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From Computer Vision Primer

Betti numbers count the number of topological features in the image:

• objects or connected components – dimension 0,
• holes or tunnels – dimension 1, and
• voids or cavities – dimension 2.

These numbers in each dimension are captured by the Betti numbers, B₀, B₁, and B₂. Examples are in the table below.

The tire (torus) has two tunnels represented by these two "cycles".

Betti numbers are combined together to produce the well-known Euler number.

Continue to Topological Features of Images.