title: What is, in simple terms, textureGrad()?
body: <p>I read the <a href="https://www.khronos.org/opengl/wiki/Sampler_(GLSL)#Gradient_texture_access" rel="noreferrer">khronos wiki</a> on this</p>
  <p>But I don;t really understand what it is saying. What exactly does textureGrad do?</p>
  <p>I think it samples multiple mipmap levels and computes some color mixing using the explicit derivative vectors given to it. But I am not sure.</p>