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The next question would be ... but how do we get the drawings from the 3D object? This will be explained over the next few pages.
Take a look at the view of a room below. It shows a table sitting on the floor, and a wall directly behind it, watch how it rotates until we get a simple view of it, as if we were on the ceiling looking down, we see the floor and the table top, the wall is now seen as a yellow line, because again we are looking straight down at it, we don't see the table legs because they are underneath the table top but from this view, we can measure the WIDTH and DEPTH of the table top, and we know how far it is from the wall.