Convex Hull or Gift Wrapping algorithm

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Pakz	Created : 21 November 2019 Language : Monkey2 Convex Hull or Gift Wrapping algorithm create lines around set of points.
#Import "<std>" #Import "<mojo>" Using std.. Using mojo.. Class MyWindow Extends Window ' create our points Field numpoints:Int=20 Field point:Vec2i[] ' our lines around the set of points Field wrap:Stack<Vec2i> Method New() wrapit() End Method Method OnRender( canvas:Canvas ) Override App.RequestRender() ' Activate this method ' Draw the convex hull or gift wrap ' First the points canvas.Color=Color.White For Local i:Int=0 Until point.Length canvas.DrawRect(point[i].x,point[i].y,2,2) Next ' here the lines around the points For Local i:Int=1 Until wrap.Length canvas.DrawLine(wrap[i].x,wrap[i].y,wrap[i-1].x,wrap[i-1].y) Next canvas.DrawLine(wrap[0].x,wrap[0].y,wrap[wrap.Length-1].x,wrap[wrap.Length-1].y) ' ' if key escape then quit If Keyboard.KeyReleased(Key.Escape) Then App.Terminate() End Method ' ' This is the wrapping routine. It creates a set of points and then ' takes the left most point and then checks from the other points for ' the next points if there are no more points on the left side. Thus ' creating a set of lines around all the other points. ' Method wrapit() SeedRnd(Microsecs()) ' create our points point = New Vec2i[numpoints] For Local i:Int=0 Until numpoints point[i].x = Rnd(100,640-100) point[i].y = Rnd(100,480-100) Next ' create our stack for the wrap lines wrap = New Stack<Vec2i> ' find our left most point Local lp:Vec2i = New Vec2i(1000,1000) For Local i:Int=0 Until numpoints If point[i].x<lp.x Then lp.x = point[i].x ; lp.y=point[i].y Next ' create our end point Local endpoint:Vec2i = New Vec2i() Repeat ' push the current left most point to the stack wrap.Push(lp) ' get a new end point endpoint = point[0] ' find the next point with no more on the left of our current (lp)left point For Local i:Int=1 Until numpoints If ((lp=endpoint) Or (orientation(lp.x,lp.y,endpoint.x,endpoint.y,point[i].x,point[i].y)=-1)) endpoint = point[i] End If Next lp=endpoint Until Not(endpoint <> wrap.Get(0)) 'when at the start point then end End Method End Class Function Main() New AppInstance New MyWindow App.Run() End Function 'Return if a point is left or center or right side of a line. 'line a to b and point left(<0) or center(0) or right(>0) is c Function orientation:Int(ax:Int,ay:Int,bx:Int,by:Int,cx:Int,cy:Int) Return Sgn((bx-ax)(cy-ay)-(by-ay)(cx-ax)) End Function --v I noticed this algorithm on the coding train a while ago. I thought it might be to complex for me to understand or to recreate but I noticed a while ago I also created a function used by this algorith,. This one the orientation function returns if a point is left or center or right of a line. This algorithm here. The gift wrapping of convex hull create lines around a series of points, wrapping as to say. It starts at the left most point and checks other points on the orientation if it is on the left side thus wrapping it. I started creating it myself at first but failing later I looked at a javascript example and was able to fix the main for loop that does the magic. I think this one also is a bit like my point in polygon collision but this one creates the polygon around the point(s).

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Convex Hull or Gift Wrapping algorithm

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