Hello,
My slide has a transparent background.
When I export the slide in png, the png file has a transparent background.
But when I export the file in webp, the webp file has a white background.
It seems the webp format does not allow transparency.
How can I export a webp file from Impress with a transparent background?
My LibreOffice version is 24.2.2.1
Thank you very much for your help, Clement
I would be inclined to export “Selection” as SVG rather than PNG if you want to keep vectors. If you do not tick Selection then the background is exported too.
I tried ticking Selection for WEBP but it cut out the shapes from the background, very odd. The webp filter is relatively recent.
if you can reproduce anytime on a small example, please report a bug
https://bugs.documentfoundation.org/enter_bug.cgi?product=LibreOffice
and meanwhile, convert the OK png to webp with an external tool 
Thank you very much for your answer. I will report a bug. Have a nice day!
Did you have the setting Force Skia software rendering ticked in Tools > Options > LibreOffice View? Try it with that unticked if it is.
There was a recent Skia bug which has been resolved in 28.8.0.0.alpha which I was testing and, maybe coincidentally, I can export transparent webp images OK as well.