糟糕的三维三角形渲染

我正在尝试使用opengl 3.1获取三维三角形。它可以很好地渲染为2D三角形，但当我添加第三个坐标时，它并不像我想要的那样。我为所有三个三角形顶点添加了0.0 z坐标。以下是代码：

void init( void )
{   
    vec3 points[NumPoints];
    /*
    points[0] = vec2(-0.9, 0.9 );
    points[1] = (vec2(-0.9, -0.9));
    points[2] = (vec2(0.9, -0.9));
   */
    points[0] = vec3(-0.9, 0.9, 0.0 );
    points[1] = vec3(-0.9, -0.9, 0.0 );
    points[2] = vec3(0.9, -0.9, 0.0 );

    // Create a vertex array object
    GLuint vao;
    glGenVertexArrays( 1, &vao );
    glBindVertexArray( vao );
    // Create and initialize a buffer object
    GLuint buffer;
    glGenBuffers( 1, &buffer );
    glBindBuffer( GL_ARRAY_BUFFER, buffer );
    glBufferData( GL_ARRAY_BUFFER, sizeof(points), points, GL_STATIC_DRAW );
    // Load shaders and use the resulting shader program
    GLuint program = InitShader( "vshader21.glsl", "fshader21.glsl" );
    glUseProgram( program );
    // Initialize the vertex position attribute from the vertex shader
    GLuint loc = glGetAttribLocation( program, "vPosition" );
    glEnableVertexAttribArray( loc );
    glVertexAttribPointer( loc, 2, GL_FLOAT, GL_FALSE, 0, BUFFER_OFFSET(0) );
    glClearColor( 1.0, 1.0, 1.0, 1.0 ); // white background
    glEnable(GL_DEPTH_TEST); 
}

void display( void )
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);  // clear the window
    glDrawArrays( GL_TRIANGLES, 0, NumPoints );    // draw the points
    glFlush();
}
int main( int argc, char **argv )
{
    glutInit(&argc, argv);
    glutInitDisplayMode( GLUT_RGBA | GLUT_DEPTH );
    glutInitWindowSize( 512, 512 );
    glutCreateWindow( "Triangle" );
    glewInit();
    init();
    glutDisplayFunc( display );
    glutKeyboardFunc( keyboard );
    glutMainLoop();
    return 0;
}