What is the SDG file format?
.sdg is a proprietary binary container used by the StarOffice Gallery feature to store clipart and image objects. It is always accompanied by two sibling files sharing the same base name: an .sdv file (which uses the Microsoft Compound File Binary Format to store the gallery index and metadata) and a .thm file (which stores thumbnail previews). All three files together form one complete gallery entry.
Each .sdg file may contain multiple image objects - both raster and vector - with each object prefixed by the ASCII marker SGA3, indicating StarOffice Gallery format version 3 or 4. Raster content is stored at up to 24-bit color depth; vector objects are resolution-independent.
The format was introduced as part of the Gallery feature shipped with StarOffice by Sun Microsystems before 2000 and carried forward into early OpenOffice.org releases. By the OpenOffice.org 2.x era the .sdg/.sdv/.thm trio was replaced by a modern XML-based gallery format, making the old format effectively obsolete. StarOffice was discontinued by Oracle around 2011.
No major modern imaging application documents support for .sdg. Users who still hold legacy StarOffice gallery data should try to open files in an old version of OpenOffice.org or LibreOffice and export content to PNG or another current format before the original software becomes inaccessible.
Security & safety
RISK: LOWSDG is a passive image data file that cannot execute. Risk is negligible - it's an obsolete image container from a discontinued product.
Format details
in a nutshellPrograms that open SDG files
Windows2 apps
XnView
Free XnView claims SDG support but real-world support is uncertain; try opening the .sdg directly. 
StarOffice (OpenOffice.org, old version)
Open-source Old OpenOffice.org versions (1.x/2.x) can read the Gallery. Place .sdg/.sdv/.thm together; open via Tools > Gallery. Apache OpenOffice 4.x may partially support it. 
Linux1 app
LibreOffice (limited)
Open-source LibreOffice dropped native SDG gallery support; may partially open old galleries via Tools > Gallery if .sdv/.thm are alongside the .sdg. Technical details
deep spec| Format type | Proprietary binary multi-object gallery/clipart container |
| File set | Always paired with same-base-name `.sdv` (gallery index, Microsoft Compound File Binary) and `.thm` (thumbnails) |
| Object signature | Each embedded image object prefixed with ASCII marker `SGA3` (Gallery format version 3/4 identifier) |
| Content types | Raster images (up to 24-bit color) and resolution-independent vector objects |
| Byte order | Little-endian |
| Compression | None documented; image data stored uncompressed per object |
| Typical file size | 10 KB - several MB depending on number and complexity of gallery images |
| Gallery versions | Format versions 1-4 known; version 4 identified by `SGA3` object marker |
| Associated OS | Windows, Linux, macOS (via StarOffice and OpenOffice.org) |
| Encoding | Binary |
| Superseded by | XML-based gallery format introduced in OpenOffice.org 3.x / LibreOffice (directory of individual image files + XML index) |
| Released | ~1990s (StarOffice Gallery versions 1-4, pre-2000) |
| Latest version | StarOffice Gallery format version 4 (last known; obsolete as of OpenOffice.org 2.x era) |
| Specification | mailman.astron.com |
SDG conversions
Community Q&A
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