FBXLoader: Add TransparencyFactor

[safu9]

I had warning message when loading fbx file,
THREE.FBXLoader: TransparancyFactor map is not supported in three.js, skipping texture.

Since TransparancyFactor seems to be another name of TransparentColor, I added it in FBXLoader.

Related: #11598

[looeee]

Since TransparancyFactor seems to be another name of TransparentColor

TransparencyFactor maps to opacity, while TransparentColor maps to alphaMap so I don't think this is correct.

There's also an FBX property Opacity which is what the loader currently uses, and TransparencyFactor is ignored. It's possible that's not correct though.

Are you testing this change with any models? If so, can you share them?

[safu9]

Sorry for late reply.
I compared with the same model exported by Blender.

Before:

After this PR:

But I found some parts are not working correctly ... (z-buffer?)

[safu9]

Model file is here. Tree.zip

[mrdoob]

@looeee what do you want to do with this one?

[looeee]

Sorry it took me so long to check this.

It seems like the easy solution here is to manually set material.alphaTest = 0.5:

Treating TransparencyFactor as an alphaMap as in this PR is not correct since that makes the leaves transparent. At least in this case, it's presumably meant to be an alpha mask map, which could be handled in the loader like this:

case 'TransparencyFactor':
        parameters.alphaMap = scope.getTexture( textureMap, child.ID );
	parameters.alphaTest = 0.5;
	break;

However, when I do this, the mask is inverted:

I'm not sure why this is happening, seem to be that

material.alphaMap = texture;
material.alphaTest = 0.5;

... and

material.map = texture;
material.alphaTest = 0.5;

...give inverse results, for the same texture.

[looeee]

I've found one more model that uses TransparencyFactor in this thread:

facebookincubator/FBX2glTF#158

However, in that case it looks like a standard transparency map. They also note there that they've never encountered a model with this property before. Both of these files are created by a recent Blender version (2.79 and 2.80) so maybe they have started to use it.

I've changed my mind on this - I think treating this the same way as TransparentColor is OK. Users will have to manually set alphaTest when required (as usual).

[looeee]

@mrdoob this can be merged now.

[mrdoob]

Thanks!

[safu9]

Thank you!

[looeee]

@safu9 thanks for your patience ^_^

And note that you probably still need to manually set material.alphaTest = 0.5; here.