title: GPUImage : YUV or RGBA impact on performance?
body: <p>I'm working on some still image processing, and GPUImage is a really awesome framework (thank you Brad Larson!).</p>
  
  <p>I understand that : </p>
  
  <ul>
  <li>some filters can be done with only 1 component. In this case, the image should be YUV (YCbCr), and we use only Y (luma = image grey level).</li>
  <li>other filters need all color information from the 3 components - R,G and B.</li>
  <li>YUV -> RGB conversion is provided (in <code>GPUVideoCamera</code>), RGB -> YUV may be hard-coded into the fragment shader (ex: <code>GPUImageChromaKeyFilter</code>)</li>
  </ul>
  
  <p>I have many image-processing steps, some which can be based on YUV, others on RGB.
  Basically, I want to mix RGB and YUV filters, so my general question is this :</p>
  
  <p>What is the cost / information-loss of such successive conversions, and would you recommend any design ?</p>
  
  <p>Thanks!</p>
  
  <p>(PS : what is the problem with iPhone4 YUV->RGB conversion &amp; <code>AVCaptureStillImageOutput</code> pixel-Format ?)</p>