What "encoding" POI supports ?

What "encoding" POI supports ?

What "encoding" POI supports ?
I am using POI XSLF API to convert PPTX file to PNG image. My PPTX file
contain some UNICODE characters like - inəˈvâSHən but when
PPTX is converted to image it just make those UNICODE characters as
rectangle boxes after converting to image. Any thought please ? I really
need to get this done.
Does not look like it supports - UTF8 otherwise would have properly
converted those UNICODE characters properly.
Not sure what encoding is used by POI.