Negative transformation scale

[JoshKlint]

https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#transformations

Implementation Note: If the determinant of the transform is a negative value, the winding order of the mesh triangle faces should be reversed. This supports negative scales for mirroring geometry.

According to what I am reading, the indice order should be reversed if a node has a negative scale in its transformation. This makes little sense, as meshes can potentially be shared across different nodes, some of which may be reversed and some not. Face direction should be a per-mesh property, not a per-node property. Am I misunderstanding this?

[JoshKlint]

I think I get it. At some point someone was asked "what happens if the scale inverts the faces" and this was the answer. It is true that you could save some VRAM if you had a separate mesh object that included the triangle order and a "mesh data" object that was shared between meshes that contains the vertex arrays, and thus instance the mesh with two different winding orders, but it seems like a really obscure optimization.

I think this decision is covering one mistake with another one.

[donmccurdy]

I think this implementation note should be removed. Negative scales will invert single-sided faces, and that's OK.

[bghgary]

This implementation note is there to support mirrored geometry. Both Unity and Unreal do this.

For the history, see #971 (comment) and the corresponding PR #985.

[JoshKlint]

Well, at least the correct behavior is explicitly stated. Thanks for the info.

[JoshKlint]

I think this implementation note should be removed. Negative scales will invert single-sided faces, and that's OK.

Now that it's in use, this is actually critical to follow. I have encountered GLTF files that use a negative scale on one axis and if you don't follow this rule the faces will be inverted from what you would expect.

[bghgary]

See https://github.com/KhronosGroup/glTF-Asset-Generator/blob/master/Output/Positive/Node_NegativeScale/README.md for a set of test cases if you are implementing a loader.

[donmccurdy]

I see, thanks – looks like this was added to three.js as well, in r89, and I'd missed it. mrdoob/three.js#12787.

@JoshKlint to your original question...

... meshes can potentially be shared across different nodes, some of which may be reversed and some not. Face direction should be a per-mesh property, not a per-node property. Am I misunderstanding this?

...the same mesh can be can be reused, uploaded to the GPU once, and rendered with different states of gl.frontFace for example. So reversed face direction does not really interfere with reuse of the mesh in practice.