libgdx – Equation of the limits of the world of the isometric game

I had a problem last night and I can not solve it since.
I create an isometry game based on tiles with equal rows and columns.
Now, I've created moving entities that should turn whenever they reach the limits of the world. As I illustrated in the photo, they return to the upper limits (green), but not to the lower limits (red).
I have tried various equations in the lower part, but I can not determine for myself the code of the lower parts. What would be an appropriate solution for shrink lines?

booleans checkBounds () {
float absX = Math.abs (getX ());
float absY = Math.abs (getY ());
if (absY * 2-absX <0 || ???) returns false
otherwise, return true;

enter the description of the image here