Microsoft Train Simulator uses a custom file format for its textures and other in-game images - ACE (no, not the data compression format). They are stored as a 2nd-level format inside the Simis file format, which means they can be compressed with zlib's DEFLATE algorithm transparently.
I'm not going to document the format publicly just yet, but instead show a preview of loading ACE images in the next version of Simis Editor:
I have also implemented a DXT1 compression algorithm which, together with the decoder, allows Simis Editor to load and save ACE files in all three of their "formats":
- Uncompressed (A)RGB.
- Zlib compressed (A)RGB.
- DXT1 compressed.
While Simis Editor will not become an image editor in its own right, I will be supporting importing/exporting as standard image types (BMP, PNG, etc.) and the new "ImageFile" command-line tool will provide for easy bulk conversions. Stay tuned for more details.