Supports multiple textures in .obj file

[theNded]

Enhancement of PR #1194.

This PR supports loading multiple textures for one single mesh, addressing #1484.

Major changes:

• Added property material_ids per face in TriangleMesh supported by tinyobjloader.
• Supports rendering multiple textures in several passes.

Note: due to the general API design in the IO module, an .obj containing multiple shapes is still not supported.

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This change is 

[update-docs]

Thanks for submitting this pull request! The maintainers of this repository would appreciate if you could update the CHANGELOG.md based on your changes.

[theNded]

Sorry that I messed up the log... Any idea how to clean this shit?
Update: alright fixed, but the delete and restore history stays. Can make a new PR if you feel that it looks dirty...

[griegler]

Thanks. I did not check the visualization part, maybe @yxlao can have a look at that.
Is the mesh from #1484 now rendering correctly?

Reviewed 3 of 9 files at r1.
Reviewable status: 3 of 9 files reviewed, 1 unresolved discussion (waiting on @griegler, @theNded, and @yxlao)

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src/Open3D/IO/FileFormat/FileOBJ.cpp, line 280 at r1 (raw file):

    if (write_triangle_uvs && mesh.HasTexture()) {
        if (mesh.textures_.size() > 1) {

Shouldn't we also support writing multiple textures. What are the blockers?

[theNded]

Reviewable status: 1 of 9 files reviewed, 1 unresolved discussion (waiting on @griegler and @yxlao)

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src/Open3D/IO/FileFormat/FileOBJ.cpp, line 280 at r1 (raw file):

Previously, griegler (Gernot) wrote…

Shouldn't we also support writing multiple textures. What are the blockers?

Done.

[germanros1987]

Here is an overview of what got changed by this pull request:

Clones removed
==============
+ src/Open3D/Visualization/Shader/TexturePhongShader.cpp  -9
+ src/Open3D/Visualization/Shader/TextureSimpleShader.cpp  -2
         

See the complete overview on Codacy

[syncle]

Reviewed 2 of 9 files at r1, 1 of 5 files at r2.
Reviewable status: 3 of 10 files reviewed, 1 unresolved discussion (waiting on @griegler, @syncle, and @yxlao)

[griegler]

Reviewed 4 of 5 files at r2.
Reviewable status: 7 of 10 files reviewed, all discussions resolved (waiting on @yxlao)

[yxlao]

Reviewed 3 of 9 files at r1, 5 of 5 files at r2.
Reviewable status: complete! all files reviewed, all discussions resolved

[sadams207]

In o3d version 0.9, python bindings for using textures worked. Only one texture per mesh was accepted as the textures attribute.
In 03d version 0.10, in python, multiple textures are accepted as the textures attribute takes a list of o3d Images;
however, there is not a clear way to set the new material_ids mesh attribute to select the appropriate texture.
The attached script demonstrates the problem, i.e. crashes with signal 11: SIGSEGV due to missing material_ids.
Need to put in some other image to run the code.

`import open3d as o3d
import numpy as np

o_image = o3d.io.read_image('Selection_028.png')

points = np.array([
[0., 0, 0],
[1, 0, 0],
[0, 1, 0]
])

triangles = np.array([
[0, 1, 2]
])

uvs = np.array([
[0., 0.],
[1, 0],
[0, 1]
])

mesh = o3d.geometry.TriangleMesh()
mesh.vertices = o3d.utility.Vector3dVector(points)
mesh.triangles = o3d.utility.Vector3iVector(triangles)
mesh.triangle_uvs = o3d.utility.Vector2dVector(uvs)
mesh.textures = [o_image]

print(mesh.has_triangle_material_ids())
o3d.visualization.draw_geometries([mesh])`