The embedded DWG preview was the whole drawing fit to a canvas, encoded
as a 24-bit uncompressed DIB (~180 KB at 256²) — the bulk of the gap
between an OCS-saved file and AutoCAD's. Three changes:
- Frame it like the screen. `from_scene` now projects every wire vertex
through the live camera into a viewport-aspect canvas, so the thumbnail
is the current pan / zoom / rotation and only the visible region, not
the full extent. Off-screen / clipped segments don't draw (a `None`
projection or NaN separator breaks the polyline run). Works for any
view. `stamp_thumbnail` passes the model pane's pixel size and the
target version.
- PNG for R2013+ (AC1027+). A line drawing on a flat background is almost
one colour, so PNG collapses to a few KB. PNG previews are only valid
from R2013, so older targets keep a BMP.
- BMP is now 8-bit palettised, BI_RLE8-compressed (was 24-bit raw). The
long single-colour runs of a preview compress to a handful of KB; a
view with >256 colours (rare) falls back to the 24-bit DIB. A full
256-entry palette is written so the reader's `(1<<bpp)*4` pixel-data
offset lands correctly.
Net: the preview drops from ~120 KB to ~2-5 KB on every version.
Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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