title: What is faster - varying or manual interpolation in fragment shader?
body: <p>What is faster - letting a varying to interpolate texture coordinates (and some other gradually changing coefficients) or calculating them manually in fragment shader?</p>
  
  <p>One would think that the answer to this is obvious, but then I stumbled upon someone mentioning (cannot find the source at the moment) that every additional varying comes at a cost and results in a performance drop.</p>
  
  <p><strong>Update:</strong> 
  I'm doing image resampling, so basically for every destination pixel I need to take several samples from original texture and then interpolate. I can pre-calculate the exact coordinates for those samples in vertex shader and pass them via varyings or I can calculate them directly in the fragment shader. In fact I've not seen anyone doing it via varyings. And I thought that there should be a reason behind this.</p>