lagrange interpolation - GameCreators Forum

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nz0

AGK Developer

16

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Joined: 13th Jun 2007

Location: Cheshire,UK

Posted: 5th Apr 2013 02:26 Edited at: 5th Apr 2013 02:28

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I finally managed to implement one of the many spline movements I was looking into and lagrange is the one I think will work best for me. However, I have a problem with reversing the prediction.

The lagrange works on a set of waypoints (my use of this is waypoints) but when I want to reverse the system, it "bounces" rather than reverses the wave.

I thought it would be easy to make a variation for a reverse case, but it has stumped me. I'm sure others will benefit from this great interpolation system, but I really need it to be reversible... can anyone help?

Here's an implementation:

I set up an array of co-ordinates which works ok until I need to return in the other direction.

SetVirtualResolution (960,640)

global TestSprite as Integer

global x# as float

type vector2
 x# as float
 y# as float
endtype

global dim path[10] as vector2

TestSprite=createSprite(0)
setSpriteScale(TestSprite,4,4)

a=createSprite(0)
SetSpriteColor(a,255,0,0,255)
setSpriteScale(a,2,2)
SetSpritePositionByOffset(a,100,160)
path[0].x#=100
path[0].y#=160
a=cloneSprite(a)
SetSpritePositionByOffset(a,200,170)
path[1].x#=200
path[1].y#=170
a=cloneSprite(a)
SetSpritePositionByOffset(a,300,180)
path[2].x#=300
path[2].y#=180
a=cloneSprite(a)
SetSpritePositionByOffset(a,400,130)
path[3].x#=400
path[3].y#=130
a=cloneSprite(a)
SetSpritePositionByOffset(a,500,220)
path[4].x#=500
path[4].y#=220
a=cloneSprite(a)
SetSpritePositionByOffset(a,600,210)
path[5].x#=600
path[5].y#=210
a=cloneSprite(a)
SetSpritePositionByOffset(a,700,230)
path[6].x#=700
path[6].y#=230

x#=100.0
xdir#=3.3

do
 print("x="+str(GetSpriteX(TestSprite)))
 print("y="+str(GetSpritey(TestSprite)))

if xdir#<0
  y#=lagrangerev(x#)
 else
  y#=lagrange(x#)
 endif

SetSpritePositionByOffset(TestSprite,x#,y#)
 x#=x#+xdir#

if x#>700.0 then xdir#=xdir#*-1.0
 if x#<100.0 then xdir#=xdir#*-1.0

Sync()

loop

end

function lagrange(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+t#*path[j].y#
 next j
endfunction y0#

function lagrangerev(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+(t#*path[j].y#)
 next j
endfunction y0#

+ Code Snippet


SetVirtualResolution (960,640)


global TestSprite as Integer

global x# as float

type vector2
 x# as float
 y# as float
endtype

global dim path[10] as vector2

TestSprite=createSprite(0)
setSpriteScale(TestSprite,4,4)

a=createSprite(0)
SetSpriteColor(a,255,0,0,255)
setSpriteScale(a,2,2)
SetSpritePositionByOffset(a,100,160)
path[0].x#=100
path[0].y#=160
a=cloneSprite(a)
SetSpritePositionByOffset(a,200,170)
path[1].x#=200
path[1].y#=170
a=cloneSprite(a)
SetSpritePositionByOffset(a,300,180)
path[2].x#=300
path[2].y#=180
a=cloneSprite(a)
SetSpritePositionByOffset(a,400,130)
path[3].x#=400
path[3].y#=130
a=cloneSprite(a)
SetSpritePositionByOffset(a,500,220)
path[4].x#=500
path[4].y#=220
a=cloneSprite(a)
SetSpritePositionByOffset(a,600,210)
path[5].x#=600
path[5].y#=210
a=cloneSprite(a)
SetSpritePositionByOffset(a,700,230)
path[6].x#=700
path[6].y#=230


x#=100.0
xdir#=3.3

do
 print("x="+str(GetSpriteX(TestSprite)))
 print("y="+str(GetSpritey(TestSprite)))

 if xdir#<0
  y#=lagrangerev(x#)
 else
  y#=lagrange(x#)
 endif

 SetSpritePositionByOffset(TestSprite,x#,y#)
 x#=x#+xdir#

 if x#>700.0 then xdir#=xdir#*-1.0
 if x#<100.0 then xdir#=xdir#*-1.0

 Sync()

loop

end

function lagrange(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+t#*path[j].y#
 next j
endfunction y0#

function lagrangerev(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+(t#*path[j].y#)
 next j
endfunction y0#

