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Canning
 Post subject: shpshoot
PostPosted: Wed Sep 21, 2011 3:38 am 
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I have an old example of a space ship moving with physics called shpshoot.zip in vb6 - http://www.canning.co.nz/shpshoot.zip

I have tried converting this to VB2010, but havent had any luck with it. Is there an updated VB2010 version of this?

Basically, I am wanting to modify this so that the user can enter a 2d coordinate, and have the space ship move there, but in VB2010.

Can I have a little bit of help with this please?

thanks
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Codehead
 Post subject: Re: shpshoot
PostPosted: Wed Sep 21, 2011 6:38 am 
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Welcome!

Have you looked at the SpaceShip Physics page on the wiki? Does that help?

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Canning
 Post subject: Re: shpshoot
PostPosted: Wed Sep 21, 2011 6:47 am 
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I have had a look. The example there is the same one that i referred to in my post... just a different name.

I am having a bit of trouble specifying a targetx and a targety coordinate to move to in this example. Is there some sample code that might be able to help me? Or maybe even a bit of advice on the formulas.

thanks
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Codehead
 Post subject: Re: shpshoot
PostPosted: Wed Sep 21, 2011 8:13 am 
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Sorry, didn't realise that you'd already looked there.

The maths on that page is beyond me, but I've done similar things in my homing missile code using vectors. Basically, a 'velocity' vector is used to represent the current speed and direction of your object. You can adjust it by applying a 'thrust' vector, possibly derived from a 'target' vector which is calculated from the position of your object and the desired destination position.

Does that make sense?

Image
This is one of my homing missile tests (yes I know they've all missed the target), you can see the flight path curving as the thrust vector adjusts the velocity vector.

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Canning
 Post subject: Re: shpshoot
PostPosted: Wed Sep 21, 2011 9:52 am 
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It does make sense. Its the math that I am a bit lost with.

Is your homing missile test done in VB? Can you give some examples of the physics that you played around with?
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Codehead
 Post subject: Re: shpshoot
PostPosted: Wed Sep 21, 2011 2:45 pm 
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Sorry, my code was C++. I'll see what I can dig up later.

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Codehead
 Post subject: Re: shpshoot
PostPosted: Thu Sep 22, 2011 8:57 pm 
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Can't find it now. However it went something like this:

  • Your ship object has a position and a currentHeading vector.
  • Each update, add the currentHeading vector to the position to calculate the new position
  • If you are looking for a target, subtract the ship position from the target position and normalise the result to get a target vector.
  • Add a fraction of the target vector to the currentHeading vector. (Because the vector is normalised, you can simply multiply each value by a float thrust value. e.g. tv.x * 0.025f, tv.y * 0.025f to get a consistent thrust effect.)
  • On the next update the currentHeading will have shifted towards the target.

You should probably normalise the currentHeading vector if the length is over 1.0 just to control the speed. Otherwise, as the currentHeading aligns with the target vector, the speed will build and build. However, don't normalise currentHeading lengths under 1.0 or your ship object will always travel at the same speed.

I hope that makes sense, I did start to knock up a little PyGame demo, but my Python-fu is weak and I was spending too much time in the documentation.

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Canning
 Post subject: Re: shpshoot
PostPosted: Fri Sep 23, 2011 8:29 am 
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OK, I have done a bit of work on this game. I now have 2 vectors, an acceleration vector and a velocity vector for the ship. When moving to a new target, I will make the ship come to a complete stop, then rotate to the new destination and then apply some thrust.

So what I am after now is the method to make the ship come to a complete stop... Do i rotate the ship to the opposite direction of its momentum and then apply some thrust until it reaches a speed of 0? Or something to that effect? If so, how can i calculate this direction? Is it something to do with the Ship.msngHeading variable?

Here is my physics code:

Code:
Private Sub Physics()

Dim sngXComp As Single  'Resultant X and Y components
Dim sngYComp As Single
Dim i As Integer
                   
    'Thrust
    If mblnUpKey Then
        mblnUpKey = False
       
        Ship.Velocity.x = Ship.msngSpeed * Sin(Ship.msngHeading)
        Ship.Velocity.y = Ship.msngSpeed * Cos(Ship.msngHeading)
       
        Ship.Acceleration.x = Ship.ACCEL * Sin(Ship.msngFacing)
        Ship.Acceleration.y = Ship.ACCEL * Cos(Ship.msngFacing)

        sngXComp = Ship.Velocity.x + Ship.Acceleration.x
        sngYComp = Ship.Velocity.y + Ship.Acceleration.y
       
