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How use (.obj) Files in Android Projects whit Min3D Framework

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Continuing with testing of 3d android frameworks, finally I could understand how use a .obj file in android.

The following steps may seem illogical but works for me!!

Steps

Download .obj zip

From pages like https://www.turbosquid.com/3d-models/free-dna-3d-model/612564

In this example, zip file contains:

  • Face.obj
  • Face.mtl
  • face_eyel_hi.jpg
  • face_eyer_hi.jpg
  • face_skin_hi.jpg
  • face_sock.jpg

UpperCase

Due to this android restriction :

Error:Execution failed for task ':app:mergeDebugResources'.
> \..\..\Android3D-min3d-Face-Rotation\app\src\main\res\drawable\EyesWhite.jpg: Error: 'E' is not a valid file-based resource name character: File-based resource names must contain only lowercase a-z, 0-9, or underscore

We need replace any upper case letter top lower case if exists from .obj , .mtl and other resources

  • From Face.obj to face.obj

Android ignore extensions

This names are not allowed by android :

  • Face.obj
  • Face.mtl

If you saw, both files are name Face. Android forces us to use a unique file name. So wee need to add obj, mtl or something as to make a unique file name. I use _suffix like :

  • from face.obj to face_obj.obj
  • from face.mtl to face_mtl.mtl

Update references

Files names are changed (images, obj and mtl), so We need to update its refereces where using them.

Update .obj

Obj files need a reference to .mtl files. So if our mtl file name are changed, We need update this. In some cases , .obj files use a asbolute path , so this also could be changed.

  • Open obj file with some text editor and create or update a line whit mtlib reference.
  mtllib /dir/some_dir/face.mtl
  o FaceGen
  g eyeL_hi
  s 1
  v 28.1446 41.1304 43.5008
  v 23.2692 38.3366 41.7695

To

  mtllib face.mtl
  o FaceGen
  g eyeL_hi
  s 1
  v 28.1446 41.1304 43.5008
  v 23.2692 38.3366 41.7695

This is quite strange :

Real name of mtl is face_mtl.mtl , so reference might be mtllib face_mtl.mtl but this does not work.

If I change it to mtllib face.mtl works.

It is as if min3D ignore _mtl of file name.

Other example from my last work : real file name : bigmax_white_mtl.mtl reference in bigmax_white_obj.obj file : mtllib bigmax_white.mtl

Maybe this is part of documentation but I can’t find it, so this crazy workaround works!!

Verify or fix texture

Open face_mtl.mtl file and verify that texture files names are correct:

Example :

newmtl Texture0
Ns 20
d 1
illum 2
map_Kd face_eyel_hi.jpg
Kd 0.7 0.7 0.7
Ks 0 0 0
Ka 0 0 0

newmtl Texture1
Ns 20
d 1
illum 2
map_Kd face_eyer_hi.jpg
Kd 0.7 0.7 0.7
Ks 0 0 0
Ka 0 0 0

Use obj file with android and Min3D

Using this template project, just put files in :

  • res/raw obj and mtl files
  • res/drawable texture jpg files.

Run in your device or android emulator. This should look like:

face_rotation

Enjoy it!!


Until the next,
JRichardsz

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