title: Camera transformation, how to distinguish object in the front and the back?
body: <p>I have a camera transformation matrix <code>K * Rt</code> where K is the intrinsic and Rt is extrinsic, which maps a 4-dimensional homogeneous coordinate X to pixel coordinate on the image Y.</p>
  <pre><code>Y = K*Rt*X
  </code></pre>
  <p>This works fine and it does map the real-world points to my image. However, for some reason, it is also mapping some points that should not be inside the image into the image, so I wonder if these points are from the back of the camera.</p>
  <p>So my question is, where will the point from the back of the camera be mapped to if we apply this transformation to them?  And if it will also be mapped to the image within the same pixel, how can we distinguish if it's from the back or front of the camera?</p>