Xuan,

Unfortunately I don't have the original problematic obj files anymore, but I just created a new one with the same export settings that I first tried (where "Materials" was unchecked in the Blender options settings").  If I try to import an obj file without that I now get an error that says "Trying to use material without a loaded material file.  Cannot load model."  Today is the first time I've ever seen that message, however I can still replicate the original problem I experienced by deleting the "usemtl" line on the obj file, which then allows the file to be imported and saved, however attempting to open the resultant l3d file gives the error "File does not contain a valid Layar3D model" (it actually just produces an empty file now).  The obj file is attached (bad1.obj).  This part seems to be less of an issue now though, because the initial error message now seems to effectively prevent the loading of an obj file without an associated mtl file.

As for the issue where Blender sometimes exports obj filed with an unnamed "usemtl", I've attached an example of that as well (test4.obj, test4.mtl).  Attempting to import test4.obj will not produce any error messages, it will simply cause the converter to hang.

After some more trial and error, I found that the shape doesn't seem to matter.  Immediately after this post I made a simple model with cubes again, and the converter failed to successfully import it.  After that I began comparing the obj and mtl files which could be converted with those which could not, and I believe I've found the solution.  Right now it involves editing both of the files, but perhaps the converter could be modified to handle this instead.

In the obj files which failed to import, there is a line that references the material with "newmtl" but has no name after it, as you can see in this snippet that I cut and paste:

v -10.000004 0.250000 0.250000
v -9.999999 0.250000 -0.250000
usemtl
s off
f 1 2 3 4
f 5 8 7 6
f 1 5 6 2

The corresponding mtl file looks like this:

# Blender MTL File: ''
# Material Count: 1
newmtl
Ns 96.078431
Ka 0.000000 0.000000 0.000000
Kd 0.640000 0.640000 0.640000
Ks 0.500000 0.500000 0.500000
Ni 1.000000
d 1.000000
illum 2

If I add a parameter name for newmtl such as "Material" in both the obj and mtl files, then the converter tools seems to work fine.  The obj snippet then looks like:

v -10.000004 0.250000 0.250000
v -9.999999 0.250000 -0.250000
usemtl Material
s off
f 1 2 3 4
f 5 8 7 6
f 1 5 6 2

and the mtl file looks like:

# Blender MTL File: ''
# Material Count: 1
newmtl Material
Ns 96.078431
Ka 0.000000 0.000000 0.000000
Kd 0.640000 0.640000 0.640000
Ks 0.500000 0.500000 0.500000
Ni 1.000000
d 1.000000
illum 2

As of now, I'm not sure why Blender sometimes names the "newmtl" and other times does not.  It seems to me that the converter is currently coded in such a way that a name MUST follow "newmtl" or else the converter hangs.

Xuan,

I'm not sure what changed, but when I tried today exporting and converting worked fine without editing the obj file provided that I use cubes.  However, if I try to create a model using cylinders then I get an obj file which causes the converter to hang when it attempts to import it.  I tried selecting the 'triangulate' option in the export settings with the same result.  It is possible to convert a model which uses cylinders?