So, how exactly does the Nintendo 3DS project images without making you wear a pair of retro red-and-cyan 3D glasses?
How 3D Works
We see 3D in real life because the placement of our eyes combines two 2D images into one 3D image. If two 2D images are taken at different angles—the average distance between our eyes—and we view them side by side while cross-eyed, the image appears to pop out at us. The trick is getting our eyes to perceive the pop-out effect without going cross-eyed. This is accomplished in several ways. The most iconic way is via red-and-cyan anaglyph glasses, which work with red-and-cyan movie projector filters. The red lens only emits cyan light, while the cyan one is for the red light. In this way, the eye only sees the light source meant for it, and the cross-eyed 3D effect is achieved without confusion or eyestrain.
Why You Don’t Need Glasses to See 3D on the 3DS
The top screen of the Nintendo 3DS utilizes a filter called a parallax barrier. One of the images necessary for seeing 3D is projected to the right and the other image to the left. The images occupy alternating vertical columns of pixels and are filtered through the parallax barrier. The barrier acts as a vent to project the images and ensure they hit your eyes at the necessary angles to produce the desired 3D effect. For the Nintendo 3DS to project its 3D illusion satisfactorily, you need to be one to two feet away from the top screen and looking directly at it. If you look too far to the side, the effect won’t work properly.
