What are the mathematical/computational principles behind this game?

Finite Projective Geometries

The axioms of projective (plane) geometry are slightly different than the Euclidean geometry:

• Every two points have exactly one line that passes through them (this is the same).
• Every two lines meet in exactly one point (this is a bit different from Euclid).

Now, add “finite” into the soup and you have the question:

Can we have a geometry with just 2 points? With 3 points? With 4? With 7?

There are still open questions regarding this problem but we do know this:

• If there are geometries with Q points, then Q = n^2 + n + 1 and n is called the order of the geometry.
• There are n+1 points in every line.
• From every point, pass exactly n+1 lines.

• Total number of lines is also Q.

• And finally, if n is prime, then there does exists a geometry of order n.

What does that have to do with the puzzle, one may ask.

Put card instead of point and picture instead of line and the axioms become:

• Every two cards have exactly one picture in common.
• For every two pictures there is exactly one card that has both of them.

Now, lets take n=7 and we have the order-7 finite geometry with Q = 7^2 + 7 + 1 . That makes Q=57 lines (pictures) and Q=57 points (cards). I guess the puzzle makers decided that 55 is more round number than 57 and left 2 cards out.

We also get n+1 = 8, so from every point (card), 8 lines pass (8 pictures appear) and every line (picture) has 8 points (appears in 8 cards).

Here’s a representation of the most famous finite projective (order-2) plane (geometry) with 7 points, known as Fano Plane, copied from Noelle Evans – Finite Geometry Problem Page

I was thinking of creating an image that explain how the above order-2 plane could be made a similar puzzle with 7 cards and 7 pictures, but then a link from the math.exchange twin question has exactly such a diagram: Dobble-et-la-geometrie-finie