"Real" 3D engines (also known as rasterizers) such as id Tech 2, Unreal Engine etc. draws their images in an entirely different way, which makes it impossible to use the rather simple Build Engine method. In most cases, a mirror in a 3D engine (like in Max Payne or Condemned: Criminal Origins) is a smaller secondary viewport projected onto geometry as a texture. This is the same method used for creating the real-time image-in-image effects as seen in e.g. Half-Life 2 and Batman: Arkham Asylum, and taken to the extremes in Portal, where the projected viewpoint is a literal connection to the depicted area that the player is able to walk through. The additional viewports are often kept in a small resolution to increase performance, which is why they often are blurry or very small.