Arrow projectile with air drag applied to feathered tail.

arrowlib.lua

-- arrow test

-- http://www.iforce2d.net/b2dtut/sticky-projectiles
-- video: http://screencast.com/t/vcPLSJR6xkIq

require("mathlib")

local sWidth, sHeight = display.actualContentWidth, display.actualContentHeight
local centerX, centerY = sWidth/2, sHeight/2

local arrows = display.newGroup()

-- original arrow image: http://content.screencast.com/users/HoraceBury/folders/Default/media/1d553487-b39f-440c-a89c-3d4e818cf20a/arrow.png?downloadOnly=true
-- if you don't have/can't get this image, comment out this line...
local outline = graphics.newOutline( 4, "arrow.png" )
-- if you don't have the image, use this instead...
--outline = {157,0,223,13,153,24,0,12,156,0}

local launchPad = display.newCircle( 500, sHeight, 50 )

local maxDamping = 10
local multi = 10
local x, y = nil, nil
local zero = {x=0,y=0}
local one = {x=1,y=0}
local dragConstant = .25
local trim = 300

--[[ -- This was some working out while trying to directly convert (yet again) then "sticky projectiles" code into Lua... (didn't work)
local dc = .5 -- dragConstant
local pointingDirection = math.rotateTo( one, 45, zero )
local flightDirection = {x=12,y=0} -- arrow:getLinearVelocity()
local flightSpeed = math.normalise( flightDirection ) -- normalises flightDirection and returns the length

local dot = math.dotProduct( flightDirection, pointingDirection )
local mass = math.polygonArea( outline ) * 0.1 -- area * density
local dragForceMagnitude = (1 - math.abs(dot)) * flightSpeed * flightSpeed * dc * mass
print(mass)
print(dot)
print( math.dotProduct( -10,10 , 10,0 ) )
print(dragForceMagnitude) -- this is always wayyyyy too high
]]--

-- apply drag to tail of each arrow
Runtime:addEventListener( "enterFrame", function()
	for i=1, arrows.numChildren do
		-- get the arrow
		local arrow = arrows[i]
		
		-- get direction the arrow is facing
		x, y = arrow:localToContent( 1, 0 )
		local pointing = { x=x-arrow.x, y=y-arrow.y }
		
		-- get direction of flight
		x, y = arrow:getLinearVelocity()
		local velocityVector = { x=x, y=y }
		
		-- get the ballistic velocity
		local velocity = math.lengthOf( x, y )
		
		-- get drag direction
		local dragDirection = { x=-x, y=-y }
		
		-- get angle between the two
		local angle = math.angleOf( {x=0,y=0}, pointing, velocityVector )
		
		-- value of angle determines how much of the drag is applied (0 - 180)
		local fraction = math.fractionOf( 180, math.abs(angle) )
		
		-- drag force
		local dragForce = { x=dragConstant * dragDirection.x, y=dragConstant * dragDirection.y }
		
		-- feathers location
		x, y = arrow:localToContent( -100, 0 )
		local feathers = { x=x, y=y }
		
		-- apply drag if it is above a certain amount (an arrow dropping to the ground should just flop down after nose diving)
		if (velocity > trim) then
			-- choose the version which you prefer...
			
			-- drag is relative to the angle between the direction of travel and facing direction
			--arrow:applyForce( fraction*dragForce.x, fraction*dragForce.y, feathers.x, feathers.y )
			
			-- drag is relative to speed
			arrow:applyForce( dragForce.x, dragForce.y, feathers.x, feathers.y )
		end
	end
end )

