title: GPUImage Different Preview to Output
body: <p>For the first time when using a different GPUImage filter I am seeing strange performance where GPUImage is showing a fairly big difference between the live preview and outputted photo.</p>
  
  <p>I am currently experiencing this with GPUImageSobelEdgeDetectionFilter as follows;</p>
  
  <p><img src="https://i.stack.imgur.com/gJRyL.png" alt="difference"></p>
  
  <p>On the left hand side I have a screenshot of the device screen and on the right, the outputted photo. It seems significantly reduce the thickness and sharpness of the detected lines outputting a very different picture.</p>
  
  <p>I have tried having SmoothlyScaleOutput on and off, but as I am not currently scaling the image this should not be effecting it.</p>
  
  <p>The filter is set up like so;</p>
  
  <pre><code>        filterforphoto = [[GPUImageSobelEdgeDetectionFilter alloc] init];
          [(GPUImageSobelEdgeDetectionFilter *)filterforphoto setShouldSmoothlyScaleOutput:NO];
          [stillCamera addTarget:filterforphoto];
  
          [filterforphoto addTarget:primaryView];
          [stillCamera startCameraCapture];
  
          [(GPUImageSobelEdgeDetectionFilter *)filterforphoto setEdgeStrength:1.0];
  </code></pre>
  
  <p>And the photo is taken like so;</p>
  
  <pre><code>        [stillCamera capturePhotoAsImageProcessedUpToFilter:filterforphoto withCompletionHandler:^(UIImage *processedImage, NSError *error){
  </code></pre>
  
  <p>Does anyone know why GPUImage is interpreting the live camera so differently to the outputted photo? Is it simply because the preview is of a much lower quality than the final image and therefore does look different on a full resolution image?</p>
  
  <p>Thanks,</p>
  
  <p>(p.s. Please ignore the slightly different sizing on the left and right image, I didn't quite light them up as well as I could have)</p>