What is the result of vector.direction ()?

[burli]

I don't understand what's the result of vector.direction (p1,p2). Can someone explain this for me?

[AnxiousInfusion]

Hmm, http://dev.minetest.net/vector doesn't have anything to say about it. Can you print p1 and p2 to console just to see what the values are?

[burli]

http://dev.minetest.net/vector doesn't have anything to say about it.

That is my problem

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print("Vector",dump(vector.direction({x=1,y=2,z=3},{x=4,y=5,z=6})))
Vector   {
   y = 0.11111111111111,
   x = 0.11111111111111,
   z = 0.11111111111111
}


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print("Vector",dump(vector.direction({x=1,y=1,z=1},{x=1,y=1,z=1})))
Vector   {
   y = nan,
   x = nan,
   z = nan
}



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print("Vector",dump(vector.direction({x=1,y=1,z=1},{x=2,y=2,z=2})))
Vector   {
   y = 1,
   x = 1,
   z = 1
}


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print("Vector",dump(vector.direction({x=5,y=3,z=4},{x=1,y=5,z=7})))
Vector   {
   y = 0.5,
   x = -1,
   z = 0.75
}


[Hybrid Dog]

There's the code:
https://github.com/minetest/minetest/bl ... or.lua#L57

Obviously it firstly subtracts pos1 from pos2,
then it gets the abs values of the numbers
after that it divides every number by the biggest abs one, if there is not one biggest but two or three, it divides two or three times

l assume it's supposed to do the same as vector.normalize(vector.subtract(pos2, pos1))
but it does something completely different, it doesn't return a unit vector,
maybe it's supposed to return a vector with coordinates which are inside [-1;1] and at least one of them equal to -1 or 1
but this only works if each number's abs is different because there "if" is used and not "elseif", so if the numbers are e.g. {x=3,y=3,z=3}, instead of returning {x=1,y=1,z=1} it returns {x=1/3^2,y=1/3^2,z=1/3^2} (1/3^2 = 1/9 = 0.111…; see your test result) because it divides by the biggest abs number three times instead of just one time

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print("Vector",dump(vector.direction({x=1,y=2,z=3},{x=4,y=5,z=6})))
print("Vector",dump(vector.direction({x=0,y=0,z=0},{x=3,y=3,z=3})))

print("Vector2",dump(vector.normalize{x=3,y=3,z=3}))
print("Vector2",dump{x=3/math.sqrt(3^2+3^2+3^2),y=3/math.sqrt(3^2+3^2+3^2),z=3/math.sqrt(3^2+3^2+3^2)})
print("Vector2",dump{x=1/math.sqrt(3),y=1/math.sqrt(3),z=1/math.sqrt(3)})


l don't know what the result of vector.direction is good for…

[burli]

duane uses this function in his valleys helper mod

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local function calc_velocity(pos1, pos2, old_vel, power)
   local vel = vector.direction(pos1, pos2)
   vel = vector.normalize(vel)
   vel = vector.multiply(vel, power)

   -- Divide by distance
   local dist = vector.distance(pos1, pos2)
   dist = math.max(dist, 1)
   vel = vector.divide(vel, dist)

   -- Add old velocity
   vel = vector.add(vel, old_vel)
   return vel
end

it is also used in the identify mod

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local function node_contents_within_reach(center, direction, position)

  local cluster = node_cuboids(center)

  local i, j = box_distance_minimum(direction, position, cluster)

  local reach = point_interval(node_ray_intersection(vector.direction(center, position), center), position)

  local value = false

  if not (i == 0) then
    if (reach.x < 4.0) and (reach.y < 4.0) and (reach.z < 4.0) then
      if (math.abs(vector.distance(box_side_intersection(direction, position, cluster[i], j), position)) < 5.0) then
        value = true
      end
    end
  elseif (i == 0) then
    value = true
  end

  return value
end

[Hybrid Dog]

l guess duane doesn't know what it does and isn't experienced with vectors
this should work better

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local function calc_velocity(pos1, pos2, old_vel, power)
   local vel = vector.subtract(pos2, pos1)
   vel = vector.normalize(vel)
   vel = vector.multiply(vel, power)

[…]

or more streamlined:

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local function calc_velocity(pos1, pos2, old_vel, power)
   local vel = vector.subtract(pos2, pos1)
   vel = vector.multiply(vel, power / vector.length(vel))

[…]

lf l understand it correctly, the identify mod uses it for a ray iteration function, so a function which calculates positions which "follow" a line,
the technic mining laser uses a ray iteration function,
as far as l know there are lots of ways to do such a rayIter function and lots of bad and wrong working ways,
l made one myself (https://github.com/HybridDog/vector_ext ... t.lua#L100), it's fairly simple but it tests each (of the 3 or less) position if it fits best, so it's not perfect

[rubenwardy]

Whatever this function does, it doesn't look like it calculates a direction. In linear algebra, a direction is usually:

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normalise(to - from)
where
    normalise(A) = A / |A|

[Hybrid Dog]

rubenwardy, so a "direction" is always a unit vector?

And the direction from pos to pos (resulting in the zero vector) is an isotropic vector?

[burli]

I'm not sure, but I think this function calculates a vector between two positions

[Ferk]

I'm not sure, but I think this function calculates a vector between two positions

I don't think so, for that you only need to subtract the initial position from the final position, vector.subtract(b, a) would return the vector from point a to point b.

Just as Hybrid Dog and Ruberwardy said, the unitary vector between two points would simply be:

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function vector.direction(a, b)
    return vector.normalize(vector.subtract(b, a))
end


And if instead of 2 points you have 2 vectors that you want to merge into one to get the resulting direction when combined, you would rather just use vector.add and then (if needed) normalize it.

[Hybrid Dog]

https://github.com/minetest/minetest/co ... 642775dR52

ShadowNinja may know more about it.