OpenGL视图在旋转时消失

我正在使用OpenGL 2.0绘制一个矩形。最初,视口是这样的,我从上面看,我可以像预期的那样看到我的矩形。 然后我开始围绕x轴旋转这个矩形。当旋转角度等于-90度(如果沿另一个方向旋转,则为+90度)时,矩形将消失。 当我旋转超过90度/-90度时,如果矩形的底面消失了,我希望看到的是什么。当上表面即将显示时,它确实会重新出现,总旋转角度为-270度(或+270度)。

如何确保我可以一直看到矩形(旋转时上下表面都必须可见)?

以下是相关代码:

- (void)touchesMoved:(NSSet *)touches withEvent:(UIEvent *)event {
    UITouch * touch = [touches anyObject];
    if ([touches count] == 1) {
        CGPoint currLoc = [touch locationInView:self];
        CGPoint lastLoc = [touch previousLocationInView:self];
        CGPoint diff = CGPointMake(lastLoc.x - currLoc.x, lastLoc.y - currLoc.y);

        rotX = -1 * GLKMathDegreesToRadians(diff.y / 2.0);
        rotY = -1 * GLKMathDegreesToRadians(diff.x / 2.0);

        totalRotationX += ((rotX * 180.0f)/3.141592f);

        NSLog(@"rotX: %f, rotY: %f, totalRotationX: %f", rotX, rotY, totalRotationX);

        //rotate around x axis
        GLKVector3 xAxis = GLKMatrix4MultiplyVector3(GLKMatrix4Invert(_rotMatrix, &isInvertible), GLKVector3Make(1, 0, 0));
        _rotMatrix = GLKMatrix4Rotate(_rotMatrix, rotX, xAxis.v[0], 0, 0);
    }
}


-(void)update{
    GLKMatrix4 modelViewMatrix = GLKMatrix4MakeTranslation(0, 0, -6.0f);
    modelViewMatrix = GLKMatrix4Multiply(modelViewMatrix, _rotMatrix);
    self.effect.transform.modelviewMatrix = modelViewMatrix;

    float aspect = fabsf(self.bounds.size.width / self.bounds.size.height);
    GLKMatrix4 projectionMatrix = GLKMatrix4MakePerspective(GLKMathDegreesToRadians(65.0f), aspect, 0, 10.0f);
    self.effect.transform.projectionMatrix = projectionMatrix;
}

- (void)setupGL {

    NSLog(@"setupGL");
    isInvertible = YES;
    totalRotationX = 0;
    [EAGLContext setCurrentContext:self.context];
    glEnable(GL_CULL_FACE);

    self.effect = [[GLKBaseEffect alloc] init];

    // New lines
    glGenVertexArraysOES(1, &_vertexArray);
    glBindVertexArrayOES(_vertexArray);

    // Old stuff
    glGenBuffers(1, &_vertexBuffer);
    glBindBuffer(GL_ARRAY_BUFFER, _vertexBuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);

    glGenBuffers(1, &_indexBuffer);
    glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, _indexBuffer);
    glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Indices), Indices, GL_STATIC_DRAW);

    glViewport(0, 0, self.frame.size.width, self.frame.size.height);

    // New lines (were previously in draw)
    glEnableVertexAttribArray(GLKVertexAttribPosition);
    glVertexAttribPointer(GLKVertexAttribPosition, 3, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Position));
    glEnableVertexAttribArray(GLKVertexAttribColor);
    glVertexAttribPointer(GLKVertexAttribColor, 4, GL_FLOAT, GL_FALSE, sizeof(Vertex), (const GLvoid *) offsetof(Vertex, Color));

    _rotMatrix = GLKMatrix4Identity;

    // New line
    glBindVertexArrayOES(0);

    initialized = 1;
}

我是OpenGL的新手,我正在使用GLKit和OpenGL 2.0

谢谢