将CIFilter应用于OpenGL渲染到纹理

我正在尝试在全屏渲染输出上应用CoreImage滤镜，但由于我将黑屏作为输出，因此似乎缺少了一些东西。

首先，我将整个场景绘制为纹理。然后，我根据该纹理创建CoreImage，最后绘制并呈现。但是我得到的只是黑屏。我遵循Apple关于绘制纹理并将CoreImage与OpenGLES集成的指导方针：WWDC2012 511和https://developer.apple.com/library/ios/documentation/3ddrawing/conceptual/opengles_programmingguide/WorkingwithEAGLContexts/WorkingwithEAGLContexts.html

以下是相关代码：

渲染器：

@interface Renderer () {
  EAGLContext* _context;
  GLuint _defaultFramebuffer, _drawFramebuffer, _depthRenderbuffer, _colorRenderbuffer, _drawTexture;
  GLint _backingWidth, _backingHeight;
  CIImage *_coreImage;
  CIFilter *_coreFilter;
  CIContext *_coreContext;
}

初始化方法：

- (BOOL)initOpenGL
{
  _context = [[EAGLContext alloc] initWithAPI:kEAGLRenderingAPIOpenGLES2];
  if (!_context) return NO;

  [EAGLContext setCurrentContext:_context];

  glGenFramebuffers(1, &_defaultFramebuffer);
  glBindFramebuffer(GL_FRAMEBUFFER, _defaultFramebuffer);

  glGenRenderbuffers(1, &_colorRenderbuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, _colorRenderbuffer);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_RENDERBUFFER, _colorRenderbuffer);

  glGenFramebuffers(1, &_drawFramebuffer);
  glBindFramebuffer(GL_FRAMEBUFFER, _drawFramebuffer);

  glGenTextures(1, &_drawTexture);
  glBindTexture(GL_TEXTURE_2D, _drawTexture);
  glFramebufferTexture2D(GL_FRAMEBUFFER, GL_COLOR_ATTACHMENT0, GL_TEXTURE_2D, _drawTexture, 0);

  glGenRenderbuffers(1, &_depthRenderbuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, _depthRenderbuffer);
  glFramebufferRenderbuffer(GL_FRAMEBUFFER, GL_DEPTH_ATTACHMENT, GL_RENDERBUFFER, _depthRenderbuffer);

  _coreFilter = [CIFilter filterWithName:@"CIColorInvert"];
  [_coreFilter setDefaults];

  NSDictionary *opts = @{ kCIContextWorkingColorSpace : [NSNull null] };
  _coreContext = [CIContext contextWithEAGLContext:_context options:opts];

  return YES;
}

每当层大小更改时（在初始化和方向更改时）的Alloc内存：

- (void)resizeFromLayer:(CAEAGLLayer *)layer
{
  layer.contentsScale = 1;

  glBindFramebuffer(GL_FRAMEBUFFER, _defaultFramebuffer);

  glBindRenderbuffer(GL_RENDERBUFFER, _colorRenderbuffer);
  [_context renderbufferStorage:GL_RENDERBUFFER fromDrawable:layer];

  glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_WIDTH, &_backingWidth);
  glGetRenderbufferParameteriv(GL_RENDERBUFFER, GL_RENDERBUFFER_HEIGHT, &_backingHeight);

  // glCheckFramebufferStatus ... SUCCESS

  glBindFramebuffer(GL_FRAMEBUFFER, _drawFramebuffer);

  glBindTexture(GL_TEXTURE_2D, _drawTexture);
  glTexImage2D(GL_TEXTURE_2D, 0, GL_RGBA, _backingWidth, _backingHeight, 0, GL_RGBA, GL_UNSIGNED_BYTE, NULL);

  glBindRenderbuffer(GL_RENDERBUFFER, _depthRenderbuffer);
  glRenderbufferStorage(GL_RENDERBUFFER, GL_DEPTH_COMPONENT16, _backingWidth, _backingHeight);

  // glCheckFramebufferStatus ... SUCCESS
}

绘制方法：

- (void)render:(Scene *)scene
{
  [EAGLContext setCurrentContext:_context];

  glBindFramebuffer(GL_FRAMEBUFFER, _drawFramebuffer);

  // Draw using GLKit, custom shaders, drawArrays, drawElements
  // Now rendered scene is in _drawTexture

  glBindFramebuffer(GL_FRAMEBUFFER, _defaultFramebuffer);
  glBindRenderbuffer(GL_RENDERBUFFER, _colorRenderbuffer);

  // Create CIImage with our render-to-texture texture
  _coreImage = [CIImage imageWithTexture:_drawTexture size:CGSizeMake(_backingWidth, _backingHeight) flipped:NO colorSpace:nil];

  // Ignore filtering for now; Draw CIImage to current render buffer 
  [_coreContext drawImage:_coreImage inRect:CGRectMake(0, 0, _backingWidth, _backingHeight) fromRect:CGRectMake(0, 0, _backingWidth, _backingHeight)];

  // Present
  [_context presentRenderbuffer:GL_RENDERBUFFER];
}

注意，在绘制场景之后，_drawTexture包含渲染的场景。我使用Xcode调试工具（捕获OpenGL ES框架）进行了检查。

编辑：如果我尝试从其他纹理中创建CIImage，然后_drawTexture，它将正确显示它。我怀疑_drawTexture可能尚未准备就绪，或者在CIContext尝试通过CIImage呈现它时被某种方式锁定。

EDIT2：我还尝试仅用视口清除来替换所有绘图代码：

  glViewport(0, 0, _backingWidth, _backingHeight);
  glClearColor(0, 0.8, 0, 1);
  glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

结果仍然是黑色的。这表明问题可能出在绘制纹理或帧缓冲区上。