-- tap to fire arrow - location and distance from launchPad will indicate direction and speed of flight
Runtime:addEventListener( "tap", function(e)
	local angle = math.angleOf( launchPad, e )
	
	local arrow = display.newImage( arrows, "arrow.png" )
	arrow.x, arrow.y = e.x, e.y
	arrow.rotation = angle
	
	physics.addBody( arrow, "dynamic", { outline=outline, friction=1, density=1, bounce=.1, } )
	arrow.angularDamping = 1
	
	arrow:setLinearVelocity( (e.x-launchPad.x)*multi, (e.y-sHeight)*multi )
end )

build.settings

settings = {
	orientation =
	{
		default = "landscapeLeft",
	},
	iphone =
	{
		plist=
		{
			UIStatusBarHidden=true,
			CFBundleIconFile = "Icon.png",
			CFBundleIconFiles = {
				"Icon.png", 
				"Icon@2x.png", 
				"Icon-72.png", 
			},
		},
	}
}

config.lua

application = 
{
	content = 
	{ 
		width = 1024*3,
		height = 768*3,
		scale = "letterbox",
		
		xAlign = "left",
		yAlign = "top",
		
		antialias = true,
		
		imageSuffix =
		{
			["-x2"] = 2,
			["-x4"] = 4,
		},
	}
}

main.lua

-- arrow test

-- http://www.iforce2d.net/b2dtut/sticky-projectiles

display.setStatusBar( display.HiddenStatusBar )

require("physics")
physics.start()
physics.setDrawMode("hybrid")
physics.setGravity( 0, 9.8 )

local sWidth, sHeight = display.actualContentWidth, display.actualContentHeight
local centerX, centerY = sWidth/2, sHeight/2

physics.addBody( display.newRect( centerX, 0, sWidth, 20 ), "static" )
physics.addBody( display.newRect( centerX, sHeight, sWidth, 20 ), "static" )
physics.addBody( display.newRect( 0, centerY, 20, sHeight ), "static" )
physics.addBody( display.newRect( sWidth, centerY, 20, sHeight ), "static" )

require("arrowlib")

mathlib.lua

-- mathlib.lua

--[[
	Maths extension library for use in Corona SDK by Matthew Webster.
	All work derived from referenced sources.
	Many of these functions are useful for trigonometry and geometry because they have developed for use within graphical user interfaces.
	Much of these are useful when building physics games
	
	twitter: @horacebury
	blog: http://springboardpillow.blogspot.co.uk/2012/04/sample-code.html
	code exchange: http://code.coronalabs.com/search/node/HoraceBury
	github: https://gist.github.com/HoraceBury
]]--
 
--[[
	References:
		http://stackoverflow.com/questions/385305/efficient-maths-algorithm-to-calculate-intersections
		http://stackoverflow.com/questions/4543506/algorithm-for-intersection-of-2-lines
		http://community.topcoder.com/tc?module=Static&d1=tutorials&d2=geometry2#reflection
		http://gmc.yoyogames.com/index.php?showtopic=433577
		http://local.wasp.uwa.edu.au/~pbourke/geometry/
		http://alienryderflex.com/polygon/
		http://alienryderflex.com/polygon_fill/
		http://www.amazon.com/dp/1558607323/?tag=stackoverfl08-20
		http://www.amazon.co.uk/s/ref=nb_sb_noss_1?url=search-alias%3Daps&field-keywords=Real-Time+Collision+Detection
		http://en.wikipedia.org/wiki/Line-line_intersection
		http://developer.coronalabs.com/forum/2010/11/17/math-helper-functions-distancebetween-and-anglebetween
		http://www.mathsisfun.com/algebra/vectors-dot-product.html
		http://www.mathsisfun.com/algebra/vector-calculator.html
		http://lua-users.org/wiki/PointAndComplex
		http://www.math.ntnu.no/~stacey/documents/Codea/Library/Vec3.lua
		http://www.iforce2d.net/forums/viewtopic.php?f=4&t=79&sid=b9ecd62533361594e321de04b3929d4f
		http://rosettacode.org/wiki/Dot_product#Lua
		http://chipmunk-physics.net/forum/viewtopic.php?f=1&t=2215
		http://www.fundza.com/vectors/normalize/index.html
		http://www.mathopenref.com/coordpolygonarea2.html
		http://stackoverflow.com/questions/2705542/returning-the-nearest-multiple-value-of-a-number
		http://members.tripod.com/c_carleton/dotprod.html/
		http://www.1728.org/density.htm
		http://www.wikihow.com/Find-the-Angle-Between-Two-Vectors
]]--

