Incomplete type/Unknown type name in particle system

Hi, I have met a problem, thanks a lot for helping!

I want to use optical flow to add acceleration to particles around me and create a class of optical flow by using the original optical flow example code. It works well in ofApp.cpp, can create an object and run functions in it. But met a problem in Particle.h.

have already included “opticalFlow.h” “ofxOpenCV.h” in Particle.h.

By the way, can I transfer the distance of movement of the pixels to acceleration added to the particles? I want to push the particles when I move by contour tracking? Is it feasible? If not, could you recommend any example or solution for me? Thanks a lot!!

where are you declaring that opticalFlow flow;?
Which library are you using for it.
maybe you are missing a namespace