I found the “edit filter settings” is greyed out when I try to save the “odt” document by “save as”
I also found “edit filter settings” is only NOT greyed out when I save as “Text - Choose Encoding (.txt)”
So, is it designed to be like this?
And how can I convert text encoding when use the libreoffice writer with “odt” format?
ODT is a file format that has strict specifications. Thus, an ODT file must comply with the specifications. That’s why you can’t edit any settings for ODT. In particular, you don’t need to (and can’t) change the character encoding because it’s always Unicode (UTF-8).
Yes, it is designed to be like this.
The checkbox is activated when an export filter reports that it has some configurable settings. A configurable setting for an import/export filter is something that affects how the data is being read from/written to a file, but which is not defined in the file itself. E.g., a text-only (TXT) file may use any possible encoding to represent characters in it, and the disk file contents would be absolutely different for different encodings, but the used encoding isn’t stored or referenced in such a file, so it has to be explicitly specified by user on opening/saving. A similar problem is which settings are used for, say, CSV, which might use different characters for separation of fields (despite its name), or different locales to encode/decode numbers (affecting decimal separators, currency, end so on).
For formats which have their settings invariant, or that store the setting values inside the file itself (like ODF, where a dedicated XML inside the package is used to keep settings related to the file), the filter need no user-configurable settings. The same holds for other advanced formats (like MS binary formats, OOXML, RTF, etc.) - so it’s normal that most filters have the checkbox disabled.