A person can use a two-dimensional picture to get a very complete idea of the distances to the depicted objects, their shapes and sizes, and thus fully perceive the three-dimensional world in all its depth. How do we achieve this?
As you know, it is the two-dimensional picture that we see directly with our eyes. What we see can be captured, for example, with a camera, printed on a sheet of paper (i.e. two-dimensional) and hung on a wall, so the image that comes to our brain from the eyes is two-dimensional.
However, when we look at real objects, photos, and videos, we manage to draw so much information from these two-dimensional pictures that they begin to seem three-dimensional to us. We are very good at perceiving the relative position of objects in space through vision alone. The type of vision that allows us to perceive the shape, size and distance of objects is called stereoscopic vision. A person has this type of vision and achieves this through the following effects:
Binocular vision. A person has two eyes. A slightly different two-dimensional image of the same three-dimensional scene is formed on the retina of each eye. Based on life experience and huge computational abilities, the brain compares these two slightly different images and forms an idea of three-dimensionality of the picture. This effect works best when looking at close objects, the distance to which is somehow comparable with the distance between the eyes. When looking at objects at a distance of more than five meters, this effect has almost no effect. Let us also say right away that since binocular vision is not the only factor that allows seeing in 3D, and since its scope is limited to a few meters, the absence of two eyes would not be a disaster for humans. We would nevertheless be able to see in 3D, it would just take us more life experience and time to learn how to apply the other effects. This assertion is proven very easily. Just close one eye. So, have you stopped seeing in 3D? No!
Shifting objects as the observer moves. As the observer moves, the picture he sees is constantly changing, with nearby objects changing their position in this picture much faster than distant objects, which slowly change their position in the observer’s field of view. Again, great life experience and the computational abilities of the brain allow us to perceive the distance to the objects in the field of vision well by the speed of their movement. By the way, in fact, moving one eye to a distance equal to the distance between the eyes can replace binocular vision, because indeed the brain will eventually be able to match the same two images as from two eyes at once. However, this method requires a lot of effort and constant movement, and also the pictures will not be captured at the same moment in time, i.e. they may already be different. So binocular vision is still a very useful option, which helps a lot when working with close objects, which is what a person usually does.
Life experience. Most people have a good idea of the size of many familiar objects, such as trees, other people, cars, windows, doors, and so on. With this knowledge, you can estimate the distance to one of these objects (and therefore to those objects that are nearby) quite well, depending on how much of the total field of view they occupy. For example, you will immediately guess that the girl in the photo below is much closer to the observer than the tower she is supposedly reaching the top of…
Smokiness of distant objects. The atmosphere does have a certain degree of opacity. Therefore, very distant objects look smoky. So by the degree of smokiness you can determine which of the distant objects is further away, and which is closer to the observer. This is a very useful effect, because for distant objects do not work well other ways to build three-dimensional images.