Prefers the stored AcDbBlockTableRecord.previewIcon (DXF/DWG group 310).
When absent — common for DXF exports without PreviewIcon — falls back to an
off-scene draw of the block definition via AcDbRenderingCache, matching
AutoCAD's gallery behavior for blocks that have no embedded icon.
Thumbnails are cached per document + block-table-record object id. The cache is
cleared on document activation. Callers that need fresh thumbnails after an
in-place block edit (same object id) should call clearCache before
capturing again.
Captures thumbnail images for block definitions.
Prefers the stored AcDbBlockTableRecord.previewIcon (DXF/DWG group 310). When absent — common for DXF exports without PreviewIcon — falls back to an off-scene draw of the block definition via AcDbRenderingCache, matching AutoCAD's gallery behavior for blocks that have no embedded icon.
Thumbnails are cached per document + block-table-record object id. The cache is cleared on document activation. Callers that need fresh thumbnails after an in-place block edit (same object id) should call clearCache before capturing again.