title: GPUImage group filters
body: <p>I am trying to implement brightness,contrast and exposure filters in a single view same as you see in iPhoto app. I have tried to put up group filter for doing the same. But it shows up a white screen instead of modified picture. Here is the code I applied.</p>
  
  <pre><code> GPUImageFilterGroup *groupFilter = [[GPUImageFilterGroup alloc]init];
      GPUImageBrightnessFilter *brightnessFilter = [[GPUImageBrightnessFilter alloc]init];
      [brightnessFilter setBrightness:brightnessValue];
      GPUImageContrastFilter *contrastFilter = [[GPUImageContrastFilter alloc]init];
      [contrastFilter setContrast:contrastValue];
      GPUImageExposureFilter *exposureFilter =[[GPUImageExposureFilter alloc]init];
      [exposureFilter setExposure:exposureValue];
      [groupFilter addFilter:brightnessFilter];
      [groupFilter addFilter:contrastFilter];
      [groupFilter addFilter:exposureFilter];
       GPUImagePicture *stillImage= [[GPUImagePicture alloc]initWithImage:self.imageToModify];
      [stillImage addTarget:groupFilter];
  
  
      [stillImage processImage];
  
      previewPicture.image = [groupFilter imageFromCurrentlyProcessedOutputWithOrientation:self.imageToModify.imageOrientation];
  </code></pre>
  
  <p>I even tried to put each individually but still it shows up white image. Is the above code I am using correct?
  I have also tried using <code>GPUImageFilterPipeline</code> instead of <code>GPUImageFilterGroup</code> but still having the same issue. 
  For the record, the image is a still image and not live feed.</p>