Saturday, August 6, 2011

Baddies 2.2–Node Positioning

Each node in the game tree has two possible options when drawing, either relative to the parent node, or with absolute coordinates respect to the screen. This is done as some things like the GUI need to be relative to the screen, regardless of where they are hooked up in the game tree, and for most  of the compound objects (like a player with a sword and a shadow) it makes it a lot easier if you just put the sword and shadow relative to the player and forget about it (when the player moves, the sword and shadow will as well.)

But this apparently simple system raises some questions. First and foremost, do we keep 2 sets of coordinates? We keep the absolute coordinates and (needed for drawing from everyone) and the relative ones? Or we recalculate one of them?
And if we recalculate one of them, do we do it on demand? Or every cycle?
And finally, how do we keep this coordinates “safe” so there are no hidden bugs when the user modifies one set and not the other?

The answer I came with is to keep just the relative coordinates as position, and a variable parentRelativePos, which indicates whether the coordinates are relative to the parent post or not.  No double set of coordinates because data replication is a sure way for innocent bugs to creep invisible into the system, and still attack months later in the development. So by elimination we end up with just one set of data.

As for recalculating them, it will be apparent by now that for the vast majority of nodes, it is only needed to recalculate the absolute position it when drawing (for those relative to the parent position), and not at all for the rest (the not relative to the parent position nodes can just draw with their normal position variable, as it’s absolute coordinates).

So, the problem boils down to recalculating when drawing, or when the position changes. Doing it when the position changes is viable ONLY if we keep the position variable private, and all changes to it go through an accessor method that updates the position and the “drawPosition”, but that is still a possible nest of bugs (maybe we need to open the level of security of position to protected, or have some unacceptable access times in derived classes from node, and this is again giving a user full access to fuck up). So we recalculate the drawPosition every draw cycle, by adding the parent drawPosition to our own position. Just 3 additions per cycle of extra cost, not so bad when the alternative is ninja bugs all over the code.

In conclusion, one variable that is position, the Boolean parentRelativePos indicates whether it is a relative or absolute position, and the drawPosition is updated every draw cycle from our position and our parents drawPosition.

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