The exporter analyzes the SKP by 'context' - 'raw-face/group-face/instance-face' and then by 'face-material/group-material/instance-material'.
The faces are triangulated and exported/grouped in material sub-sets.
Faces on visible layers within the same 'context' are not segregated by layer in the export.
You do not need to 'group' faces yourself - beyond what you need to assist in your modeling - so for example within a room there may be several walls with different materials, apply the materials to the faces,
Applying materials to a group/instance affects all faces with 'nil' material applied within that 'container'.
You cannot 'map' Textures in the SKP that have been applied to 'containers' - only when applied directly to faces.
'Images' [as opposed to materials with Textures that are made from images] will be successfully exported from the SKP but again an Image is a flat rectangular face in the SKP and exported as a group of two triangulated faces using the image as their material. An 'Image' cannot be mapped or used on non-rectangular surfaces - unless it is exploded into a Material beforehand.
When the SKP's triangulated faces are exported each one is assigned the front-material that it has, the mapping of that material in the SKP is replicated on each facet. Mapping only applies to Materials with Textures, as plain-colors need no mapping.
All faces using a Mapped Texture that has been simply [re]positioned/scaled/rotated will export using the one texture-image file, will suitable mapping code in the exported files.
Any Textures that have been 'distorted/skewed' cannot be 'mapped' using the same texture-image file. as a triangulated facet in the SKP has insufficient vertices in OBJ format [3 v 4] - so in that case the distorted-texture is exported as a separate distorted image file and that is used [with no mapping]: this is the same result as using 'Make Unique Texture' in the SKP - where a distorted texture can be reused in a new material that will always display the texture with that distortion.
If you have a very faceted form with many 'projected' [i.e. then distorted] textures then this can result in a lot of individual images being created during the export to resolve the mapping.
This is a limitation of this exporter AND Sketchup Pro's own OBJ exporter...
There are several excellent 3rd party organic modeling tools, some with mapping capabilities [e.g. Artisan and TGi3d].
There are also several UV-mapping apps that can make a single unwrapped 'image' to reuse as your forms image later [e.g. http://forums.sketchucation.com/viewtop ... 18#p339218
forums for many such discussions and links about these aspects...
e.g. http://www.sketchucation.com/forums/vie ... e0#p300284http://www.youtube.com/watch?v=VPf0EIO1iBohttp://www.youtube.com/watch?v=8q9TE62bksYhttp://www.youtube.com/watch?v=r8Ubwh5F1d8http://www.youtube.com/watch?v=Bm7P3swwbUchttp://www.youtube.com/watch?v=fAtCkjXQOtg