Photogrammetry is a technique allowing measurements of features visible on photographs taken from different points of view.

The eyes of an observer send slightly different images to his brain. The visual cortex analyses the differences between these images to build a virtual 3D model of the scene

Using 2 camera placed at a distance equivalent to the distance between our eyes (about 65mm), we can also get 2 slightly different images:

To build a virtual 3D model from these photos, the left eye must see the left image and the right eye must see the right image. This can be achieved with some practice, but not easily, because in normal circumstances, the two axes of the human eyes converge to a point placed at focusing distance.

To be able keeping vision axes parallel while focusing on a close distance, we need some optical assitance in the form of lenses, mirrors, prisms, etc.

Another solution to visually merge 2 different images is based on using colour filters. This is the principle of analglyph displays, based on glasses equipped with one red and one cyan filter. This principle works with printed images, but also on video screens and projected images, still or moving. One drawback of the analglyph method is to slightly degrade the colours of the scene.

More sophisticated systems are based on using polarizing filters or shutter lenses synchronized to alternated display of left and rigth images on the same screen.