Law of closeness
What's missing is replenished.
Let's have a look at the illustration with the dots again: In the first image, we see several rows of dots (Law of proximity. The image below shows the same dots, which are replenished by a few dots in the vertical axis. The law of closeness comes into effect. Our minds interpret this image as a frame of dots which is divided by rows of dots.
If we see an isosceles triangle where one side is missing, our brain amends this side and the shape is recognized as a triangle anyway. Again, the law of closeness allowed us to see a complete triangle, because we are so familiar with triangles that the missing line was amended instantly.