= Triple-junction angle in isotropic segmentation
{title2=$\theta_1=\theta_2=\theta_3=120^\circ$}
At a freely moving regular junction of three smooth <image edges> with equal isotropic length weights, the endpoint <first variation> requires the sum of their outward unit tangent vectors to vanish. Three unit vectors with zero sum have pairwise angles of $120$ degrees. Different interface weights instead give a weighted force balance, and constrained or singular junctions require separate hypotheses.
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