implement the KHR_draco_mesh_compression extension

[stephengold]

Khronos has specified an extension to glTF for loading models that employ Draco mesh compression.

Currently, attempting to load such a model results in warnings and an empty result:

Oct 17, 2023 11:50:50 AM com.jme3.scene.plugins.gltf.CustomContentManager readExtension
WARNING: Could not find loader for extension KHR_draco_mesh_compression

Since the extension may be widely used in the future, it would be nice if jme3-plugins could handle it.

[javagl]

There are not too many options for decoding Draco in Java.

I tried using the official WASM distribution of Draco with GraalWASM, but ... that doesn't seem like a promising path right now.

There's always the option to use JNI, and that could be a reasonable option, but always carries some overhead for compiling and deploying the native libraries. (Maybe LWJGL will jump in here. They already created bindings for Meshoptimizer, meaning that support for EXT_meshopt_compression may be in reach, and maybe even relatively easy... ).

In the meantime: I had some success with decoding Draco-compressed data from glTF with https://github.com/fileformat-drako/FileFormat.Drako-for-Java . I can not yet strongly "reommend" that library, because it's relatively new, and I can not yet say much about stability and performance. But it's promising, and I'll continue to investigate it. Maybe someone here also wants to have a look.

[stephengold]

At the present time, I believe our best solution is to use MonkeyWrench. Of course, as its author I may be biased.

[javagl]

@stephengold From quickly skimming over that, it looks like it is using Assimp for the actual import/loading of the glTF asset, which, in turn uses the LWJGL Assimp bindings.

(Iff this is correct, then it already does boil down to LWJGL...)

I don't know the details about how well Assimp itself supports glTF. I think I heard some rumours that the import may ~"have some issues", but I don't know how true that is or how severe these issues are. The Fileformats.md lists glTF 2.0 with a few extensions, where Draco is not included. When doing a quick code search for "draco", then it seems to appear in the code in the context of glTF. So I don't know what the truth is.

In any case: If MonkeyWrench->(LWJGL)Assimp "works" for glTF+Draco, then this may very well be the most reasonable path here. And the fact that the Draco decoding itself would then be "hidden" behind the Assimp API may, in fact, be a plus: When something's wrong and you have to choice of "Fixing it" or "Opening an issue in another repo", the latter tends to be easier...

[stephengold]

Yes, MonkeyWrench boils down to LWJGL and Assimp. I believe Assimp supports Draco because I tested it on Draco models found in KhronosGroup/glTF-Sample-Assets and KhronosGroup/glTF-Sample-Models.

However, depending on MonkeyWrench for our Draco support isn't ideal because of the 2-year lag for getting Assimp bugfixes into MonkeyWrench. I'd be much happier if jme3-plugins also supported Draco.

[riccardobl]

I think there is a java implementation now https://products.openize.com/drako/java/

[javagl]

@riccardobl Yes, that's the https://github.com/openize-com/openize-drako-java that I already linked to above. I tried it out for a few input data sets, but not "deeply, properly" integrated into any form of glTF loader. Somewhat recently, someone asked about how to use that library in the context of glTF, at openize-com/openize-drako-java#4 , and I added a small comment and code snippet there as well - maybe a starter for people who want to try it out.

[github-actions]

💰 This issue has a bounty

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If you want to start working on this, comment on this issue with your intent.
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edit: reservation removed at the author’s request.

[stephengold]

I promise not to work on this issue for at least a month. If someone wants the bounty, there's no need to wait.

[javagl]

I actually already started to ""work"" on this. But this started at 0 for now - cloning jMonkeyEngine, trying to set things up (never having used Gradle before) ... Now... I can start TestGltfLoading, and trying to load a draco-compressed asset already prints some
System.out.println("Trying to handle draco...");
that I inserted into some (otherwise empty) DracoMeshCompressionExtensionLoader. Maybe it's not sooo hard to fill this with life, but 1. I cannot make any promises or "strong" committment to actually doing (i.e. 'resolving') this, and 2. I don't want to "block" anyone from grabbing the bounty. First come, first serve, I guess 🤷