`models/LICENCE.md` recorded, on 2026-08-21, that no YOLO model trained on
ADE20K existed in usable form — the stuff classes photography cares about,
sky and vegetation and water, had no model to come from. Re-checked
2026-08-30: Ultralytics now ships a `semantic` task with ADE20K
checkpoints, so `models/scene/` holds `yolo26s-sem-ade20k`.
This is an addition, not a replacement. A semantic model labels every
pixel but merges same-class pixels into one region, so it cannot tell
three people apart — which is exactly what clicking a subject needs, and
exactly what `segment/`'s COCO instance model already does. The scene tab
grades per category and does not care that instances are merged. Keeping
both is the point.
## The export is truncated, deliberately
Ultralytics ends the graph with `Resize -> ArgMax -> Cast` and hands back
a `[1, 640, 640]` u8 label map. The script cuts that tail and exposes the
classifier's `[1, 150, 80, 80]` f32 logits instead, for two reasons.
Cost: the Resize materialises 150 x 640 x 640 x f32, 246 MB, and ArgMax
then reduces across the channel axis, striding 409,600 elements per
comparison. On one loaded machine the full graph ran ~1160 ms against
~500 ms truncated — roughly four fifths of the time spent on work the
application discards. Those numbers were measured under contention and
are upper bounds, but the ratio is structural.
Softness: ArgMax destroys the per-class scores, and the scene tab needs
them. Softmax over the 150 channels, summed within each photographic
category, yields per-category weights summing to 1 at every pixel.
Feathering a partition of unity cannot double-grade a boundary, whereas
feathering hard labels outward from two adjacent categories paints both
grades into the overlap and haloes every horizon.
The discarded upsample was never information: the graph's true spatial
resolution is the 80x80 logit grid, and the application can resample from
that itself.
The tail is matched by op type and asserted before cutting, so an
upstream graph change fails loudly in the exporter rather than quietly
shipping a differently-shaped model.
Nothing reads these weights yet — the decode path, the category
descriptor grouping 150 classes into ~8 photographic ones, and the scene
tab are still to come. At 24 MB this model also wants the runtime-asset
treatment `models/face/` already gets on Android rather than
`include_bytes!`; embedding it would put ~35 MB of weights in the binary.
Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
