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At the company where I work we model literally thousands of 3DS models for all types of objects. Most don't have transparent textures applied to them, but a few do. The issue is that a texture may have an alpha channel defined, yet its 3DS material definition is not marked as being an alpha source, or transparent. This can happen for any number of modelling reasons (so I'm told) but the net effect is that when a 3DS object is loaded with such a material applied, the stateset creation for the 3DS geometry omits to add the GL_BLEND attribute as neither of the conditions that it currently tests for are satisfied under this scenario. However, one thing that is apparently an indication of transparency/alpha information that we do make use of when modelling, but that the 3DS reader omits to test, is that we assign to the transparent texture to the opacity_map of the 3DS material structure. Now, having discussed with our modellers the theory behind the opacity map, it seems that it is safe to say that if an opacity map is specified, then the reader should really apply the GL_BLEND attribute as it is an indication of some form of blending being required between materials. With this in mind, I have made a minor change to the function createStateSet below, from the ReaderWriter3DS.cpp file, that checks for an opacity map having been specified, and in such a scenario the GL_BLEND attribute is applied to the geometry. This fixed our issue, and thought it might be helpful to others."
This 3ds loader is based on lib3ds, details below. ReaderWriter3DS.cpp is the OSG code which adapts the library into a osg plugin. Robert Osfield, Feburary 2001. -- Lib3ds is a free alternative to Autodesk's 3DS File Toolkit for handling 3DS files It's main goal is to simplify the creation of 3DS import and export filters. This project is not related in any form to Autodesk. The library is based on unofficial information about the 3DS format found on the web. This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU Lesser General Public License for more details. The official Lib3ds Homepage can be found under: http://lib3ds.sourceforge.net J.E. Hoffmann <je-h@gmx.net>