I saw elsewhere people suggesting that for pixel-perfect 3d object picking (mouse clicking), ray-casting per-triangle is the best way. What I had in mind was key-colouring (or color-keying

The scene is drawn in an off-screen buffer. Every separate clickable entity is drawn with its own flat distinct color, with no texturing, lighting, anti-aliasing or complex shaders. For speed purposes, the buffer could only be a small area around the position of the mouse cursor (3x3 or even 1x1 area

So, when there is a mouse click, it's co-ordinates are translated to the off-screen buffer and by checking the color beneath it, we know instantaneously which object is right below the cursor with pixel-perfect accuracy.
How efficient is this method? How fast would it be to draw a complex scene with this dead-simple rendering every frame, alongside normal rendering?
Kind regards,
Bill Kotsias