--[[
	Deprecated functions (see revisions for code):
		rad = convertDegreesToRadians( degrees )
		deg = convertRadiansToDegrees( radians )
		polygonFill( points, closed, perPixel, width, height, col )
]]--

--[[
	Multiplication & Fractions Functions:
		
]]--

--[[
	Point Functions:
		
]]--

--[[
	Angle Functions:
		
]]--

--[[
	Line Functions:
		
]]--

--[[
	Polygon Functions:
		
]]--

--[[
	Point Functions:
		
]]--

-- rounds up to the nearest multiple of the number
local function nearest( number, multiple )
	return math.round( (number / multiple) ) * multiple
end
math.nearest = nearest

-- Returns b represented as a fraction of a.
-- Eg: If a is 1000 and b is 900 the returned value is 0.9
-- Often the returned value would be used in a multiplication of another value, usually a distance value.
local function fractionOf( a, b )
        return b / a
end
math.fractionOf = fractionOf

-- Returns b represented as a percentage of a.
-- Eg: If a is 1000 and b is 900 the returned value is 90
-- Use: This is useful in determining how far something should be moved to complete a certain distance.
-- Often the returned value would be used in a division of another value, usually a distance value.
local function percentageOf( a, b )
        return fractionOf(a, b) * 100
end
math.percentageOf = percentageOf

-- return a value clamped between a range
local function clamp( val, low, high )
	if (val < low) then return low end
	if (val > high) then return high end
	return val
end
math.clamp = clamp
 
-- rotates point around the centre by degrees
-- rounds the returned coordinates using math.round() if round == true
-- returns new coordinates object
local function rotateAboutPoint( point, degrees, centre )
	local pt = { x=point.x - centre.x, y=point.y - centre.y }
	pt = math.rotateTo( pt, degrees )
	pt.x, pt.y = pt.x + centre.x, pt.y + centre.y
	return pt
end
math.rotateAboutPoint = rotateAboutPoint

-- rotates a point around the (0,0) point by degrees
-- returns new point object
-- center: optional
local function rotateTo( point, degrees, center )
	if (center ~= nil) then
		return rotateAboutPoint( point, degrees, center )
	else
		local x, y = point.x, point.y

		local theta = math.rad( degrees )

		local pt = {
			x = x * math.cos(theta) - y * math.sin(theta),
			y = x * math.sin(theta) + y * math.cos(theta)
		}
		
		return pt
	end
end
math.rotateTo = rotateTo

--[[ Support values for angles ]]--
local PI = (4*math.atan(1))
local quickPI = 180 / PI
math.PI, math.quickPI = PI, quickPI

--[[
	Returns the angle.
	
	Params:
		a : Returns the angle of the point at a relative to (0,0) (east is the virtual base)
		a, b Params: Returns the angle of b relative to a
		a, b, c Params: Returns the angle found at a for between b and c
]]--
local function angleOf( ... )
	local a, b, c = arg[1], arg[2], arg[3]
	
	if (#arg == 1) then
		-- angle of a relative to (0,0)
		return math.atan2( a.y, a.x ) * quickPI -- 180 / PI -- math.pi
	elseif (#arg == 2) then
		-- angle of b relative to a
		return math.atan2( b.y - a.y, b.x - a.x ) * quickPI -- 180 / PI -- math.pi
	elseif (#arg == 3) then
		-- angle between b and c found at a
		local deg = angleOf( a, b ) - angleOf( a, c ) -- target - source
		
		if (deg > 180) then
			deg = deg - 360
		elseif (deg < -180) then
			deg = deg + 360
		end
		
		return deg
	end
	
	-- wrong set of parameters
	return nil
end
math.angleOf = angleOf

-- Brent Sorrentino
-- Returns the angle between the objects
local function angleBetween( srcObj, dstObj )
	local xDist = dstObj.x - srcObj.x
	local yDist = dstObj.y - srcObj.y
	local angleBetween = math.deg( math.atan( yDist / xDist ) )
	if ( srcObj.x < dstObj.x ) then
		angleBetween = angleBetween + 90
	else
		angleBetween = angleBetween - 90
	end
	return angleBetween
end
math.angleBetween = angleBetween

--[[
	Calculate the angle between two lines.
	