You can see I set up another version of the function to test reverse scenarios

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Markus

Valued Member

20

Years of Service

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Joined: 10th Apr 2004

Location: Germany

Posted: 5th Apr 2013 09:48

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you can better see what happens if you paint the positions:

SetVirtualResolution (960,640)

global TestSprite as Integer

global x# as float

type vector2
 x# as float
 y# as float
endtype

global dim path[10] as vector2

TestSprite=createSprite(0)
setSpriteScale(TestSprite,1,1)

a=createSprite(0)
SetSpriteColor(a,255,255,255,255)
setSpriteScale(a,2,2)
SetSpritePositionByOffset(a,100,160)
path[0].x#=100
path[0].y#=160
a=cloneSprite(a)
SetSpritePositionByOffset(a,200,170)
path[1].x#=200
path[1].y#=170
a=cloneSprite(a)
SetSpritePositionByOffset(a,300,180)
path[2].x#=300
path[2].y#=180
a=cloneSprite(a)
SetSpritePositionByOffset(a,400,130)
path[3].x#=400
path[3].y#=130
a=cloneSprite(a)
SetSpritePositionByOffset(a,500,220)
path[4].x#=500
path[4].y#=220
a=cloneSprite(a)
SetSpritePositionByOffset(a,600,210)
path[5].x#=600
path[5].y#=210
a=cloneSprite(a)
SetSpritePositionByOffset(a,700,230)
path[6].x#=700
path[6].y#=230

x#=100.0
xdir#=3.3

do

setspritecolor(TestSprite,0,255,0,192)
 for x#=100.0 to 700.0 step 10.0
  y#=lagrange(x#)
  SetSpritePositionByOffset(TestSprite,x#,y#)
  drawsprite(TestSprite)
 next

setspritecolor(TestSprite,255,0,0,192)
 for x#=700.0 to 100.0 step -10.0
  y#=lagrangerev(x#)
  SetSpritePositionByOffset(TestSprite,x#,y#)
  drawsprite(TestSprite)
 next

Sync()

loop

end

function lagrange(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+t#*path[j].y#
 next j
endfunction y0#

function lagrangerev(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+(t#*path[j].y#)
 next j
endfunction y0#

+ Code Snippet

SetVirtualResolution (960,640)

global TestSprite as Integer

global x# as float

type vector2
 x# as float
 y# as float
endtype

global dim path[10] as vector2

TestSprite=createSprite(0)
setSpriteScale(TestSprite,1,1)

a=createSprite(0)
SetSpriteColor(a,255,255,255,255)
setSpriteScale(a,2,2)
SetSpritePositionByOffset(a,100,160)
path[0].x#=100
path[0].y#=160
a=cloneSprite(a)
SetSpritePositionByOffset(a,200,170)
path[1].x#=200
path[1].y#=170
a=cloneSprite(a)
SetSpritePositionByOffset(a,300,180)
path[2].x#=300
path[2].y#=180
a=cloneSprite(a)
SetSpritePositionByOffset(a,400,130)
path[3].x#=400
path[3].y#=130
a=cloneSprite(a)
SetSpritePositionByOffset(a,500,220)
path[4].x#=500
path[4].y#=220
a=cloneSprite(a)
SetSpritePositionByOffset(a,600,210)
path[5].x#=600
path[5].y#=210
a=cloneSprite(a)
SetSpritePositionByOffset(a,700,230)
path[6].x#=700
path[6].y#=230


x#=100.0
xdir#=3.3

do

 setspritecolor(TestSprite,0,255,0,192)
 for x#=100.0 to 700.0 step 10.0
  y#=lagrange(x#)
  SetSpritePositionByOffset(TestSprite,x#,y#)
  drawsprite(TestSprite)
 next

 setspritecolor(TestSprite,255,0,0,192)
 for x#=700.0 to 100.0 step -10.0
  y#=lagrangerev(x#)
  SetSpritePositionByOffset(TestSprite,x#,y#)
  drawsprite(TestSprite)
 next

