Problem: with a trained model installed, Object Matte's optional box reached
MobileSAM only as background points along the box's edges (an approximation),
although the model's prompt encoder takes a box prompt directly.
Fix: a new Roto Brush stroke kind, Box (two opposite corners), that
stroke_prompt turns into the model prompt's bbox (scaled to the matte grid,
corners in any order, degenerate or non-finite boxes ignored). The graph cut
never rasterizes it. Object Matte emits it instead of the edge strokes when a
model segments; the classical fallback keeps the edge strokes. A prior matte's
box no longer replaces a box the user drew on that frame.
Tests: effects object_matte::the_box_reaches_the_model_as_a_box_prompt (a
stand-in model sees the box, at full and half resolution, and no edge points);
track roto::tests::box_stroke_is_the_model_box_prompt.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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