Game Component: Spaceship editor, part 3

I've been trying out a few ideas with quick & dirty demos to see if
they're at all promising as part of a game. Two posts ago I talked
about a spaceship editor, where you could place the thrusters, and the
“AI” would learn how to pick thrusters that most closely matched your
desired motion. In the last post I spent some time describing how I
spent some time trying to figure out the characteristics of the ship,
how I learned more about the problem I needed to solve, and some of
the things I tried to find a “nice” solution to the math. I was really
happy with the things I had worked on. However…

The real questions I wanted to answer were:

1. 
   Is it actually fun to fly a ship with this physics model?
   



2. 
   Is it actually fun to design a ship? In other words, are there interesting designs and tradeoffs?
   

Somewhere during the process I realized that getting the “nice”
solution wasn't that important, and I had probably spent too much time
on it. (Reading this
article
reminds me that I get distracted too easily on fun problems and I have
trouble completing things.) I wasn't getting closer to answering the
important questions. Instead, I was having fun learning some math that
I had forgotten 15 years ago. Having fun learning math isn't bad; it's
just not the main goal.

To answer the first question, I spent some time in various ships just
flying around. My conclusion was yes, it's fun to fly around in a
world by itself, but no, it doesn't seem like it'd be fun in an
actual game, where there are other things to do. And that's when it
hit me: all that time I spent thinking about the math might have been
a waste of time (except that it was a lot of fun to learn some math);
I should first make sure I can make the ships fun to fly.

I flew around a lot and thought about what bugged me. The main problem
was that inertia was fun at first but it seemed to get annoying
after a little while. For example, if I'm moving forwards with
W, and then I rotate left with A, I'm still
moving, but my jets are pointed at an angle. To stop moving you need
to use a combination of S and Q. Even worse,
since it's a keyboard instead of an analog controller, you can't hit
the right mix of the two to make yourself stop.

In addition, it wasn't clear to me whether the controls should set
acceleration, velocity, or target position. Acceleration was the most
natural thing, and that's what I started with. But that means when you
hit any key, you start out slow, then go faster and faster, until it's
out of control. That might be realistic but it's not much fun. I
switched it to setting velocity. But what velocity should I target? I
arbitrarily chose some multiple K of the acceleration, so that after K
time you'd reach that velocity.

I also needed to do more tuning. The mass, moment of inertia,
friction, and rotation/translation tradeoffs are set arbitrarily. I
had tried adjusting this but none of the parameters were quite right,
and every time I changed the physics it got worse, and I had to tune
to make it better again.

I tried to answer the second question (whether there are interesting
ship tradeoffs) by creating several ships. Based on that experience,
my answer is maybe. The ships I made are noticeably different but
I have a clear favorite. If there's only one best ship then the ship
editor's not going to be interesting. The problem is that the answer
to the second question depends on what I do with the physics.

So I decided to work on the physics first.

I tried tackling inertia directly, with some ideas from a friend:

• 
  Inertia in World Coordinates gives me realistic physics for
  flying ships. This is what I had started with, and this is
  what I was unhappy with. When you're going north, and turn
  left, you keep flying north. The inertia keeps moving you
  north, even if you face west.
  

  
  
  




• 
  Inertia in Ship Coordinates gives me something that behaves
  more like a vehicle on wheels. When you're going north, and
  turn left, you start going west. The inertia keeps moving you
  forwards, whatever direction that may be.
  

  
  
  




• 
  No Inertia would mean that you only move when thrusters are
  fired. This is the most extreme change but if inertia is really
  a problem then it's worth a try.
  

  
  
  

I also tried treating rotation differently from x and y, because
rotation seems to lead to some of the situations that make the ship
less fun to control.

It was only after playing with the various options that I learned that
I do really want inertia in world coordinates. Sometimes I just have
to try something to help me learn something (also see href="http://paulbuchheit.blogspot.com/2009/01/communicating-with-code.html">this
blog post). Having no inertia, or inertia in ship coordinates,
just didn't feel fun to fly at all, and that's not what I would've
predicted. I had the right form of inertia; something else was wrong.

After all the testing, I realized I wanted inertia but not the full
effect. At low speeds, inertia is great, but at high speeds, inertia
is less fun. With inertia alone it takes as long to speed up as it
does to slow down again. It's okay if it takes a second or two to
reach a high speed. But when I let go of the keys I want the ship to
come to a stop pretty quickly. I added a force to slow the ship down.
I tried three approaches:

• 
  Constant force decreases velocity V by up to K. This can be
  interpreted as surface friction.
  

  
  
  




• 
  Linear force decreases velocity V by K*V. I don't know what
  this might correspond to in physics, but in the calculations it
  corresponds to “reducing” inertia.
  

  
  
  




• 
  Quadratic force decreases velocity V by KVV. This can be
  interpreted as air resistance.
  

  
  
  

All three of them helped. After trying and tuning the three I decided
that the linear force reduction was the most pleasant, but still not
ideal. With quadratic, the problem I ran into (which I might have
predicted if I were smarter) was that you can't increase your maximum
velocity much if you add thrusters. Instead, it was largely determined
by the air resistance. That would be fine if I were creating just one
ship, but for the ship editor to be interesting, I need the number and
power of the thrusters to actually matter! Constant friction left too
much of your motion determined by inertia at higher speeds. The
linear slowdown felt the best. I can't justify it if I were going for
realism, but I'm going for fun, not realism.

I think flying the ship can be made fun. The next question to tackle
is whether there are interesting tradeoffs in ship design.