Testpoints and borders

 Testpoints are the oldest check technique I've been using to implement collisions in my games. From Calimero to Bilou and even Crazy Brix. The idea was simply to check the color of a specific point on screen to decide whether it is solid or not. In Calimero, white was solid, and black was not. In Bilou (QuickBasic), pixels with all their 4 lower bits set were non-solid. Everything else was.

But by 2000, it was no longer sounding sufficient. I had a long technical discussion with Gedeon by then, who was enthusiast about Crazy Brix, but pointed out that ball/bricks collisions were imperfect. I had found (by browsing and floppy-swapping- and printing and reading) mask-based techniques for sprite-to-sprites collisions through shifting and masking. But I was still missing a way to do the same for ground and walls.

Then I came up with a paradigm shift: instead of doing checks on a bitmap, I could have a vector-based description of the level boundary and wrote a letter to Gedeon to present him how I'd do it.

There are still testpoints associated with sprite positions, but now they can be attached to the closest level boundary. They can be split in 4 casts depending on whether they should block as floor, ceiling or walls. Once a testpoint is associated with one border, checking whether we're in or out is trivial. One test, and that's it.

My idea was that we could build a graph or a network of boundaries for each cast, so we can switch from one boundary to its neighbour.

I had high hope to use that in my Ultimate Game Maker, but it although I said to Gedeon that navigating the 'network' shouldn't be that hard, just having a Mario-pipe in the level and falling below its height would make our 'right wall' (initially far away) become very near, and possibly skipping many other obstacles that are "shadowed" by the pipe.

edit: by re-typing this on 2021, it turns out that the solution to boundaries network might simply be to use something inspired by the 'portals' used in 3D engines (Ogre ?): above my Mario-pipe, you'd have an "empty" right-wall. That means when you're jumping and have no walls in front of you, the scanning is halted by a 'right-portal' that just serves the purpose of saying 'there's a pipe right below me'. Only when you go past the horizontal position of the pipe, you unlock the wall-boundaries that are further away.

Ah. Bin c'était supposé être un post de traduction d'un vieux post pour le tag-de-la-semaine et à y re-réfléchir, j'en viens à me dire que la clé, pour pouvoir utiliser ces bordures, c'est de rajouter des bordures-portail pour prolonger les bordures réelles. ça conserve le potentiel de décrire la structure du niveau avec moins de données qu'une tilemap, mais par contre il risque d'y avoir du travail pour l'édition de niveaux...

Et en cherchant un peu (autre chose), je retombe sur des illustrations qui doivent dater d'après la lettre à Gédéon, mais d'avant la tentative de moteur 3D pour Bilou.