 Sync()

loop

end

function lagrange(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+t#*path[j].y#
 next j
endfunction y0#

function lagrangerev(x0#)
 y0#=0.0
 for j=0 to 6
  t#=1.0
  for i=0 to 6
   if i<>j then t#=t#*(x0#-path[i].x#)/(path[j].x#-path[i].x#)
  next i
  y0#=y0#+(t#*path[j].y#)
 next j
endfunction y0#

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Marl

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Joined: 19th Nov 2011

Location: Bradford, UK

Posted: 5th Apr 2013 21:37 Edited at: 5th Apr 2013 21:39

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Looks to work ok to me.

(offsetting the return journey by 100 so it doesn't hide the green)

+ Code Snippet

SetSpritePositionByOffset(TestSprite,x#,y# + 100)

or am I not understanding the problem. (or indeed did Markus fix the problem with the recode?)

Edit: wait are you wanting it to continue down as it comes back?

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nz0

AGK Developer

16

Years of Service

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Joined: 13th Jun 2007

Location: Cheshire,UK

Posted: 5th Apr 2013 21:48

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Yes, I think painting them misses the point.
Look at my first one again and see that the movement bounces back.
I would want the return journey to be a smooth interpolation, so when going through the last point, it should curve back up.

I think this might be impossible with lagrange now, as it uses the whole set of points, so the solution may be to replot the points for the reverse journey.

I would want to do this anyway if I was going to use it in pathing, but I think messing with the points would interfere with the flow as soon as you did it?

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nz0

AGK Developer

16

Years of Service

User Offline

Joined: 13th Jun 2007

Location: Cheshire,UK

Posted: 6th Apr 2013 01:58

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On further investigation, this isn't any good for what I want. The lagrange formula is useful if you *must* intersect the points, but it has a couple of fails:

can't use the same Y value on the set or it breaks
uses the whole set of X to plot the Y.

After messing with beziers, this seems to be better and more flexible (but harder to implement)

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Markus

Valued Member

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Joined: 10th Apr 2004

Location: Germany

Posted: 6th Apr 2013 11:06 Edited at: 6th Apr 2013 11:55

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maybe you can add a last point that the curve go flat.

here a bezier interpolation in agk example but the control points are different.

rem
rem AGK Application 1.08B9

rem Bezier Curve Example
rem MR 06.04.2013

SetDisplayAspect(4/3)
SetVirtualResolution(800,600)

type TVector3
    x as float
    y as float
    z as float
endtype
global VectorMax=6
global dim path[VectorMax] as TVector3 //vector list
global bezret as TVector3 //return from Bezier until AGK can handle types correct

global TestSprite as Integer
TestSprite=createSprite(0)
setSpriteScale(TestSprite,1,1)

path[0].x=100
path[0].y=300

path[1].x=200
path[1].y=300+50

path[2].x=300
path[2].y=300-100

path[3].x=400
path[3].y=300+50

path[4].x=500
path[4].y=300+200

path[5].x=600
path[5].y=300-200

path[6].x=700
path[6].y=300-50

do
 Print("Hello Bezier Curve")

setspritecolor(TestSprite,255,255,255,255)
 for i = 0 to VectorMax
  SetSpritePositionByOffset(TestSprite,path[i].x,path[i].y)
  drawsprite(TestSprite)
 next

setspritecolor(TestSprite,0,255,0,128)
 for mu#=0.0 to 1.0 step 0.01
  Bezier(mu#)
  SetSpritePositionByOffset(TestSprite,bezret.x,bezret.y)
  drawsprite(TestSprite)
 next

Sync()
loop
end

Function Bezier(mu as float)