	Params:
		lineA - The first line { a={x,y}, b={x,y} }
		lineA - The first line { a={x,y}, b={x,y} }
]]--
local function angleBetweenLines( lineA, lineB )
	local angle1 = math.atan2( lineA.a.y - lineA.b.y, lineA.a.x - lineA.b.x )
	local angle2 = math.atan2( lineB.a.y - lineB.b.y, lineB.a.x - lineB.b.x )
	return math.deg( angle1 - angle2 )
end
math.angleBetweenLines = angleBetweenLines

-- returns the smallest angle between the two angles
-- ie: the difference between the two angles via the shortest distance
-- returned value is signed: clockwise is negative, anticlockwise is positve
-- returned value wraps at +/-180
-- Example code to rotate a display object by touch:
--[[
	-- called in the "moved" phase of touch event handler
	local a = mathlib.angleBetweenPoints( target, target.prevevent )
	local b = mathlib.angleBetweenPoints( target, event )
	local d = mathlib.smallestAngleDiff( a, b )
	target.prev = event
	target.rotation = target.rotation - d
]]--
local function smallestAngleDiff( target, source )
	local a = target - source

	if (a > 180) then
		a = a - 360
	elseif (a < -180) then
		a = a + 360
	end

	return a
end
math.smallestAngleDiff = smallestAngleDiff

-- Returns the angle in degrees between the first and second points, measured at the centre
-- Always a positive value
local function angleAt( centre, first, second )
	local a, b, c = centre, first, second
	local ab = math.lengthOf( a, b )
	local bc = math.lengthOf( b, c )
	local ac = math.lengthOf( a, c )
	local angle = math.deg( math.acos( (ab*ab + ac*ac - bc*bc) / (2 * ab * ac) ) )
	return angle
end
math.angleAt = angleAt

-- Returns true if the point is within the angle at centre measured between first and second
local function isPointInAngle( centre, first, second, point )
	local range = math.angleAt( centre, first, second )
	local a = math.angleAt( centre, first, point )
	local b = math.angleAt( centre, second, point )
	-- print(range,a+b)
	return math.round(range) >= math.round(a + b)
end
math.isPointInAngle = isPointInAngle

-- Forces to apply based on total force and desired angle
-- http://developer.anscamobile.com/code/virtual-dpadjoystick-template
local function forcesByAngle(totalForce, angle)
	local forces = {}
	local radians = -math.rad(angle)

	forces.x = math.cos(radians) * totalForce
	forces.y = math.sin(radians) * totalForce

	return forces
end
math.forcesByAngle = forcesByAngle

-- returns the distance between points a and b
-- b is optional. assumes distance from 0,0 if b is nil
local function lengthOf( a, b )
	if (type(a) == "number") then
		a = {x=a,y=b}
		b = nil
	end
	if (b == nil) then
		b = {x=0,y=0}
	end
	local width, height = b.x-a.x, b.y-a.y
	return (width*width + height*height)^0.5 -- math.sqrt(width*width + height*height)
	-- nothing wrong with math.sqrt, but I believe the ^.5 is faster
end
math.lengthOf = lengthOf

--[[
	Description:
		Extends the point away from or towards the origin to the length of len.
	
