title: What does "pre-image" mean in this context?
body: <p>I am comparing the performance of feature detection algorithms like Harris, ORB, MSER (OpenCV Python). I have two images of the same object taken from different viewpoints. Since I am just a beginner to this area, I am having trouble understanding what &quot;pre-image&quot; means in this context. How do I get the &quot;pre-image&quot;?</p>
  <blockquote>
  <p>Detecting regions covariant with a class of transformations has now reached some maturity in the computer vision literature. The requirement for these regions is that they should correspond to the same pre-image for different viewpoints, i.e., their shape is not fixed but automatically adapts, based on the underlying image intensities, so that they are the projection of the same 3D surface patch.</p>
  </blockquote>