GPUImage imageFromCurrentFramebuffer 有时为 GPUImageLookupFilter 及其子类返回 nil

我一直在使用GPUImage对于我的项目，我遇到了这个问题imageFromCurrentFramebuffer对于某些返回 nilGPUImageLookupFilter's.

我子类化了GPUImageFilterGroup就像在GPUImageAmatorkaFilter我的代码如下：

-(MTLookupFilter *) initWithLookupImageName:(NSString *) lookupImageName {
self = [super init];
if (self) {
    UIImage *image = [UIImage imageNamed:lookupImageName];
    self.lookupImageSource = [[GPUImagePicture alloc] initWithImage:image];
    GPUImageLookupFilter *lookupFilter = [[GPUImageLookupFilter alloc] init];
    [self addFilter:lookupFilter];
    [self.lookupImageSource addTarget:lookupFilter atTextureLocation:1];
    [self.lookupImageSource processImage];
    self.initialFilters = [NSArray arrayWithObjects:lookupFilter, nil];
    self.terminalFilter = lookupFilter;
}
return self; 
}

我将此类的几个对象添加到数组中，并使用：

- (IBAction)filterAction:(id)sender {
      NSInteger index = arc4random()%self.filtersArray.count;
      id filter = self.filtersArray[index];
      GPUImagePicture *stillImageSource = [[GPUImagePicture alloc] initWithImage:self.fullImage];
      UIImage *filteredimage = nil;
      [stillImageSource addTarget:filter];
      [stillImageSource processImage];
      [filter useNextFrameForImageCapture];
      filteredimage = [filter imageFromCurrentFramebuffer];

      if (filteredimage) {
          self.imageView.image = filteredimage;
      } else {
          NSLog(@"Filtered image is nil");
      }
  }

返回的图像来自imageFromCurrentFramebuffer有时为零，我不明白其原因。我将不胜感激任何帮助。有时即使对于过滤器图像也为零，GPUImageAmatorkaFilter, GPUImageSoftEleganceFilter和`GPUImageMissEtikateFilter，所以我知道这不是我的子类的问题。