Allowing glTFs to be loaded that don't have uvs and normals

[julhe]

Issue

Bevy will panic when rendering a glTF mesh that doesn't have UVs and normals

Fix

UVs and normal attributes will now be filled up with zeros.

Generally I think it is a better approach to fill up missing attributes with zeros. That will make it easier to deal with different meshs from render side.

Risks

It is now mandatory for the mesh to contain a position attribute. I really can't think of a case were a mesh wouldn't contain that, but may someone else has. 🤔

[cart]

This is a nice temporary fix, but I think I would actually prefer it if loaded GLTFs only contained the attributes that have been specified.

The Mesh type was designed to be flexible in terms of vertex buffer attributes. You can pass in a VertexBufferDescriptor and it will output a gpu buffer that matches the format. The missing piece is that currently it will only do that for attributes that are in the Mesh.

I think we should extend Mesh to (optionally) fill in missing attributes with zeros.

[cart]

get_vertex_buffer_bytes is the method where this would happen.

[julhe]

Okay, so something like this:

pub enum MissingVertexAttributeBehaviour{
    /// Attribute doesn't need to contain meaningfull data. Example: VertexColor
    DefaultValue(VertexAttributeValues //TODO),
    /// Attribute is optional, but must contain meaningfull data. 
    /// Example: eg. SecondaryUV for lightmaps which can also be the PrimaryUV
    /// Cool Trick?: instead of cloning the fallback buffer, bind it to the same slot as the missing one
    FallbackAttribute(Cow<'static, str>),
    /// attribute is required. Example: VertexPosition
    Panic, 
}

Or do you think thats too verbose?

[cart]

Hmm I think short term I'd prefer something simpler like this.

fn get_vertex_buffer_bytes(&self, vertex_buffer_descriptor: &VertexBufferDescriptor, fill_in_missing_attributes: bool) -> Result<Vec<u8>, MeshToVertexBufferError> {
  /* existing logic here, but fill in missing attributes with zeros */
}

If later consumers decide that its overly limiting, we can revisit it. But I'm guessing this will be "good enough".

[julhe]

okay, new commit online 🙂

[cart]

haha i totally forgot we were already generating arrays filled with zeros 😄