        Ship.msngSpeed = Sqr(sngXComp ^ 2 + sngYComp ^ 2)
        If Ship.msngSpeed > 5 Then Ship.msngSpeed = 5
        'Calculate the resultant heading, and adjust for arctangent by adding Pi if necessary
        If sngYComp > 0 Then Ship.msngHeading = Atn(sngXComp / sngYComp)
        If sngYComp < 0 Then Ship.msngHeading = Atn(sngXComp / sngYComp) + PI
    End If
   
    Ship.msngX = Ship.msngX + Ship.Velocity.x
    Ship.msngY = Ship.msngY - Ship.Velocity.y
   
    If Ship.msngX < 0 Then Ship.msngX = ScreenWidth
    If Ship.msngY < 0 Then Ship.msngY = ScreenHeight
    If Ship.msngX > ScreenWidth Then Ship.msngX = 0
    If Ship.msngY > ScreenHeight Then Ship.msngY = 0


End Sub
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Canning
 Post subject: Re: shpshoot
PostPosted: Fri Sep 23, 2011 11:35 am 
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OK, I have got the ship stopping. Now I want to turn to the new target, accelerate towards it and when I get close, turn and apply the thrust again to stop dead on the target.
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Codehead
 Post subject: Re: shpshoot
PostPosted: Fri Sep 23, 2011 1:52 pm 
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This is where my maths starts to fail. This isn't something I had to worry about with homing missles, they don't have to worry about slowing down as they approach the target.

If you're stopping the ship, I take it you've inverted the velocity vector to create a thrust vector in the opposite direction?

Working out the magnitude of thrust to apply to slow the ship down would have to be calculated from the distance to the target. Obviously, if the ship is at the target position, the thrust vector would have to be equal and opposite to the velocity in order to cancel it out. However, at a distance the thrust required would have to be smaller in order to slow the ship down gradually.
Perhaps the length of the target vector before normalisation could be used to dictate the level of thrust required?

[edit]
After looking at the code, the msngHeading variable seems to be an angle, so we're using polar coordinates. You can subtract 180 degrees (or 3.14 radians) to find the opposite thrust direction.

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Jasmine
 Post subject: Re: shpshoot
PostPosted: Fri Sep 23, 2011 11:00 pm 
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Calculate stopping distance for the speed the ship is currently travelling at.

Calculate the distance to the target.

Allow a little extra on the stopping distance because you don't want to cut things too fine.

If one is bigger than two, then decelerate, otherwise accelerate.

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Canning
 Post subject: Re: shpshoot
PostPosted: Sat Sep 24, 2011 3:20 am 
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To calculate when to turn and apply thrust in my opposite heading to stop on the target, I presume I will have to use a formula that takes into account my current heading and my ships acceleration.

What would that formula be?
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Jasmine
 Post subject: Re: shpshoot
PostPosted: Sat Sep 24, 2011 10:59 am 
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If I'm reading this correct, the player will push the up key, and the ship will accelerate along the bearing (Ship.msngHeading).

Even though the ship may be facing a different direction (Ship.msngFacing) when the key is pressed, motion remains constrained along the bearing Ship.msngHeading, and the ship cannot move off that line.

Is that correct?

Next, do you want the ship to automatically slow itself down to a stop when it approaches it's destination? What if the Up key is pressed during this time? What if the player attempts to turn the ship during this time?

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Canning
 Post subject: Re: shpshoot
PostPosted: Sat Sep 24, 2011 11:39 am 
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I have a ship that has a Facing angle, a Heading, a velocity and an acceleration.
When the 'up' key is pressed, the ship thrusts in its facing direction. When the ship is moving, you can rotate it around without affecting its momentum. This only changes when you press the thrust key.

While moving to a destination, if the thrust key is pressed, it will cancel its movement. You can still rotate the ship while moving to a target.

Quote:
Next, do you want the ship to automatically slow itself down to a stop when it approaches it's destination?


Yes, that is what I am up to.
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Jasmine
 Post subject: Re: shpshoot
PostPosted: Sat Sep 24, 2011 12:11 pm 
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Well for a start, I think your physics code can be simplified to this:

Code:
Private Sub Physics()
    'Thrust
    If mblnUpKey Then
        mblnUpKey = False
        Ship.Velocity.x = Ship.Velocity.x + Ship.ACCEL * Sin(Ship.msngFacing)
        Ship.Velocity.y = Ship.Velocity.y + Ship.ACCEL * Cos(Ship.msngFacing)
        Ship.msngSpeed = Sqr(Ship.Velocity.x  * Ship.Velocity.x + Ship.Velocity.y * Ship.Velocity.y)
        If Ship.msngSpeed > 5 Then  SF = 5/Ship.msngSpeed :  Ship.Velocity.x = Ship.Velocity.x * SF : Ship.Velocity.y = Ship.Velocity.y * SF
    End If
 
    Ship.msngX = (Ship.msngX + Ship.Velocity.x + ScreenWidth) MOD ScreenWidth
    Ship.msngY = (Ship.msngY + Ship.Velocity.y + ScreenHeight) MOD ScreenHeight
End Sub



Canning wrote:
Quote:
Next, do you want the ship to automatically slow itself down to a stop when it approaches it's destination?


