public float getViewToProjectionZ(float viewZPos) {
        float far = getFrustumFar();
        float near = getFrustumNear();
        float a = far / (far - near);
        float b = far * near / (near - far);
        return a + b / viewZPos;
    }
Computes the z value in projection space from the z value in view space
Note that the returned value goes non-linearly from 0 to 1.
For more explanation of non-linear z buffer see
http://www.sjbaker.org/steve/omniv/love_your_z_buffer.html

@param viewZPos the z value in view space.
@return the z value in projection space.
jme3-core/src/main/java/com/jme3/renderer/Camera.java