Moving an object from point A to B

[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

[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]

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.

[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]

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