Comment #1 on issue 156 by
pascal.m...@gmail.com: undefined reference to
`WebPEncodeRGB' function
http://code.google.com/p/webp/issues/detail?id=156
[i assume unix platform here. Or, are you using Windows?]
did you include the '-lwebp' on the linker command line?
Another possibility is the your system-wide library is not installed in a
standard location, and that you're linking against an old library
instead... (although that would be strange, since WebPEncodeRGB() has been
there in the API since the very first version)