`Pruning`¶

Removes the outermost branches of thin objects in an image.

Pruning[image, n] runs n passes, each of which deletes every endpoint — a foreground pixel at the tip of a branch. Here that is the two ends of the bar and the pixel sticking out of its middle:

$ wo 'ImageData[Pruning[Image[{{0, 0, 1, 0, 0}, {1, 1, 1, 1, 1}}], 1]]'
{{0., 0., 0., 0., 0.}, {0., 1., 1., 1., 0.}}

Every pass eats one more pixel off each branch, so n passes remove every branch at most n pixels long:

$ wo 'ImageData[Pruning[Image[{{0, 0, 1, 0, 0}, {0, 0, 1, 0, 0}, {0, 0, 1, 0, 0}, {1, 1, 1, 1, 1}}], 2]]'
{{0., 0., 0., 0., 0.}, {0., 0., 0., 0., 0.}, {0., 0., 1., 0., 0.}, {0., 0., 1., 0., 0.}}

Infinity keeps going until nothing more falls away:

$ wo 'ImageData[Pruning[Image[{{0, 0, 1, 0, 0}, {0, 0, 1, 0, 0}, {1, 1, 1, 1, 1}}], Infinity]]'
{{0., 0., 0., 0., 0.}, {0., 0., 0., 0., 0.}, {0., 0., 1., 0., 0.}}

A connected piece that never branches is a shape rather than a branch, so Pruning[image, n] leaves it whole however many passes are asked for:

$ wo 'ImageData[Pruning[Image[{{1, 1, 1, 1, 1}}], Infinity]]'
{{1., 1., 1., 1., 1.}}

A pixel with no lit neighbor at all is an isolated point, not the tip of a branch, and survives:

$ wo 'ImageData[Pruning[Image[{{1, 0}, {0, 0}}]]]'
{{1., 0.}, {0., 0.}}

Only the pruned pixels go black; the survivors keep their gray value:

$ wo 'ImageData[Pruning[Image[{{0.4, 0.6, 0.8}}]]]'
{{0., 0.6000000238418579, 0.}}

A branch length that is not a non-negative integer is reported:

$ wo 'Pruning[Image[{{1, 0}}], -1]'

Pruning::invniter: Expecting a non-negative integer value for the number of iterations.
Pruning[-Image-, -1]

Pruning¶

`Pruning`¶