[libredwg] [bug #59782] dwgbmp doesn't work in newer DWG version

[Lorenzo Marcantonio]

URL:
  <https://savannah.gnu.org/bugs/?59782>

                 Summary: dwgbmp doesn't work in newer DWG version
                 Project: LibreDWG
            Submitted by: mrclnz
            Submitted on: Wed 30 Dec 2020 10:55:24 AM UTC
                Category: None
                Severity: 3 - Normal
              Item Group: None
                  Status: None
                 Privacy: Public
             Assigned to: None
             Open/Closed: Open
         Discussion Lock: Any

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Details:

More than a bug report this could be a contribution. I did a thumbnail
provider for dwg files, somewhat inspired by dwgbmp.

dwgbmp doesn't work on many post R2000 dwg files, for some different reasons:
- Maybe it can't find the preview block
- It only supports type 1 (BMP) DIBs in 256 color paletted mode
- Newer formats actually have type 6 previews (not documented in the reverse
engineered format), which are simply PNG files stored in the preview block.

I solved the first issue simply by looking for the 16-byte signature (not
elegant but it works); the second issue would need interpreting the DIB header
but I've only seen a couple of file around with a different DIB format. The
third issue is simply handled writing the block as-is as a png file.

I use it with ARES Commander (up to R2018 format) and it seems to work fine.

The tool is here: https://github.com/mrclnz/dwgthumb
It's GPL3 so there shouldn't be any problem

Thanks for the very good work.




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