//MR 06.04.2013

//General Bezier curve
 //mu 0.0 to 1.0

k as integer ,kn as integer,nn as integer,nkn as integer
 blend as float,muk as float,munk as float

px as float,py as float,pz as float

n=VectorMax //< global

bezret.x=0.0
   bezret.y=0.0
   bezret.z=0.0

muk = 1.0
   munk = pow(1.0-mu,IntegerToFloat(n))

For k=0 To n
      nn = n
      kn = k
      nkn = n - k
      blend = muk * munk
      muk =muk * mu
      munk = munk / (1.0-mu)
      While nn => 1
         blend=blend * nn
         nn=nn-1
         If kn > 1
            blend=blend / IntegerToFloat(kn)
            kn=kn-1
         EndIf
         If nkn > 1
            blend=blend / IntegerToFloat(nkn)
           nkn=nkn-1
         EndIf
      endwhile
      bezret.x=bezret.x+path[k].x * blend
      bezret.y=bezret.y+path[k].y * blend
      bezret.z=bezret.z+path[k].z * blend
   Next

EndFunction

function IntegerToFloat(i as integer)

f#=i
    exitfunction f#

endfunction f#

Function pow(a as float,b as float)

//C Like :-)

exitfunction a^b

EndFunction c#

+ Code Snippet

rem
rem AGK Application 1.08B9

rem Bezier Curve Example
rem MR 06.04.2013

SetDisplayAspect(4/3)
SetVirtualResolution(800,600)

type TVector3
    x as float
    y as float
    z as float
endtype
global VectorMax=6
global dim path[VectorMax] as TVector3 //vector list
global bezret as TVector3 //return from Bezier until AGK can handle types correct

global TestSprite as Integer
TestSprite=createSprite(0)
setSpriteScale(TestSprite,1,1)

path[0].x=100
path[0].y=300

path[1].x=200
path[1].y=300+50

path[2].x=300
path[2].y=300-100

path[3].x=400
path[3].y=300+50

path[4].x=500
path[4].y=300+200

path[5].x=600
path[5].y=300-200

path[6].x=700
path[6].y=300-50

do
 Print("Hello Bezier Curve")

 setspritecolor(TestSprite,255,255,255,255)
 for i = 0 to VectorMax
  SetSpritePositionByOffset(TestSprite,path[i].x,path[i].y)
  drawsprite(TestSprite)
 next

 setspritecolor(TestSprite,0,255,0,128)
 for mu#=0.0 to 1.0 step 0.01
  Bezier(mu#)
  SetSpritePositionByOffset(TestSprite,bezret.x,bezret.y)
  drawsprite(TestSprite)
 next

 Sync()
loop
end

Function Bezier(mu as float)

 //MR 06.04.2013

 //General Bezier curve
 //mu 0.0 to 1.0

 k as integer ,kn as integer,nn as integer,nkn as integer
 blend as float,muk as float,munk as float

 px as float,py as float,pz as float

 n=VectorMax //< global

   bezret.x=0.0
   bezret.y=0.0
   bezret.z=0.0

   muk = 1.0
   munk = pow(1.0-mu,IntegerToFloat(n))

   For k=0 To n
      nn = n
      kn = k
      nkn = n - k
      blend = muk * munk
      muk =muk * mu
      munk = munk / (1.0-mu)
      While nn => 1
         blend=blend * nn
         nn=nn-1
         If kn > 1
            blend=blend / IntegerToFloat(kn)
            kn=kn-1
         EndIf
         If nkn > 1
            blend=blend / IntegerToFloat(nkn)
           nkn=nkn-1
         EndIf
      endwhile
      bezret.x=bezret.x+path[k].x * blend
      bezret.y=bezret.y+path[k].y * blend
      bezret.z=bezret.z+path[k].z * blend
   Next

EndFunction

function IntegerToFloat(i as integer)

    f#=i
    exitfunction f#

endfunction f#

Function pow(a as float,b as float)

 //C Like :-)

 exitfunction a^b

EndFunction c#

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AppGameKit Classic Chat / lagrange interpolation

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