JSON must use UTF-8 encoding without BOM

[bghgary]

@lexaknyazev

https://github.com/KhronosGroup/glTF/tree/master/specification/2.0#json-encoding

1. JSON must use UTF-8 encoding without BOM.

Is there a specific reason why we restrict it to "without BOM"? Can we add some validation to the validator?

#831 seems to talk about this, but I see no discussion on BOM.

[lexaknyazev]

From RFC-7159 (JSON), section 8.1

Implementations MUST NOT add a byte order mark to the beginning of a JSON text. In the interests of interoperability, implementations that parse JSON texts MAY ignore the presence of a byte order mark rather than treating it as an error.

Also it allows loaders to not support full UTF-8 charset because all glTF-defined strings are ASCII-representable.

The validator will refuse to load an asset starting with BOM in some cases, I'll investigate to make that consistent across all builds/inputs.

[bghgary]

The validator will refuse to load an asset starting with BOM in some cases

http://github.khronos.org/glTF-Validator/ doesn't fail.

Here is an asset with a UTF-8 BOM:
AssetWithBOM.zip

[lexaknyazev]

@bghgary What kind of validation result would you prefer?

• refuse to read an asset as if JSON is broken; or
• read as usual but issue an additional error.

[bghgary]

I would prefer the second if it's not too much trouble.

[lexaknyazev]

The validator update will land today. Do we need any spec changes?

[bghgary]

Do we need any spec changes?

I don't think we should remove the restriction from the spec, but it might be useful to have an implementation note similar to what you referenced earlier in RFC-7159 (JSON), section 8.1.