I have no idea what this means. Assuming the deck is randomized you can’t guarantee only stacking 3 images per locus.
With variable image stacking, the last pair’s image IS the marker. As soon as you reach a red-first pair, make that the last element at the locus and then start the next locus with the next pair. If that one is red-first as well, make it the only element at that locus and move to the next.
This way logically:
If there are multiple elements at a locus, the last one represents a red-first pair.
If there is only one element at a locus, it represents a red-first pair.
You can create your own marker system if you want like splattering all the red-first pairs with blood or fire or lava or whatever. In my experience, this becomes very difficult to manage as you’ll have many such modified images in your scenes and its very easy to get confused about which ones had the modifications. If you are going for speed it will be even harder to do this because you’ll have to take extra time to recognize and adjust your images.
Yes. If every pair begins with a red card, you’d need 26 loci. The odds of this happening are astronomically small. In fact, the odds that you need more than 19 or 20 loci are in the one in millions range.
There was a thread where we did the math on this a while back: