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Can anyone spot the obvious bug with this math (trig)? — Gideros Forum
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    Can anyone spot the obvious bug with this math (trig)?

    Tom2012Tom2012 Guru
    edited August 2013 in General questions
    Hey everyone. Got what should be a simple problem but I'm pulling out my hair as to why I'm getting the results I am.

    A character walks at an angle to a point on the screen:

    local nextX = self.scene.path[self.id].vertices.x[self.pathCounter]
    local nextY = self.scene.path[self.id].vertices.y[self.pathCounter]

    I'm using 'distance' to stop him when he gets with a range.

    But he never actually gets there. Instead, there is always a distance of around 16 pixels.

    Any help very much appreciated.
    function CreeperBug:onEnterFrame()
 
	local currentX,currentY = self:getPosition()
 
	local nextX = self.scene.path[self.id].vertices.x[self.pathCounter]
	local nextY = self.scene.path[self.id].vertices.y[self.pathCounter]
 
	local xDiff = nextX - currentX
	local yDiff = nextY - currentY
 
	local distance = math.sqrt((xDiff*xDiff)+(yDiff*yDiff))
 
	local angle = math.deg(math.atan2(yDiff,xDiff))
 
	local nextX = currentX + (math.cos(angle) * 1)
	local nextY = currentY + (math.sin(angle) * 1)
 
	if(self.phase == "walk") then
		self:setPosition(nextX, nextY)
	end
 
end
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    Comments

    • ar2rsawseenar2rsawseen Maintainer
      @Tom2012
      my most common mistake is forgetting that Gideros has Cartesian coordinate system, thus y axis is always inverted

      Other usual problem is that some math function might work not from the 0 to 360 degrees as expected, but for example from -180 to 180 degrees.

      This is my usual way on getting angle:
      local angle = math.acos(yDiff/distance)
if(xDiff >0) then
	angle = -angle
end

      Likes: Tom2012

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    • Bill5567Bill5567 Member
      Distance between 2 points

      local x1, y1 = 100, 100
      local x2, y2 = 200, 200

      local distanceBetween = math.ceil(math.sqrt( ((y1-y2) ^ 2) + ((x1-x2) ^ 2) ))

      Likes: Tom2012

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    • Tom2012Tom2012 Guru
      edited August 2013
      Thanks guys, still having problems with this (tried both suggestions)

      Here's a simple code that produces the problem I have:
      local apple = Bitmap.new(Texture.new("apple.png"))
apple:setAnchorPoint(0,0)
stage:addChild(apple)
 
local pumpkin = Bitmap.new(Texture.new("pumpkin.png"))
pumpkin:setAnchorPoint(0,0)
stage:addChild(pumpkin)
pumpkin:setPosition(100,100)
 
local angle = 45
local speed = 5
 
local x1, y1 = apple:getPosition()
local x2, y2 = pumpkin:getPosition()
 
local nextX = x1+(math.cos(angle) * speed)
local nextY = y1+(math.sin(angle) * speed)
 
Timer.delayedCall(1000, function()
	apple:setPosition(nextX,nextY)
end)
      Lost Caverns - my little game on Play Store - https://play.google.com/store/apps/details?id=com.firecroc.lostcaverns
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    • adityaaditya Member
      edited August 2013 Accepted Answer
      apple:setPosition is being called once in your code.

      This should work.
       
local apple = Bitmap.new(Texture.new("apple.png"))
apple:setAnchorPoint(0,0)
stage:addChild(apple)
 
local pumpkin = Bitmap.new(Texture.new("pumpkin.png"))
pumpkin:setAnchorPoint(0,0)
stage:addChild(pumpkin)
pumpkin:setPosition(100,100)
 
 
local speed = 5 
local angle,x1,y1,x2,y2,nextX,nextY
 
function checkReached(x1,y1,x2,y2)
	local res = false
	if((x1>=x2-2 and x1<=x2+2) and (y1>=y2-2 and y1<=y2+2)) then
		res = true
	end
	return res
end
 
function onEnterFrame(e)
	x1, y1 = apple:getPosition()
	x2, y2 = pumpkin:getPosition()
	print(x1 .. " " .. y1 .. "   |   " .. x2 .. " " ..y2)
 
	if(checkReached(x1,y1,x2,y2)) then
		stage:removeEventListener(Event.ENTER_FRAME, onEnterFrame)
	else
		angle = math.atan2(y2-y1,x2-x1)
 
		nextX = x1+(math.cos(angle) * speed)
		nextY = y1+(math.sin(angle) * speed)
 
		apple:setPosition(nextX,nextY)
	end
 
end
 
stage:addEventListener(Event.ENTER_FRAME, onEnterFrame)

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    • Tom2012Tom2012 Guru
      @aditya

      Thank you! That works perfectly. Will be using that code in my game.

      I'm embarrassed to say I was passing degrees rather than radians to math.cos / sin.

      I thought was getting better at this stuff. :\">

      Thank you again for the replies and help folks.
      Lost Caverns - my little game on Play Store - https://play.google.com/store/apps/details?id=com.firecroc.lostcaverns
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