3d Rayracing basics
I'm afraid they are quite correct on the matter of doubling render time for stereoscopics.
Yes, the geometry of the objects has not changed, but the camera position has, and no matter which way you cut it, that will require a full frame render, independant of the other eye angle.
Where you may be getting understandably confused is the common 'tricks' and 'shortcuts' used in software and hardware for realtime 3D effects, commonly used in directx and openGL. While they can work pretty well, they will still leave telltale glitches.
The best example I could give is anti-aliasing. (the feathering of an objects outline to remove "jaggies").
In realtime 3D, the code in hardware/software to do this, while lightyears beyond my rudimentry programming abilities, is crude to but it mildly. This is because these algorythms rely a great deal on guesstimating what the final output should look like.
Most raytracing software will re-render huge portions of the image to acheive this. In its simplest mode, the ray tracer will 'jostle' the virtual camera about and re-render the entire frame several times, then merge these multiple images together. Most software is optimised now to only re-render aspects of the image that will significatly change, or to detect and only render the elements of the image that have noticable high contrast changes between pixels.
The problem is, there are limits to this predictive ability because ray-tracing a photo-realistic image is highly unpredictable on a pixel-by-pixel basis. reflection, diffusion, specularity, refraction, liminosity, radiosity, lens distortion, motion blur, volumetrics and even lens flares are impossible to be predicted ACCURATELY without going through all the donkey work of actually calculating them.
Moving the camera by a few inches throws every single one of the prior calculations out the window. Hence, two eyes, two angle, two renders. Yes, you can try to cheat, but the difference in the quality shows.
25 years on, and this is the singular lesson that has been well learnt by the ray tracing industry. You want quality? Don't take shortcuts, just throw more horsepower at it.
I often smile when I hear this. I'm afraid to say that you have been tricked many times by CGI, you've only spotted the shoddy workmanship. Just like every single image in a magazine has been photoshopped, every major movie in the last ten years contains a very large number of CGI elements that you don't even notice.
An extra car here, the filling out of a crowd there, a few extra helicopters, explosions, and the replacement of stuntmen. Everybody likes to think they can instantly spot every bit of CGI in a movie, and everybody is wrong. Yes, your eye picks up on the cgi human bodies when they perform physically impossible feats, but what you don't spot is where the human begins, and the cgi ends.
Here's a quick game for you. Can you tell me which explosions in 24 are real, and which ones are digital? Go, just for fun, see if you can draw up a list. I did this one to my friend, while we watched a season back-to-back.
He was gutted when I threw his carefully drawn up list in the bin and explained they ALL were.