocateDirect(quad.size * 4)
        .order(ByteOrder.nativeOrder()).asFloatBuffer()
        .put(quad).apply { position(0) }
    GLES30.glBufferData(GLES30.GL_ARRAY_BUFFER, quad.size * 4, buf,
                        GLES30.GL_STATIC_DRAW)   // STATIC:传一次不再改

    // 告诉 GPU:location 0 的属性 = 每顶点 2 个 float,紧密排列
    GLES30.glEnableVertexAttribArray(0)
    GLES30.glVertexAttribPointer(0, 2, GLES30.GL_FLOAT, false, 0, 0)

    GLES30.glBindVertexArray(0)   // 收工,解绑
}

private var startNs = System.nanoTime()
private var width = 0; private var height = 0

override fun onSurfaceChanged(gl: GL10?, w: Int, h: Int) {
    width = w; height = h
    GLES30.glViewport(0, 0, w, h)
}

override fun onDrawFrame(gl: GL10?) {
    GLES30.glClear(GLES30.GL_COLOR_BUFFER_BIT)
    GLES30.glUseProgram(program)               // 用哪套 Shader

    // 传本书标准 uniform(location 查询下一章会优化成缓存)
    val time = (System.nanoTime() - startNs) / 1e9f
    GLES30.glUniform1f(GLES30.glGetUniformLocation(program, "uTime"), time)
    GLES30.glUniform2f(GLES30.glGetUniformLocation(program, "uResolution"),
                       width.toFloat(), height.toFloat())

    GLES30.glBindVertexArray(vao)
    GLES30.glDrawArrays(GLES30.GL_TRIANGLE_STRIP, 0, 4)   // 开画!
    GLES30.glBindVertexArray(0)
}