Second Life Physics Scripting - Pinball #20 - Complete Puck Flipper System

Is this ever going to end? 20 posts already and there still seems like a ton of game code to add. This next step was all about trying to figure out where the ball crossed what I'm now calling the "puck flipper". I just typed it in once and it stuck, but now that I look at it I don't think it's the best name since it seems like the brain really wants to swap the f and p. Maybe it's just my brain? Oh well, it's in the code now.

Removing the flipper and hooking up the new puck flipper to the machine was a pretty simple exercise and only took a few minutes.


Anyway, it was pretty easy to setup the collision system, but actually a little hard to figure out where it crosses when you think about it in larger terms. Look at this first step.

   collision_start(integer total_number)
    {
        if (llDetectedName(0) == "ball")
        {
            // positions are in global coordinates

            // find the position of the collision...
            vector colpos = llDetectedPos(0);
            //llOwnerSay("colpos = "+(string)colpos);

            // find the location of the puck prim
            vector locpos = llGetPos();
            //llOwnerSay("locpos = "+(string)locpos);

            // subtract two to get local coordinates on the puck
            colpos = locpos - colpos;
        }
    }

This is pretty straight forward. Take the positions of the two elements and subtract their positions. This will give you the coordinates local to the puck. If you divide the y size by 4, this gives you the size of each puck positions. Multiply by the current puck and position and add a little bit in each direction and you can then compare that to the Y collision position.

Easy you say. Until you start to rotate the pinball machine around. I've built it aligned along the X axis, but if you rotate it 90 degrees into the Y axis then all the calculations will no longer work! You have to take this rotation into account. This next piece shows that code.

   collision_start(integer total_number)
    {
        if (llDetectedName(0) == "ball")
        {
            // find the position of the collision...
            vector colpos = llDetectedPos(0);
            //llOwnerSay("colpos = "+(string)colpos);

            // find the location of the puck prim
            vector locpos = llGetPos();
            //llOwnerSay("locpos = "+(string)locpos);

            // subtract two to get local coordinates on the puck
            colpos = locpos - colpos;
            llOwnerSay("colpos = "+(string)colpos);
            
            // divide by the global rotation so we are back
            // in the original build orientation
            rotation locrot = llGetRot();
            colpos = colpos / locrot;
            llOwnerSay("colpos after rot = "+(string)colpos);
        }
    }

Don't think I'm some sort of genuis and this just fell out of my brain. I spent at least an hour tweaking to get this right. For the longest time I had what I originally wrote colpos = colpos * locrot;. After I got my view port inside the pinball machine and dropped the ball at both edges a bunch of times I realized the Y values were giving opposite results when the table was rotated. I figured that using division of the rotation might swap the signs and sure enough it did and now I can move on.

I really like the puck mechanic so far and think it will be really effective. I think it will be even better if I used it with more than 4 positions. Given a bunch of positions it might even look just like a pong puck and allow for some really cool game mechanics. I've not seen anyone use this yet, so I'm excited to see this in action.