No Data values for Enums

[lilleyse]

One limitation of using No Data values with ENUM properties is that it needs to be one of the enumerations. It would be nice if you could use a value not in the list like an empty string and some corresponding integer value for binary values.

From the 3D Metadata Specification:

For ENUM types, a noData value should contain the name of the enum value as a string, rather than its integer value.

[javagl]

The examples are usually pretty explicit with introducing a special value ("Unknown", -1) for this case. The validator also checks that it is a valid enum name.

It could indeed be simplified, but it might require additional constraints and corner cases to think through. You causally said

and some corresponding integer value

But it's not immediately obvious where this should be. One the purposes of the enum.value.schema.json is to establish the mapping between the name and the (integer) value (which I sometimes refer to as the "enum value value" for clarity 🙂 ). When the noData value is not the name of an enum value, we'd need some sort of noDataValue in addition to that...

[lilleyse]

Maybe the maximum value could be used, e.g. 255 for UNSIGNED_BYTE, and those values would be restricted from normal usage.

[lilleyse]

But I think noData + noDataValue would be ok too, and less likely to cause problems.