Calculating the distance between two planes in rviz

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the distance between two planes is always zero unless they are parallel. so i assume in your case the two planes are parallel. the mathematical expression of a plane is ax+by+cz+d=0. if two planes are parallel, they should have the same coefficients a, b and c. so let's say plane 1 is: ax+by+cz+d1=0, plane 2 is: ax+by+cz+d2=0. in this case the distance between these two planes is: abs(d1-d2)/sqrt(a * a+b * b+c * c). if you are using PCL to fit the plane, you will get four coefficient for a plane, which just correspond to a, b, c and d as mentioned above.