	Params:
		max =
			If param max is nil then the lenOrMin value is the distance to calculate the point's location
			If param max is not nil then the lenOrMin value is the minimum clamping distance to extrude to
		lenOrMin = the length or the minimum length to extrude the point's distance to
		max = the maximum length to extrude to
	
	Returns:
		{x,y} = extruded point
]]--
local function extrudeToLen( origin, point, lenOrMin, max )
	local length = lengthOf( origin, point )
	if (length == 0) then
		return origin.x, origin.y
	end
	local len = lenOrMin
	if (max ~= nil) then
		if (length < lenOrMin) then
			len = lenOrMin
		elseif (length > max) then
			len = max
		else -- the point is within the min/max clamping range
			return point.x, point.y
		end
	end
	local factor = len / length
	local x, y = (point.x - origin.x) * factor, (point.y - origin.y) * factor
	return x + origin.x, y + origin.y, x, y
end
math.extrudeToLen = extrudeToLen

--[[
	Performs unit normalisation of a vector.
	
	Description:
		Unit normalising is basically converting the length of a line to be a fraction of 1.0
		This function modified the vector value passed in and returns the length as returned by lengthOf()
	
	Note:
		Can also be performed like this:
		function Normalise(vector)
			local x,y = 	x/(x^2 + y^2)^(1/2), y/(x^2 + y^2)^(1/2)
			local unitVector = {x=x,y=y}
			return unitVector
		end
	
	Ref:
		http://www.fundza.com/vectors/normalize/index.html
]]--
local function normalise( vector )
	local len = math.lengthOf( vector )
	vector.x = vector.x / len
	vector.y = vector.y / len
	return len
end
math.normalise = normalise

-- calculates the area of a polygon
-- will not calculate area for self-intersecting polygons (where vertices cross each other)
-- points: table of {x,y} points
-- ref: http://www.mathopenref.com/coordpolygonarea2.html
local function polygonArea( points )
	if (type(points[1]) == "number") then
		points = math.tableToPoints( points )
	end
	
	local count = #points
	if (points.numChildren) then
		count = points.numChildren
	end
	
	local area = 0 -- Accumulates area in the loop
	local j = count -- The last vertex is the 'previous' one to the first
	
	for i=1, count do
		area = area +  (points[j].x + points[i].x) * (points[j].y - points[i].y)
		j = i -- j is previous vertex to i
	end
	
	return math.abs(area/2)
end
math.polygonArea = polygonArea

-- Returns true if the dot { x,y } is within the polygon defined by points table { {x,y},{x,y},{x,y},... }
local function pointInPolygon( points, dot )
        local i, j = #points, #points
        local oddNodes = false
 
        for i=1, #points do
                if ((points[i].y < dot.y and points[j].y>=dot.y
                        or points[j].y< dot.y and points[i].y>=dot.y) and (points[i].x<=dot.x
                        or points[j].x<=dot.x)) then
                        if (points[i].x+(dot.y-points[i].y)/(points[j].y-points[i].y)*(points[j].x-points[i].x)<dot.x) then
                                oddNodes = not oddNodes
                        end
                end
                j = i
        end
 
        return oddNodes
end
math.pointInPolygon = pointInPolygon

-- Return true if the dot { x,y } is within any of the polygons in the list
local function isPointInPolygons( polygons, dot )
        for i=1, #polygons do
                if (pointInPolygon( polygons[i], dot )) then
                        return true
                end
        end
        return false
end
math.isPointInPolygons = isPointInPolygons
 
-- Returns true if the points in the polygon wind clockwise
-- Does not consider that the vertices may intersect (lines between points might cross over)
local function isPolygonClockwise( pointList )
        local area = 0
        
        if (type(pointList[1]) == "number") then
        	pointList = math.pointsToTable( pointList )
        	print("#pointList",#pointList)
        end
        
        for i = 1, #pointList-1 do
                local pointStart = { x=pointList[i].x - pointList[1].x, y=pointList[i].y - pointList[1].y }
                local pointEnd = { x=pointList[i + 1].x - pointList[1].x, y=pointList[i + 1].y - pointList[1].y }
                area = area + (pointStart.x * -pointEnd.y) - (pointEnd.x * -pointStart.y)
        end
        
        return (area < 0)
end
math.isPolygonClockwise = isPolyClockwise

-- returns true if the middle point is concave when viewed as part of a polygon
-- a, b, c are {x,y} points
local function isPointConcave(a,b,c)
	local small = smallestAngleDiff( math.angleOf(b,a), math.angleOf(b,c) )

	if (small < 0) then
		return false
	else
		return true
	end
end
math.isPointConcave = isPointConcave

