I was wondering about the following variation of the opaque forest problem (see here and there for previous questions) :
What is the least length set of segments that will intersect every straight line passing through a unit square $]0,1[^2$ in at least two different points ? $\tag 1$
The best known solution for the original problem does not satisfy the condition, since the grey line intersects only one time the set of $4$ segments :
Actually I didn't found a better solution to $(1)$ than a "trivial" one, which has total length 4. Does anyone can find a better solution, or does it seem to be the best we can do? I tried to do it by contradiction, but I got no result.
Thank you in advance for your comments !