Flight Controller Logic

Over the summer, I worked on the flight controller. The purpose of the flight controller is to make it easy for any ordinary human or AI to fly an unbalanced spaceship in a game with simulated physics. In space, the laws of inertia become very significant, as anything put in motion will tend to stay in motion. The flight controller would do two things: compensate for inertia so that the craft is easier to control, and allow an unbalanced spaceship to fly straight.

The first step I took, was to try and create an algorithm that would take take a direction/torque as an input, and would accelerate the assembly in said direction/rotation. I figured once I had this, the rest would be relatively easy.

The current way the algorithm works is as follows:

1. The force and torque vectors each engine will exert on the assembly are calculated and stored.
2. These force/torque vectors are then grouped together to form 6 dimensional vectors, one for each engine.
3. These vectors are then transferred into a matrix class as column vectors.
4. An additional target column vector is added to the far right of the matrix, this is the "target acceleration" input vector.
5. An algorithm puts the matrix into reduced row form using Gaussian elimination. (if you think of the matrix as a system of equations, it essentially solves for the unknowns)
6. The last column vector in the matrix now has a list of values. Essentially these are the ratios of the engines to each other required to accelerate in the input vector's direction.
7. These values are then scaled to create the maximum thrust possible and the enignes are turned on.

While the algorithm technically works, there are some flaws:

1. It does not take into account the fact that engines cannot fire in reverse, so some of the output values can be negative depending on the input and the engines available. This results in engines firing backwards.
2. This solution works for only 6 engines exactly, and usually only if their force/torque vectors are linearly independent (ie only one engine for each direction).

So far I have found a partial workaround for the second flaw. If the number of engines is fewer then 6, multiplying the target vector and the original matrix of force/torque values by the transpose of the original matrix creates a matrix with as many solutions as you have engines. This can then be solved to get ratio values that will fire the engines with a resultant force/torque as close as possible to the input target force/torque vector.

Video Demo

Finally found some time to make a short demo video, by comparison to that last one I think its coming out very well!

Basically the video shows a few things:

• The new graphics/art all that good stuff
• Camera tracking
• Camera rotation tracking
• The current input system
• Engine functionality
• Laser effects
• Toggling in and out of edit mode
• Selecting different assemblies
• How hard it is to fly

The camera rotation tracking basically rotates the camera as if it was parented to the selected assembly, the location tracking does the same except minus the rotating part. The cool thing about the rotation tracking part is that you can still orbit the assembly normally with the mouse while it is rotating.

It is currently very difficult to control a built ship, part of this is because the only way to rotate the ship is by using engines, which are too powerful except for large assemblies/ships. The other reason is because of the physics, once you start rotating or increasing the speed, rotational and directional inertia takes over and you continue spinning/moving until there is a counter force or torque.

To solve the first issue with rotation, I will likely add some RCS thrusters (basically thrusters for small changes in momentum or rotation) and a reaction wheel (magic science devise used to exert a rotational force on an object, for the science behind it, here)
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The second issue is a little more difficult, but the basic principle behind it is to have a flight computer that tells the engines to slow the ship down after you accelerate, or slows the rotation down after you rotate. The computer would use available engines, and would be able to increase or decrease the thrust on each one as needed. In this way non symmetrical ships would be flyable. Using the flight computer, the ship would move more like people are used too regarding simpler games, while still using physics. The computer of course would be optional for those who want direct control. I plan to make it so both the flight computer and direct control can be used at the same time.

Update

 I have been working on the multiplayer aspect of the game for quite a bit, as a result, most of the infrastructure for  it is now in place. As it is right now it is playable, however there are a few things missing, mainly client side prediction. So if you go too fast, the client receives positional update information that is to old, this causes your ship to jump backwards and jitter a lot when you are going fast.

The camera control has been much improved, now the camera will track to the center of mass of your assembly, and in addition there is a key that will toggle rotation tracking. Rotation tracking just causes the camera to rotate with the assembly if the assembly rotates. 

Below are some screen shots showing some significant graphics updates, a bit of the logic system, and the laser module:

Physics Multiplayer

Its been a while since I posted anything here, been very busy, both in college and working on this project. While I have been working on the project quite a bit, I haven't updated this blog much. Much has been changed, its practically a playable demo at this point.

Multiplayer, has been the most recent feature I have been trying to implement. Adding multiplayer to a game like this is very challenging for several reasons:

1. This game is physics based.
2. It has to be as lag free as possible for direct user feedback

That's it. 

Only problem is, those two things generally don't go together, especially when using unitys physics, here is why:

In most multiplayer games, you have a server and a client, the server runs the official version of the game, and the client runs a similar version that the server updates to be in sync with the server. Even with a very good internet connection, the delay between sending information to a server and receiving the resulting update of the simulation can range from .2-.4 seconds. Say if you press the space key to jump, the signal is sent from your computer to the server in .1-.2 seconds. The server receives the signal, tells your player to jump, then sends the updated position back to the client. So it takes .2-.4 seconds for your command to appear on the clients display.

To fix this delay, client side prediction can be used, how this works, is your player moves  immediately after receiving a command, and sends the command to the server. The server then sends its results back to the player. However, this data from the server cannot be used to update the player, its .2-.4 seconds old. The server is constantly sending out its idea of where the player should be. So if this old data is used to correct the players position, then client side prediction will be pretty much invalidated.
To Compensate for this, the data the server sends can be "checked" with what the players position was in the past. If the two don't match closely enough, then an extrapolation is made of where the player would be assuming the two did match. The player is then moved there.

The server also has to update the other players, since each of them also has a simulation of the world. The flowchart above shows roughly how this would work.

One of the problems with client side prediction though, is you need to predict something based off of old information. With unitys physics, you cannot do this. Unitys physics cannot "rewind" or "resimulate" nor "predict" any part of the physics system. That why making a physics multiplayer is very challenging, especially when using unity.

To get around this problem, you could simulate some data yourself, you would have to have your own simple physics engine essentially. What my project will do, is use a custom rigidbody system that supports "predicting" where it will be in the future. Unfortunately, it will not be possible to check collisions, so those will have to be handled server side, which will likely result in lag glitches during a collision.

This being a space ship game though, there should be few enough collisions that this should not be a problem.