-- returns true if the polygon is concave
-- assumes points are {x,y} tables
-- returns nil if there are not enough points ( < 3 )
-- can accept a display group
local function isPolygonConcave( points )
	local count = points.numChildren
	if (count == nil) then
		count = #points
	end
	
	if (count < 3) then
		return nil
	end
	
	local isConcave = true
	
	for i=1, count do
		if (i == 1) then
			isConcave = isPointConcave( points[count],points[1],points[2] )
		elseif (i == count) then
			isConcave = isPointConcave( points[count-1],points[count],points[1] )
		else
			isConcave = isPointConcave( points[i-1], points[i], points[i+1] )
		end
		
		if (not isConcave) then
			return false
		end
	end
	
	return true
end
math.isPolygonConcave = isPolygonConcave

-- returns list of points where a polygon intersects with the line a,b
-- assumes polygon is standard display format: { x,y,x,y,x,y,x,y, ... }
-- returns collection of intersection points with the polygon line's index {x,y,lineIndex}
-- sort: true to sort the points into order from a to b
local function polygonLineIntersection( polygon, a, b, sort )
	local points = {}
	
	for i=1, #polygon-3, 2 do
		local success, pt = math.doLinesIntersect( a, b, { x=polygon[i], y=polygon[i+1] }, { x=polygon[i+2], y=polygon[i+3] } )
		
		if (success) then
			pt.lineIndex = i
			points[ #points+1 ] = pt
		end
	end
	
	if (sort) then
		table.sort( points, function(a,b) return math.lengthOf(e,a) > math.lengthOf(e,b) end )
	end
	
	return points
end
math.polygonLineIntersection = polygonLineIntersection

--[[
	Products
]]--

--[[
        Calculates the dot product of two lines.
        This function implements the simple form of the dot product calculation: a · b = ax × bx + ay × by
        The lines can be provided in 3 forms:
        
        Parameters:
        	a: {x,y}
        	b: {x,y}
        
        Example:
        	print( dotProduct( {x=10,y=10}, {x=-10,y=10} ) )
        
        Parameters:
        	a: {a,b}
        	b: {a,b}
        
        Example:
                print( dotProduct(
                        { a={x=10,y=10}, b={x=101,y=5} },
                        { a={x=10,y=-10}, b={x=51,y=10} }
                ))
        
		Params:
			lenA: Length A
			lenB: Length B
			deg: Angle between points A and B in degrees
		
		Example:
			print( dotProduct( 23, 10, 90 ) )
		
        Ref:
		http://www.mathsisfun.com/algebra/vectors-dot-product.html
		http://members.tripod.com/c_carleton/dotprod.html/
		http://www.mathsisfun.com/algebra/vector-calculator.html
]]--
local function dotProduct( ... )
	local ax, ax, bx, by
	
	if (#arg == 2 and arg[1].a == nil) then
		-- two vectors - get the vectors
		ax, ay = arg[1].x, arg[1].y
		bx, by = arg[2].x, arg[2].y
		
	elseif (#arg == 2 and a.x == nil) then
		-- two lines - calculate the vectors
		ax = arg[1].b.x - arg[1].a.x
		ay = arg[1].b.y - arg[1].a.y
		bx = arg[2].b.x - arg[2].a.x
		by = arg[2].b.y - arg[2].a.y
		
	elseif (#arg == 3 and type(arg[1]) == "number") then
		-- two lengths and an angle: lenA * lenB * math.cos( deg )
		return arg[1] * arg[2] * math.cos( arg[3] )
		
	elseif (#arg == 4 and type(arg[1]) == "number") then
		-- two lines, params are (x,y,x,y) - get the vectors
		ax, ay = arg[1], arg[2]
		bx, by = arg[3], arg[4]
	end
	
	-- multiply the x's, multiply the y's, then add
	local dot = ax * bx + ay * by
	return dot
end
math.dotProduct = dotProduct

--[[
        Description:
                Calculates the cross product of a vector.
        
