VTI specification

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Dear all,

I was wondering if there is any official specification of the VTK XML file formats, particularly for the VTI image format.
The reason I’m asking are the attributes of the <ImageData> tag. For VTK9, it contains a Direction attribute which is missing for VTK8:

<ImageData WholeExtent="0 16 0 16 0 16" Origin="0 0 0" Spacing="1.0 1.0 1.0" Direction="1 0 0 0 1 0 0 0 1">

vs

<ImageData WholeExtent="0 16 0 16 0 16" Origin="0 0 0" Spacing="1.0 1.0 1.0">

I think it is obvious that WholeExtend, Origin, and Spacing are mandatory. Direction seems to allow deviation from default ordering of x,y,z (that’s how I understand 1 0 0 0 1 0 0 0 1).

My general question is now:

  • How can I know which attributes are mandatory in XML files, potentially in dependence of the version string (which is 0.1 in files from VTK8 and VTK9)?

The specific question is then:

  • Is a <ImageData> tag without Direction valid now and in future?

Note: I’m using the python bindings in case that it matters.