Yes, that is what I am up to.


The ship isn't necessarily going to be heading towards the destination point when the "Automatic Slow Down" kicks in. The ship may sail past the destination on it's port or starboard side. So in addition to slowing it down to a stop, you'll also have to line it up on course with the destination.

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Canning
 Post subject: Re: shpshoot
PostPosted: Tue Oct 04, 2011 9:53 am 
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OK, before I write the code to stop, I am after some help to rotate my ship to a destination angle.

When I want to start turning, this is the code I run:

Code:
            Ship.DestAngle = System.Math.Atan2(Ship.Target.y - Ship.msngY, Ship.Target.x - Ship.msngX) + PI / 2
            Ship.turning = True


And in the physics code I have the following:

Code:
        If Ship.turning = True Then

            a = System.Math.Sin(Ship.msngHeading + PI)
            b = System.Math.Cos(Ship.msngHeading + PI)
            c = System.Math.Sin(Ship.msngFacing)
            d = System.Math.Cos(Ship.msngFacing)

            det = a * d - b * c

            If det > 0 Then
                Ship.msngFacing = Ship.msngFacing + Ship.ROTATION_RATE * PI / 180
                If Ship.msngFacing > Ship.DestAngle Then
                    Ship.msngFacing = Ship.DestAngle
                    Ship.turning = False
                End If
            Else
                Ship.msngFacing = Ship.msngFacing - Ship.ROTATION_RATE * PI / 180
                If Ship.msngFacing < Ship.DestAngle Then
                    Ship.msngFacing = Ship.DestAngle
                    Ship.turning = False
                End If
            End If
        End If


Sometimes the ship turns nicely, other times it 'jumps' to the destination angle. I think this has something to do with the sign changing. Can I have some advice please on how to fix this?

thanks

Canning
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Jasmine
 Post subject: Re: shpshoot
PostPosted: Tue Oct 04, 2011 11:06 am 
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The problem is this:

Ship.msngFacing > Ship.DestAngle

You shouldn't use inequalities with bearings because their algebra uses modular arithmetic. It's like asking if 3 o'clock is before 10 o'clock... well it depends which way round the clock we go. :P

The correct way to do this is to measure the angle to turn through both clockwise and anticlockwise, and +/- 2π so each is bound to the range (0, 2π) .

Choose whichever has smaller magnitude and rotate in that direction. If the above is calculated correctly, then crossing the 0 or 2π mark should cause no jumping.

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Canning
 Post subject: Re: shpshoot
PostPosted: Tue Oct 04, 2011 11:22 am 
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Quote:
The correct way to do this is to measure the angle to turn through both clockwise and anticlockwise


Is there a simple formula to do this? If so, can you please enlighten me?

Canning
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Jasmine
 Post subject: Re: shpshoot
PostPosted: Tue Oct 04, 2011 11:31 am 
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It's very simple...

Anticlockwise angle = target bearing - current bearing
clockwise angle = current bearing - target bearing

As I said above, +/- 2π so each is bound to the range (0, 2π).

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Meneliki
 Post subject: Re: shpshoot
PostPosted: Fri Oct 14, 2011 10:16 pm 
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Hi CodeHead,

I see you have a missile tracking thing going on, but none of your missiles hit home

I actually had some success a long time ago with an old (partial)game I made. It was a spaceship game. I could spawn enemies, who would acurately follow and shoot at you. I think the same follow logic could apply to your missiles.

I can only provide you with logic, not code.. since your in C++ and im in C#. Anyway, it went something like this:

1) Determine the vector between enemy ship and player ship
2) Determine the quickest way to face the target (turning left, or right? which is the smaller angle?)
3) Once I'm facing the target, throttle up and chase him down!
4) Oh crap, im getting too close.. back off a bit. (naturally a missile wouldn't do this.. but you get the idea)

I hope this helps.

In all likelyhood you've already thought 50 steps beyond this, but if not, hopefully this logic is helpfull. Worked for me.

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