Is brightness and contrast function in OF openCV working?

Hello!
I realize there are many thread on the subject but non are new.
It seems to me the brightness and contrast function in oF openCV is working weird or I am using them wrong.
I’ve got this:

  
ofxCvGrayscaleImage 	grayImage;  

Later;

  
grayImage.brightnessContrast(-.9,0);  

I tried to set two variables in order to check whether the command actually works and I get weird behavior.
Has anybody had this kind of problem?

Thank you.