Hi,
Here is a new tool (still in beta version) that allow you to see .btm file.
It show texture, heightmap & resampled titles bitmaps.
It can also render the 3D-Voxel (no effects).
You can download it here :
OCBtmViewer
Usage,
1) unpack a .pak file to a directory.
2) launch ocbtmviewer and select a .btm file ;o)
