Hi Massimo,
thank you for your post and for noticing that tricky bit.
The fuzzy output is indeed what you expect, but what you see is not a bug. The “fuzzy output” shown in QtFuzzyLite v4.0 refers to the membership value of the defuzzified value over the terms in the variable, that is, you are seeing the fuzzified output value (\mu(y)). I have addressed this bit in QtFuzzyLite v5.0 by showing both the fuzzy output value that you refer to (\tilde{y}), and the fuzzified output value (\mu(y)). QtFuzzyLite v5.0 will be released early July, 2014, and it will be truly amazing. Stay tuned!
I have mentioned this bit in the forum of known issues:
Fuzzy Output Text in qtfuzzylite
Let me know if that answers your question.
Cheers,
Juan
