Spaceship movement parameters update

Math and Physics Programming

Started by pythedev May 20, 2012 04:27 PM

4 comments, last by taby 11 years, 11 months ago

101

Author

May 20, 2012 04:27 PM

Hello!

I'm making a pretty basic spaceship game. Every time I update or change a main parameter, that affects others(acceleration, acceleration change speed, direction change speed) then I want to recalculate all parameters.

I have the following parameters at the beginning of the movement:



		long currentTime = System.currentTimeMillis();

		double timeDelta = (currentTime - lastUpdate) / 1000.0;

		lastUpdate = currentTime;

		double posX = params.get(Param.POS_X);

		double posY = params.get(Param.POS_Y);

		double accX = params.get(Param.SPEED_X_PIX_SEC); // ship acceleration x axis.

		double accY = params.get(Param.SPEED_Y_PIX_SEC); // ship acceleration y axis.

		double dir = params.get(Param.DIRECTION_RAD);

		double dirSpeed = params.get(Param.DIRECTION_SPEED_RAD_SEC);

		double thrust = params.get(Param.THRUST_PIX_SEC); // acceleration change speed. Depends on direction change.

And now I want to set the parameter values to what they should be after a certain amount of time has passed(timeDelta). Two parameters don't need updating since they are there only to affect other parameters - thrust and dirSpeed.
It would be a piece of cake to figure it out when there would not be any direction change...

ChristerSwahn

198

May 20, 2012 04:39 PM

double accX = params.get(Param.SPEED_X_PIX_SEC); // ship acceleration x axis. double accY = params.get(Param.SPEED_Y_PIX_SEC); // ship acceleration y axis. double dir = params.get(Param.DIRECTION_RAD); double dirSpeed = params.get(Param.DIRECTION_SPEED_RAD_SEC); double thrust = params.get(Param.THRUST_PIX_SEC); // acceleration change speed. Depends on direction change.

I suspect you are confusing acceleration with velocity (speed) since you're assigning "speed" values to accX and accY?

Anyway:

pos2x = posX + (velX * timeDelta) + (accX * timeDelta * timeDelta) / 2;

vel2x = velX + (accX * timeDelta);

And dito for Y.

Note - you may want to treat thrust as a two-dimensional vector as well (as you do with pos and acc) and derive accX, accY from it via F = m * a.

http://mmoarch.blogspot.com/

http://www.spaceborne.se/

pythedev

101

Author

May 20, 2012 05:01 PM

Yes, sorry about this, I meant velocity under "acceleration" and thrust is the same as acceleration.
I thought that I would calculate the acceleration of the thrust first(accelerating acceleration) in both axes:



	    double dirAtEnd = dir + dirSpeed * timeDelta;

	    double thrustAccX = thrust * (Math.sin(dirAtEnd) - Math.sin(dir)) / timeDelta;

	    double thrustAccY = thrust * (Math.cos(dirAtEnd) - Math.cos(dir)) / timeDelta;

And now I should be able to calculate the x and y position using the derivative of acceleration?

pythedev

101

Author

May 20, 2012 05:05 PM

Let me fix my last post:
I calculated the velocity of the thrust:



	    double dirAtEnd = dir + dirSpeed * timeDelta;

	    double thrustVelX = thrust * (Math.sin(dirAtEnd) - Math.sin(dir)) / timeDelta;

	    double thrustVelY = thrust * (Math.cos(dirAtEnd) - Math.cos(dir)) / timeDelta;

pythedev

101

Author

May 20, 2012 09:22 PM

Okay, I think I figured it out correctly:



	private void updateParameters() {

		long currentTime = System.currentTimeMillis();

		double timeDelta = (currentTime - lastUpdate) / 1000.0;

		lastUpdate = currentTime;

		if(timeDelta == 0.0)

			return;

		double posX = params.get(ShipPhysicalParam.POS_X);

		double posY = params.get(ShipPhysicalParam.POS_Y);

		double velX = params.get(ShipPhysicalParam.SPEED_X_PIX_SEC);

		double velY = params.get(ShipPhysicalParam.SPEED_Y_PIX_SEC);

		double dir = params.get(ShipPhysicalParam.DIRECTION_RAD);

		double dirSpeed = params.get(ShipPhysicalParam.DIRECTION_SPEED_RAD_SEC);

		double thrust = params.get(ShipPhysicalParam.THRUST_PIX_SEC);

		double dirAtEnd = dir + dirSpeed * timeDelta;

		double thrustX = thrust * Math.sin(dir);

		double thrustY = thrust * Math.cos(dir);

		double thrustAtEndX = thrust * Math.sin(dirAtEnd);

		double thrustAtEndY = thrust * Math.cos(dirAtEnd);

		double thrustVelX = (thrustAtEndX - thrustX) / timeDelta;

		double thrustVelY = (thrustAtEndY - thrustY) / timeDelta;

		posX += velX * timeDelta + thrustX * timeDelta * timeDelta / 2.0 +

				thrustVelX * timeDelta * timeDelta * timeDelta / 6.0;

		posY += velY * timeDelta + thrustY * timeDelta * timeDelta / 2.0 +

				thrustVelY * timeDelta * timeDelta * timeDelta / 6.0;

		velX += thrustX * timeDelta + thrustVelX * timeDelta * timeDelta / 2.0;

		velY += thrustY * timeDelta + thrustVelY * timeDelta * timeDelta / 2.0;

		params.put(ShipPhysicalParam.POS_X, posX);

		params.put(ShipPhysicalParam.POS_Y, posY);

		params.put(ShipPhysicalParam.SPEED_X_PIX_SEC, velX);

		params.put(ShipPhysicalParam.SPEED_Y_PIX_SEC, velY);

		params.put(ShipPhysicalParam.DIRECTION_RAD, dirAtEnd);

	}

Tested and seems to work pretty nicely.
Problem solved!

taby

1,505

May 20, 2012 09:42 PM

I'm curious to know the reasoning behind the use of factors like timeDelta^2, timeDelta^3, 1/2 and 1/6:



posX += velX * timeDelta + thrustX * timeDelta * timeDelta / 2.0 +

        thrustVelX * timeDelta * timeDelta * timeDelta / 6.0;

posY += velY * timeDelta + thrustY * timeDelta * timeDelta / 2.0 +

        thrustVelY * timeDelta * timeDelta * timeDelta / 6.0;



velX += thrustX * timeDelta + thrustVelX * timeDelta * timeDelta / 2.0;

velY += thrustY * timeDelta + thrustVelY * timeDelta * timeDelta / 2.0;

Spaceship movement parameters update

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