If I hadn't shown you were wrong you would have answered the question already.
You won't because you know it has shown that you are wrong, and there is no way out for you.
You haven't shown any question to be valid.
You simply asked me what stops a blue line.
I asked you to tell me what you were viewing through and how you were viewing.
And I explained it all, repeatedly. But because you can't answer without showing yourself to be wrong, you use whatever dishonest BS you can to refuse to answer. (And here you can again see the difference between how an honest person handles it compared to how you just repeat the same pathetic lies. Notice how that because I actually have an answer, and an argument that shows you are wrong, I can provide it yet again, whereas you just repeat the same lies or use other dishonest BS to avoid answering extremely simple questions).
Again, the diagram is incredibly simple. Even a complete moron can understand it. So why do you continue to pretend you can't?
Here it is again, see if you can figure it out this time:
This is a side on, cross sectional view.
On the left, there is an eye, to represent the eye of the person looking through the tube. The tube is the purple box.
The left and right side of the tube are entirely open. They are the ends of the tube, i.e. the holes.
The top and bottom of the rectangle would be solid, the walls of the tube.
I even have a purple line extended out level with the bottom of the tube.
We can clearly see the eye is at the middle of the tube, and the tube is level.
So this is clearly the exact situation you started going down this rabbit hole with.
The solid coloured lines indicate different potential paths for light to travel along.
We can see the green, travelling level, just goes straight in to the eye. This means light travelling along it will travel along just fine and reach the eye, allowing you to see an object in that direction.
We see the orange travels up and enters the box from the bottom. This would indicate it hits the wall of the tube. Assuming the wall of the tube is opaque, this means the wall of the tube will block the light and stop it reaching the eye. This means you cannot see an object through this particular level tube in the direction of the orange line.
But they are not the only options.
There is also the blue line. This line passes through the end of the tube. It does not hit the wall of the tube and thus there is no reason for light travelling along it to be stopped. It will travel, just like the green line, entering the end of the tube and reaching the eye. This means through this tube, you can see light coming in from the direction of the blue line.
Note this is not moving the eye up to the top of the tube and looking down. This is the eye remaining at the centre of the tube, looking through it. It is simply due to the fact that there is a FOV, that they eye doesn't just magically see in 1D along a line, it has a FOV. This allows this eye to see a small amount in all directions around straight out level. This includes up and down.
The blue line is just one of many such lines.
The limits are the 2 red lines.
This shows were a beam of light would just miss the wall of the tube and instead enter through the end. Light from anywhere in the region bounded by the 2 red lines would be able to travel into the tube and reach the eye.
Importantly note that this includes regions below the level of the tube.
There can be an object below the tube (such as the ground) which has light scatter off it (or projects its own light) which travels along the blue line and hits the eye.
This means the eye can see that object.
That means you can see the ground when looking through a level tube, even with that ground below the tube (as long as it is in the FOV).
Now stop playing dumb and either accept you are wrong or explain what magic stops the light travelling along the blue eye from reaching the eye.