Stereo depth

The formula

Z = f · B / d

• f: focal length, in pixels.
• B: baseline, the distance between the two cameras.
• d: disparity, how many pixels the object shifts between the two images.

Example: f = 700 px, B = 0.12 m, d = 20 px → Z = 700 · 0.12 / 20 = 4.2 m.

Consequences

• A wider baseline or closer objects → more disparity → more precise depth.
• Far away, the disparity is only a few pixels and a one-pixel error changes the distance a lot.

Stereo vs lidar

Lidar measures distance directly with light and works at night. Stereo calculates it by comparing two images: it’s cheap and gives colour, but depends on light and on there being texture.