Union with empty set results in empty set about xmipp HOT 4 CLOSED

Mathematically this is not correct:

from xmipp.

Here is a minimal case:
md1.xmd:

data_noname
loop_
_itemId

md2.xmd:

data_noname
loop_
_itemId
1
2
3
4

xmipp_metadata_utility -i md1.xmd --set union md2.xmd:

data_noname
loop_
_itemId

xmipp_metadata_utility -i md2.xmd --set union md1.xmd: (OK)

data_noname
loop_
_itemId
1
2
3
4

from xmipp.

In fact, when adding -o <mdOut> parameter, the output file is not created if the input is empty. This is the actual root cause of the previous problem. For any empty input metadata, the output is not created, regardless of the operation performed.

from xmipp.

Fixed #714
Issue closed.

from xmipp.