0050-intro

What this is, how it’ll go.

0075-folder

0100-lets-make-things-move

## Let's move something

The purpose of this little document is to describe one way of making objects “move” in Second Life. We’ll start from some basics and work our way up.

### How can things move in Second Life?

There are at least five ways to make things move in Second Life.

### Physical objects

Objects can use “physics”, an add-on to Second Life which simulates the physics of the real world. Using physics, you can apply forces to an object to make it move around. There are settings to ...

0200-improvement-1

Physical, Vehicles, SetPos. We’ll do the last one.

0300-first-refactoring

As we go through these examples, we’ll be emphasizing finding simple solutions, then improving them along the dimensions we care about. Don’t try to bite off the whole problem. Take small bites and chew them carefully.

0400-smoother

Let’s move back and forth. First llSetPos, then llSLPPF.

BELOW HERE IS DRAFTY

This is just some text we’re typing in.

It looks double spaced.

This still looks double spaced

Now it isn’t double spaced

0600-the-question-of-paths

So in our little program, we have a path from XYZ to X’Y’Z’. We’ll have the problem of representing paths. Before we go too far, though, let’s learn some more basics.

0700-forming-our-first-script

Maybe we should build up our first script incrementally?

0800-jump-script

## First script: Popping back and forth.

Here’s our script. It’s far more elaborate than is really needed for this simple example, but I want to go over all the details as they’ll matter in the future.

// back-and-forth-jump

// JR 2018-05-21

// Just jumps <2,1,0> and back, repeatedly.

integer Running = FALSE;

vector Home;

vector Jump = <2,1,0>;

vector Pos;

start_running() {

Home = llGetPos(); // in case we've been moved.

Pos = Home;

llSetTimerEvent(1.0);

}

stop_running() {

Pos = Home;...

0900-jump-script-params

Now I hope you ran the preceding script. If you did, you probably noticed that SL makes it look as if the little cube moves smoothly from place to place. That’s called “tweening” and usually SL makes it look pretty good. However, it goes pretty fast and we really would like our object to move a bit slower, so in our next example, we’ll move incrementally.

But first we need to do something more complicated: llSetLinkPrimitiveParamsFast(). Maybe I’ll call that llSLPPF or Params Fast sometimes. The...

1000-refactoring

change move to call adjust position and set position, combined method?

what else needs cleaning?

1100-the-timer

why do we do this?

1200-moving-incrementally

start by refactoring to start-end? do refactoring right along?

1300-tracing-a-circle

Suppose we want our object to go around a circle. How can we calculate XYZ around a circle? Solve triangles, or use r,theta.

1400-representing-a-path

lines and arcs? A little language.

1500-what-about-following-the-curve

Our little cube goes around, but it doesn’t follow the curve by rotating. Let’s fix that. Set rotation angle + something. Theta*baseRot?

1600-where-do-these-lines-and-arcs-come-from

Sensing a track. Tracing a track to embed. Or a notecard.

1700-pros-and-cons-for-paths

Embedded takes 2x the storage! Digress to read a notecard.

1800-issues-with-sensing

A straight in front of you is easy enough to sense. An arc’s center is way off to the side. Sharp curves hard to sense.

1900-line-representation

(a,b) then (b,c) then (c,d) vs just b -> c -> d

2000-but-arcs

How do arcs fit into this scheme? Radius and delta-theta (from current angle?) Where does this go?

2100-what-about-using-only-straight

A curve is really just a bunch of tiny straight moves anyway: note we moved straight between points. Extend that distance?

2200-looks-like-jerking

Now the orientation of the locomotive seems to shift jerkily. The linear straight motion might not be noticeable but the angle sure is. Slerp!

2300-what-about-speed

If we only move distance d on every tick, speed looks constant. How can we calculate d for various Line and Arc elements?

2400-note-consider-never-doing-arc

Perhaps approximations are the thing right along. Curve = lots of straights.

250-representing-end-positions-on-a-segment

Start/End. But prims have a center. Sensing section?

2600-points-on-a-circle

Degrees and Radians

2700-moving-on-a-timer

Each tick, some variable needs to move from end to end, or zero to 1, or something. 0 to 1 might be a good representation.

2800-synchronizing-things

Follower objects. Pure time. Read distance.

2900-region-boundaries

Difficult problem. Here’s what we know.