Point following

[Delibrete]

Hello,

I'm here to ask as to how one would go about making an object 'walk' to an x and y coordinate.

Say this object started at x = 300, y = 400 and you wanted that object to walk to x = 600, y = 800 at a speed of 16 pixels a second. How would you do this?

[timmeh42]

Others can correct me if I'm wrong, but this would be what you want:

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function love.load()
	object_x = 300
	object_y = 400
	point_x = 600
	point_y = 800
	object_speed = 16
end

function love.update(dt)
	if ( (((point_x - object_x)^2 + (point_y - object_y)^2)^0.5) < (speed*dt) ) then
		object_x = point_x
		object_y = point_y
	else
		local angle = math.rad(math.atan2(point_x - object_x, point_y - object_y))
		object_x = object_x + math.cos(angle) * (object_speed/dt)
		object_y = object_y + math.tan(angle) * (object_speed/dt)
	end
end

To break it down, we have:

Code: Select all

function love.load()
	object_x = 300
	object_y = 400
	point_x = 600
	point_y = 800
	object_speed = 16
end

We assign the values you specified to variables.

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if ((((point_x - object_x)^2 + (point_y - object_y)^2)^0.5) < (object_speed*dt)) then
		object_x = point_x
		object_y = point_y

We check whether the object is closer than a single "step" to the destination point - this means that we can jump straight there without computing most of the trig functions. "(((x2-x1)^2+(y2-y1)^2)^0.5)" was taken from the wiki page for Additional Math, and computes the distance to the point.

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local angle = math.rad(math.atan2(point_x - object_x, point_y - object_y))

Again taken from the Additional Math page, this computes the angle from the object to the point.

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		object_x = object_x + math.cos(angle) * (object_speed/dt)
		object_y = object_y + math.tan(angle) * (object_speed/dt)

Finally, the object is moved a single "step" towards the point. A step is equal to the distance it should travel in one step, ie, its speed divided by the Delta Time (dt)

Be warned, I have not actually tried this, but theoretically it should work. I may have swapped some values around, which could make the object move away from the point instead. In that case, swap around the point_x/y and objectx/y values where the angle is computed.

[Delibrete]

Yes that is what I wanted, thank you. But for some reason it's really slow.

[timmeh42]

Ah, was still editing when you posted. May as well leave the explanation there in case others want it. I also had to fix some things where I forgot to use the variable names, so its for the best.

Can't think why it's slow, though - I don't have any experience in optimisation of code.

[Robin]

Can't think why it's slow, though - I don't have any experience in optimisation of code.

The speed is 16 pixels per second. That means to travel the width of the default window (800px), it would take 50 seconds.

[bartbes]

Actually, in irc we discussed the real problem,

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		object_x = object_x + math.cos(angle) * (object_speed/dt)
		object_y = object_y + math.tan(angle) * (object_speed/dt)

That should be object_speed*dt, of course.

[trubblegum]

viewtopic.php?f=4&t=8661 for more of the same.