        Ref:
                http://www.math.ntnu.no/~stacey/documents/Codea/Library/Vec3.lua
]]--
local function crossProduct( a, b )
        local x, y, z
        x = a.y * (b.z or 0) - (a.z or 0) * b.y
        y = (a.z or 0) * b.x - a.x * (b.z or 0)
        z = a.x * b.y - a.y * b.x
        return { x=x, y=y, z=z }
end
math.crossProduct = crossProduct

--[[
        Description:
                Perform the cross product on two vectors. In 2D this produces a scalar.
        
        Params:
                a: {x,y}
                b: {x,y}
        
        Ref:
                http://www.iforce2d.net/forums/viewtopic.php?f=4&t=79&sid=b9ecd62533361594e321de04b3929d4f
]]--
local function b2CrossVectVect( a, b )
        return a.x * b.y - a.y * b.x;
end
math.b2CrossVectVect = b2CrossVectVect

--[[
        Description:
                Perform the cross product on a vector and a scalar. In 2D this produces a vector.
        
        Params:
                a: {x,y}
                b: float
        
        Ref:
                http://www.iforce2d.net/forums/viewtopic.php?f=4&t=79&sid=b9ecd62533361594e321de04b3929d4f
]]--
local function b2CrossVectFloat( a, s )
        return { x = s * a.y, y = -s * a.x }
end
math.b2CrossVectFloat = b2CrossVectFloat

--[[
        Description:
                Perform the cross product on a scalar and a vector. In 2D this produces a vector.
        
        Params:
                a: float
                b: {x,y}
        
        Ref:
                http://www.iforce2d.net/forums/viewtopic.php?f=4&t=79&sid=b9ecd62533361594e321de04b3929d4f
]]--
local function b2CrossFloatVect( s, a )
        return { x = -s * a.y, y = s * a.x }
end
math.b2CrossFloatVect = b2CrossFloatVect

--[[
	Polygons
]]--

--[[
        Description:
                Calculates the average of all the x's and all the y's and returns the average centre of all points.
                Works with a display group or table proceeding { {x,y}, {x,y}, ... }
        
        Params:
                pts = list of {x,y} points to get the average middle point from
        
        Returns:
                {x,y} = average centre location of all the points
]]--
local function midPoint( ... )
	local pts = arg
	
	local x, y, c = 0, 0, #pts
	if (pts.numChildren and pts.numChildren > 0) then c = pts.numChildren end
	for i=1, c do
		x = x + pts[i].x
		y = y + pts[i].y
	end
	return { x=x/c, y=y/c }
end
math.midPoint = midPoint

--[[
        Description:
                Calculates the average of all the x's and all the y's and returns the average centre of all points.
                Works with a table proceeding {x,y,x,y,...} as used with display.newLine or physics.addBody
        
        Params:
                pts = table of x,y values in sequence
        
        Returns:
                x, y = average centre location of all points
]]--
local function midPointOfShape( pts )
        local x, y, c, t = 0, 0, #pts, #pts/2
        for i=1, c-1, 2 do
                x = x + pts[i]
                y = y + pts[i+1]
        end
        return x/t, y/t
end
math.midPointOfShape = midPointOfShape

-- returns true when the point is on the right of the line formed by the north/south points
local function isOnRight( north, south, point )
        local a, b, c = north, south, point
        local factor = (b.x - a.x)*(c.y - a.y) - (b.y - a.y)*(c.x - a.x)
        return factor > 0, factor
end
math.isOnRight = isOnRight

