add SPLAT decoder to be able to encode .splat files to .drc files

[patrikant]

I am using SPLAT files in addition to ply files. Currently, this datatype is not supported by the Draco library. A few months ago, I raised an issue regarding this topic (#1082), and there is another one concerning Gaussian splatting (#1020).

I am aware that SPLAT files are not standardized yet. This version accepts SPLAT files with the following attributes:

struct Gaussian {
	float position[3];
	float scale[3];
	uint8_t color[4];
	uint8_t rotation[4];
}; 

I attempted to encode the scale and rotation attributes as GeometryAttribute::GENERIC or custom attributes. However, I encountered some issues with the draco web loader in viewing these attributes. The two "new" attributes could not be decoded correctly. As a result, I am encoding position and scale together, as well as color and rotation. The implementation works perfectly for me.

Some examples for the compression using CL 10 and QP 11:
SPLAT file with 33MB -> DRC file with 10MB
SPLAT file with 4.6MB -> DRC file with 1.4MB
SPLAT file with 10MB -> DRC file with 3MB

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[waveviewer]

Following this PR, I compressed a splat file from 187M to 41M. Could you provide the code for decoding the DRC back to splat? Thanks! @patrikant

[patrikant]

Could you provide the code for decoding the DRC back to splat?

@waveviewer I do not need a local Draco decoder to decode DRC to Splat. So, I haven't customized these files yet, unfortunately. If it's necessary for this PR to be included in the next build, I can do it.
I am using the DRC files solely to view them in a WebGL viewer. I utilize a custom function from DracoLoader.js. Since they deprecated the function for using custom attributes to decode the geometry (dracoLoader.load is deprecated, so dracoLoader.decodeGeometry only uses the standard attributes), I don't know why this decision was made...
The decoded geometry is then utilized with a SplatLoader to be added to the scene.

[ondys]

is there any official SPLAT file specification / definition that we could link to here?

[patrikant]

I am using the definition by antimatter15: https://github.com/antimatter15/splat