NPC chasing the player

[Head Over Heels]

Hello,

I don't want to spam the forum with topics but I am in need of assistance. I'm trying to find examples of a script/function that would allow me to have a number of 'enemies' chase the player around a fixed area/room, basically flocking to wherever the player has moved to. I suppose a bit like they do in Geometry Wars. Can anyone point me in the right direction here?

I'm running through the tutorials but can't really find anything that will help me with this, the closest being the Hamster Ball example but obviously that is designed to enact player control rather than NPC movement. I haven't used LUA for 4 years or so and I've basically forgotten everything!

Any help appreciated.

[nevon]

I'm on my phone, so I'm not gonna give you any example code, but the easiest way to solve it is to simply get the vector between the player and each enemy, and then move the enemy along that vector. Of course you'd have to calculate a new vector each time, as long as the player is moving. This simple approach does not take into account any obstacles or walls. For that, you'd have to take a look at pathfinding algorithms.

[Taehl]

A couple functions you may want to use:

Code: Select all

-- Returns the distance between two points.
function math.dist(x1,y1, x2,y2) return ((x2-x1)^2+(y2-y1)^2)^0.5 end

-- Returns the angle between two points
function math.getAngle(x1,y1, x2,y2) return math.atan2(x2-x1, y2-y1) end

-- Averages angles.
function math.averageAngles(...)
	local x,y = 0,0
	for i=1,select('#',...) do local a= select(i,...) x, y = x+math.cos(a), y+math.sin(a) end
	return math.atan2(y, x)
end

Then you could do something like this:

Code: Select all

function love.update(dt)
	for k,v in ipairs(NPCs) do
		local a = math.getAngle(v.x, v.y, player.x, player.y)
		v.x = v.x + math.cos(a) * v.speed * dt
		v.y = v.y + math.sin(a) * v.speed * dt
	end
end

[vrld]

Then you could do something like this:

Code: Select all

stuff

Or you could do it with vectors to avoid trigonometric funtions. Like nevon suggested:

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for _,enemy in ipairs(NPCs) do
    -- direction is the vector from the enemy to the player
    local direction_x = player.x - enemy.x
    local direction_y = player.y - enemy.y
    local distance = math.sqrt(direction_x * direction_x + direction_y * direction_y)

    if dist ~= 0 then -- avoid division by zero
        enemy.x = enemy.x + direction_x / distance * enemy.speed * dt
        enemy.y = enemy.y + direction_y / distance * enemy.speed * dt
    end
end

Shameless plug: With hump's vector class this becomes:

Code: Select all

for _,enemy in ipairs(NPCs) do
    local direction = player.pos - enemy.pos
    enemy.pos = enemy.pos + direction:normalized() * enemy.speed * dt
end

[Taehl]

Vrld, wouldn't that let enemies move twice as fast diagonally than they could horizontally or vertically?

[slime]

The length of a normalized vector is always 1, so no.

(more on vectors here)

[Head Over Heels]

Thanks alot guys, I appreciate the help.

[Taehl]

That vector stuff sounds neat. Would it be faster than using trig?

Also to test my understanding, this would correctly normalize a list of numbers, yes?

Code: Select all

function math.normalize(...)
	local n, m, s = select('#',...), 0, "return "
	for i=1, n do m = m + select(i,...) end
	for i=1, n-1 do s = s..select(i,...)/m.."," end
	return loadstring(s..select(n,...)/m)()
end

For instance, math.normalize(2,4,10) returns 0.125, 0.25, 0.625. Also, is there a better way to return a vararg expression than my loadstring hack?

[bartbes]

A table + unpack? This loadstring stuff is unreadable.

[Robin]

A challenge!

Code: Select all

    function math.sum(...)
        if select('#', ...) == 0 then
            return 0
        end
        return (...) + math.sum(select(2, ...))
    end

    function math.normalize(...)
       local total = math.sum(...)
       local t = {...}
       for i = 1, #t do
           t[i] = t[i]/total
       end
       return unpack(t)
    end

Untested.