Deprecate big endian transfer syntaxes

[CPBridge]

Big endian transfer syntaxes are retired in the standard, so we should not support them. Furthermore, they considerably complicate working with certain value representations.

Now, passing a Big endian transfer syntax to the highdicom.SOPClass constructor will raise a ValueError. As this is a breaking change, there is a brief description in the release notes.

[hackermd]

Thanks @CPBridge. That looks good to me.

How shall we deal with a big endian transfer syntax upon decoding? In theory, we would need to check the file_meta information for every pydicom.Dataset passed to the from_dataset() factory method of any class derived from highdicom.SOPClass, wouldn't we?

[CPBridge]

It's a good question... I think that may be stricter than we need to be. I don't see why we couldn't allow parsing of attributes that are either: a) unaffected by endianess or b) handled by pydicom, which presumably does correctly account for endianess correctly (e.g. pixel data in segs and pms). Maybe we should just put checks on the methods and properties that would actually be affected, e.g. .lut_data? In fact I think these LUTs may be the only part of the decoding API that would be handling this incorrectly, apart from possibly something in the ann, module, which I'd have to double check. What do you think?

[hackermd]

I think it's a bit more complex. There are several binary attributes in Annotations, Parametric Maps, Softcopy Presentation States, Segmentation, etc. We would need to check any (current and potential future) use of numpy.frombuffer().

[CPBridge]

I searched the entire repo for frombuffer earlier and only found usage in pr and `ann. The other binary attributes are decoded by pydicom

[hackermd]

Values of some data element (e.g., ICC Profile) are decoded by other libraries and we are not sure whether they are (will be) handled correctly. Therefore, I would argue that we assert little endian when parsing a given data set.

[hackermd]

There is the complication that a pydicom.Dataset may not contain file_meta information, e.g., when it was constructed from DICOM JSON metadata. So there may be situations where we cannot check the transfer syntax. In that case, we may want to log a warning or debug (?) message and inform the caller that little endian will be assumed.

[CPBridge]

@hackermd I have implemented checks for endianess as you proposed on the subclasses of SOPClass that have a from_dataset method. It will log a warning as suggested if the transfer syntax, and hence endianess, cannot be determined.