-- reflect point across line from north to south
local function reflect( north, south, point )
        local x1, y1, x2, y2 = north.x, north.y, south.x, south.y
        local x3, y3 = point.x, point.y
        local x4, y4 = 0, 0 -- reflected point
        local dx, dy, t, d
 
        dx = y2 - y1
        dy = x1 - x2
        t = dx * (x3 - x1) + dy * (y3 - y1)
        t = t / (dx * dx  +  dy * dy)
 
        x = x3 - 2 * dx * t
        y = y3 - 2 * dy * t
 
        return { x=x, y=y }
end
math.reflect = reflect

-- This is based off an explanation and expanded math presented by Paul Bourke:
-- It takes two lines as inputs and returns true if they intersect, false if they don't.
-- If they do, ptIntersection returns the point where the two lines intersect.
-- params a, b = first line
-- params c, d = second line
-- param ptIntersection: The point where both lines intersect (if they do)
-- http://local.wasp.uwa.edu.au/~pbourke/geometry/lineline2d/
-- http://paulbourke.net/geometry/pointlineplane/
local function doLinesIntersect( a, b, c, d )
        -- parameter conversion
        local L1 = {X1=a.x,Y1=a.y,X2=b.x,Y2=b.y}
        local L2 = {X1=c.x,Y1=c.y,X2=d.x,Y2=d.y}
        
        -- Denominator for ua and ub are the same, so store this calculation
        local d = (L2.Y2 - L2.Y1) * (L1.X2 - L1.X1) - (L2.X2 - L2.X1) * (L1.Y2 - L1.Y1)
        
        -- Make sure there is not a division by zero - this also indicates that the lines are parallel.
        -- If n_a and n_b were both equal to zero the lines would be on top of each
        -- other (coincidental).  This check is not done because it is not
        -- necessary for this implementation (the parallel check accounts for this).
        if (d == 0) then
                return false
        end
        
        -- n_a and n_b are calculated as seperate values for readability
        local n_a = (L2.X2 - L2.X1) * (L1.Y1 - L2.Y1) - (L2.Y2 - L2.Y1) * (L1.X1 - L2.X1)
        local n_b = (L1.X2 - L1.X1) * (L1.Y1 - L2.Y1) - (L1.Y2 - L1.Y1) * (L1.X1 - L2.X1)
        
        -- Calculate the intermediate fractional point that the lines potentially intersect.
        local ua = n_a / d
        local ub = n_b / d
        
        -- The fractional point will be between 0 and 1 inclusive if the lines
        -- intersect.  If the fractional calculation is larger than 1 or smaller
        -- than 0 the lines would need to be longer to intersect.
        if (ua >= 0 and ua <= 1 and ub >= 0 and ub <= 1) then
                local x = L1.X1 + (ua * (L1.X2 - L1.X1))
                local y = L1.Y1 + (ua * (L1.Y2 - L1.Y1))
                return true, {x=x, y=y}
        end
        
        return false
end
math.doLinesIntersect = doLinesIntersect

-- returns the closest point on the line between A and B from point P
local function GetClosestPoint( A,  B,  P, segmentClamp )
    local AP = { x=P.x - A.x, y=P.y - A.y }
    local AB = { x=B.x - A.x, y=B.y - A.y }
    local ab2 = AB.x*AB.x + AB.y*AB.y
    local ap_ab = AP.x*AB.x + AP.y*AB.y
    local t = ap_ab / ab2
 
    if (segmentClamp or true) then
         if (t < 0.0) then
                t = 0.0
         elseif (t > 1.0) then
                t = 1.0
         end
    end
 
    local Closest = { x=A.x + AB.x * t, y=A.y + AB.y * t }
 
    return Closest
end
math.GetClosestPoint = GetClosestPoint

-- converts a table of {x,y,x,y,...} to points {x,y}
local function tableToPoints( tbl )
	local pts = {}
	
	for i=1, #tbl-1, 2 do
		pts[#pts+1] = { x=tbl[i], y=tbl[i+1] }
	end
	
	return pts
end
math.tableToPoints = tableToPoints

-- converts a list of points {x,y} to a table of coords {x,y,x,y,...}
local function pointsToTable( pts )
	local tbl = {}
	
	for i=1, #pts do
		tbl[#tbl+1] = pts[i].x
		tbl[#tbl+1] = pts[i].y
	end
	
	return tbl
end
math.pointsToTable = pointsToTable