Moving an object from point A to B

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Frostfalk
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Posts: 6
Joined: Tue Feb 10, 2009 9:42 am
Location: Sweden

Moving an object from point A to B

Post by Frostfalk »

Hi! I'm currently trying to get into game programming again and I'm having a bit of a problem at the moment. I'm trying to move a rectangle from point A to point B and it sort of works but the rectangle accelerate and deaccelerate instead of moving in a constant speed from A to B. Here's my code in its entirety:

Anyone know how to make the object move at a constant speed instead of accelerating/deaccelerate toward a position?

Thanks in advance!

Code: Select all

function love.load()
	X = 30; -- initial X
	Y = 30; -- initial Y
	speed = 0.1;

end


function love.update( dt )
	if (love.keyboard.isDown("right")) then
		Xn = X + 128; -- destination X
		Yn = Y + 45; -- destination Y

		dX = Xn - X; -- deltaX
		dY = Yn - Y; -- deltaY
		distance = math.sqrt(dX^2 + dY^2); -- distance from A to B
		moving = true;
	end
	if (moving == true) then
		dX = Xn - X; -- update deltaX along path
		dY = Yn - Y; -- update deltaY along path

		X = X + dX * speed; -- update position
		Y = Y + dY * speed; -- update position
		if (math.sqrt(dX^2 + dY^2) > distance) then -- check whether object
			X = Xn;									-- has moved the entire
			Y = Yn;									-- distance
			moving = false;
		end
	end
end


function love.draw()
	love.graphics.setColor(0, 100, 100);
	love.graphics.rectangle('fill', X, Y, 30, 30);
end
Solstickan
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Jeeper
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Joined: Tue Mar 12, 2013 7:11 pm
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Re: Moving an object from point A to B

Post by Jeeper »

To change a variable at a constant speed is quite easy;

Code: Select all

function love.load()
 changeSpeed = 100
 variable = 0
end


function love.update(dt)
 variable = variable + changeSpeed * dt
end
Frostfalk
Prole
Posts: 6
Joined: Tue Feb 10, 2009 9:42 am
Location: Sweden

Re: Moving an object from point A to B

Post by Frostfalk »

How do you use that method when moving a point from one place to another. in myexample, the distance from x1 to x2 is not the same as y1 to y2. I tried this:

Code: Select all

X = X + dX * speed * dt;
Y = Y + dY * speed * dt;
but I got the same result as before.
Solstickan
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BlueWolf
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Joined: Fri May 29, 2015 7:55 pm

Re: Moving an object from point A to B

Post by BlueWolf »

That's because you compute dx and dy based on distance remaining. Thus your object is moving faster at the start and gradually slows down as it comes closer to the destination.
Frostfalk
Prole
Posts: 6
Joined: Tue Feb 10, 2009 9:42 am
Location: Sweden

Re: Moving an object from point A to B

Post by Frostfalk »

Oh yeah that's right, thanks for pointing that out Bluewolf.

I did a bit of extra research and found out that I need to normalize the vector before I can move the object along the line the way I want it. I rewrote it and now it works like I want it to:

Code: Select all

function love.load()
	X = 250; -- initial X
	Y = 250; -- initial Y

	speed = 500;
	moving = false;
end


function love.update(dt)
	if (moving == false) then
		if (love.keyboard.isDown("down")) then moveto(X, Y + 32, 100) end
		if (love.keyboard.isDown("up")) then moveto(X, Y - 32, 100) end
		if (love.keyboard.isDown("left")) then moveto(X - 32, Y, 100) end
		if (love.keyboard.isDown("right")) then moveto(X + 32, Y, 100) end
	end
	if (moving == true) then
		X = X + dirX * speed * dt;
		Y = Y + dirY * speed * dt;
		if ( math.sqrt((X-oldX)^2 + (Y-oldY)^2) >= distance) then
			X = newX;
			Y = newY;
			moving = false;
		end
	end
end


function love.draw()
	love.graphics.setColor(0, 100, 100);
	love.graphics.rectangle('fill', X-15, Y-30, 30, 60);
end

function moveto(movex,movey,movespeed)
	oldX = X;
	oldY = Y;
	newX = movex;
	newY = movey;

	dX = newX - oldX;
	dY = newY - oldY;

	distance = math.sqrt(dX^2 + dY^2);
	dirX = dX / distance;
	dirY = dY / distance;

	speed = movespeed;

	moving = true;
end
Solstickan
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