title: GPUImage imageFromCurrentFramebuffer returning nil sometimes for GPUImageLookupFilter and it's subclasses
body: <p>I have been using <code>GPUImage</code> for my project and I am getting into this problem where <code>imageFromCurrentFramebuffer</code> returns nil for some of the <code>GPUImageLookupFilter's</code>.</p>
  
  <p>I subclassed <code>GPUImageFilterGroup</code> like in the <code>GPUImageAmatorkaFilter</code> my code is as follows:</p>
  
  <pre><code>-(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; 
  }
  </code></pre>
  
  <p>I have several of the objects of this class added into an array and I use:</p>
  
  <pre><code>- (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");
        }
    }
  </code></pre>
  
  <p>The returned image from <code>imageFromCurrentFramebuffer</code> is sometimes nil and I do not understand it's cause. I would be thankful for any help. Sometimes the image is nil even for the filters, <code>GPUImageAmatorkaFilter</code>, <code>GPUImageSoftEleganceFilter</code> and `GPUImageMissEtikateFilter so I know it is not a problem with my subclass.</p>