Short version: those “dirty seams” aren’t a Bakery bug—the lightmap is bleeding across UV island borders.
Lightmap Pack in Blender makes a lot of tiny charts with very little space between them. After the mesh is packed into Unity/Bakery’s lightmap atlas (and then filtered/denoised/mip-mapped/compressed), texels near a UV border sample pixels from outside the island, which are usually darker → visible dark cracks.
Why it shows up:
- Too little padding in pixels. Blender’s margin is in UV space; once the mesh is packed into a larger atlas your few-percent margin can become 1–2 pixels. Bilinear filtering, denoising and (if enabled) mipmaps/compression pull in the dark background or a neighboring chart.
- Lots of tiny/sliver islands. Lightmap Pack often splits everything; skinny islands end up < 2–3 texels wide at your chosen resolution, which guarantees bleeding.
- Normals/hard edges not matching seams (sometimes). When a smooth normal spans a UV split, small lighting differences across the split can be emphasized.
How to fix (pick as many as apply):
- Give charts more pixels.
- Raise Bakery resolution (Texels Per Unit / Lightmap Resolution).
- Or scale the object’s Scale in Lightmap up (Unity import setting).
- Increase padding / edge dilation.
- In Bakery, raise Padding/Fix seams (edge dilation). Start around 8–16 px; go higher if you still see it.
- If you unwrap in Blender, repack with a larger Margin for the final atlas size you’ll bake to (e.g., for a 1024 map, aim for ~10–16 px between islands).
- Reduce the number of islands.
- Avoid “Lightmap Pack everything.” Use Angle-Based/Conformal unwraps and mark seams where they make sense; merge tiny islands.
- Split hard edges in geometry and align UV seams with them instead of splitting every triangle.
- Avoid tiny/sliver triangles. Tidy the mesh or mark them as non-GI (if they’re invisible).
- Texture import tweaks:
- Disable compression on Bakery lightmaps (compression can re-introduce bleeding).
- If you enabled mipmaps on baked lightmaps, turn them off (they’re usually off by default for lightmaps).
- Denoiser: If you use a strong denoiser, try a lower strength or run with more samples so the denoiser doesn’t pull across seams.
Rule of thumb: at your final bake resolution, every island should have ≥ 8–16 px of clear padding and be several texels wide everywhere. If you do that and use Bakery’s “Fix seams”/padding generously, the dark cracks will disappear.