;Author - Matt Lloyd 2015
;
;Function - whenwillicollide#()
;
;Used to determine the time at which a moving object (straight line) will collide with another moving object (straight line)
;
;The example (see example function) lets the user move a little 'spaceship' around the map by clicking on the screen, the AI unit
;will 'chase' and shoot at the player when they have a likely shot....of course - if the user changes direction then it will miss! (I'm not
;a fortune teller!)
;
;
;Hopefully this comes in useful - I will find it useful anyway....
;
;test example of usage...
example()
;function definition..
Function whenwillicollide#(dimensions,posx#,posy#,posz#,velx#,vely#,velz#,targetposx#,targetposy#,targetposz#,targetvelx#,targetvely#,targetvelz#,mintime#,maxtime#,tolerance#=0.1)
;Parameters are:
;
;Dimensions is a value from 1 to 3....indicates the number of dimensions to use...
;For example - a 2d game would use '2', a 3d game would use '3', and I'm not sure why you would use less than 2 dimensions...but hey...
;
;
;Position of first entity x,y,z (floats)
;Velocity of first entity in x,y,z (floats)
;
;Position of second entity x,y,z (floats)
;Velocity of second entity in x,y,z (floats)
;
;Minimum time to accept (float)
;Maximum time to accept (float)
;
;Tolerance (float) - allowable difference between times in each coordinate system...optional value........
;
;Return values are:
;
;-1 if the vectors never intersect (either because they are skew lines or because they intersect outside of the allowable range or because they do not
;intersect on the same axis within the specified tolerance
;
;-2 if the mintime or maxtime are set incorrectly (ie less than zero or maxtime is less than mintime)
;
;-3 if the dimensions has an invalid value (must be either 2 or 3)
;
;
;Assumes two entities are NOT at the same location..ie not already collided!
;
Local intersectiontime#[2] ;array containing the intersection time of each coordinate....
;0 = x index, 1 = y index, 2 = z index
If(dimensions<2 Or dimensions>3) Then Return -3
If(mintime<0 Or maxtime<0 Or maxtime<mintime) Then Return -2
If(targetvelx - velx)=0 Then Return -1
intersectiontime[0] = (posx - targetposx) / (targetvelx - velx)
If(intersectiontime[0]<mintime Or intersectiontime[0]>maxtime) Then Return -1
If(targetvely - vely)=0 Then Return -1
intersectiontime[1] = (posy - targetposy) / (targetvely - vely)
If(intersectiontime[1]<mintime Or intersectiontime[1]>maxtime) Then Return -1
If (Abs(intersectiontime[1]-intersectiontime[0]))>tolerance Then Return -1
If(dimensions=2) Then Return (intersectiontime[0]+intersectiontime[1])*0.5
If(targetvelz - velz)=0 Then Return -1
intersectiontime[2] = (posz - targetposz) / (targetvelz - velz)
If(intersectiontime[2]<mintime Or intersectiontime[2]>maxtime) Then Return -1
If (Abs(intersectiontime[2]-intersectiontime[1]))>tolerance Then Return -1
If (Abs(intersectiontime[2]-intersectiontime[0]))>tolerance Then Return -1
Return (intersectiontime[0]+intersectiontime[1]+intersectiontime[2])*0.33333
End Function
Function example()
;
;
;2 dimensional example....
Local maxvel = 2.0,maxbulletvel = 4.0
Local playerX#,playerY#,playervelX#,playervelY#,playeraccelX#,playeraccelY#
Local enemyX#,enemyY#,enemyvelX#,enemyvelY#,enemyaccelX#,enemyaccelY#
Local enemybulletX#,enemybulletY#,enemybulletvelX#,enemybulletvelY#,enemybulletlife
Local startedchasing=0
Graphics 512,512,0,2
playerx = Rnd(256)+128
playery = Rnd(256)+128
playervelx = 0
playervely = 0
playeraccelx = 0
playeraccely = 0
enemyx = Rnd(256)+128
enemyy = Rnd(256)+128
enemybulletx = -1
enemybullety = -1
enemybulletlife = 0
SetBuffer BackBuffer()
Repeat
playerx = playerx + playervelx
playery = playery + playervely
playervelx = playervelx + playeraccelx
playervely = playervely + playeraccely
If(playervelx*playervelx+playervely*playervely)>maxvel*maxvel Then
playervelx = playervelx * maxvel/Sqr((playervelx*playervelx+playervely*playervely))
playervely = playervely * maxvel/Sqr((playervelx*playervelx+playervely*playervely))
EndIf
enemyx = enemyx + enemyvelx
enemyy = enemyy + enemyvely
enemyvelx = enemyvelx + enemyaccelx
enemyvely = enemyvely + enemyaccely
If(enemyvelx*enemyvelx+enemyvely*enemyvely)>maxvel*maxvel Then
enemyvelx = enemyvelx * maxvel/Sqr((enemyvelx*enemyvelx+enemyvely*enemyvely))
enemyvely = enemyvely * maxvel/Sqr((enemyvelx*enemyvelx+enemyvely*enemyvely))
EndIf
;move the bullet....
If(enemybulletlife>0) Then
enemybulletx = enemybulletx + enemybulletvelx
enemybullety = enemybullety + enemybulletvely
enemybulletlife = enemybulletlife - 1
EndIf
If(MouseDown(1)) Then ;player provides thrust towards point pressed by mouse....
dx# = MouseX() - playerx
dy# = MouseY() - playery
length# = dx*dx+dy*dy
If(length>0)
length = Sqr(length)
dx = 0.65*dx/length
dy = 0.65*dy/length
playeraccelx = dx
playeraccely = dy
startedchasing = 1
EndIf
EndIf
;enemy tracks player....and moves around chasing them......(after the player starts moving)
If(startedchasing=1) Then
dx# = playerx - enemyx
dy# = playery - enemyy
length# = dx*dx+dy*dy
If(length>0)
length = Sqr(length)
dx = 0.55*dx/length
dy = 0.55*dy/length
enemyaccelx = dx
enemyaccely = dy
startedchasing = 1
EndIf
If(enemybulletlife<=0) Then
;;;see if we should fire a bullet.....
;bullet starts from enemy pos
enemybulletx = enemyx
enemybullety = enemyy
;bullet starts with same velocity as enemy (ie good old vector addition - assuming non relativistic speeds!
enemybulletvelx = enemyvelx
enemybulletvely = enemyvely
dx# = playerx - enemybulletx
dy# = playery - enemybullety
length# = dx*dx+dy*dy
If(length>0)
length = Sqr(length)
dx = dx / length
dy = dy / length
;bullet velocity will be faster than ships - it will be..twice as fast as the maxvel...
enemybulletvelx = enemybulletvelx + dx
enemybulletvely = enemybulletvely + dy
If(enemybulletvelx*enemybulletvelx+enemybulletvely*enemybulletvely)>(maxbulletvel)*(maxbulletvel) Then
enemybulletvelx = enemybulletvelx * (maxbulletvel) / Sqr((enemybulletvelx*enemybulletvelx+enemybulletvely*enemybulletvely))
enemybulletvely = enemybulletvely * (maxbulletvel) / Sqr((enemybulletvelx*enemybulletvelx+enemybulletvely*enemybulletvely))
EndIf
;okay but will we hit our target? if so then we want to fire...
;now....the life of the bullet we will say is...60 frames....just an arbitrary number...can be anything....but most games I've written usually have a maximum 'lifetime' of the bullet (especially useful if it continues to fly off into deep space!)
;this is where I call the function above - if it returns a non zero value then I want to fire!
enemybulletlife = whenwillicollide(2,enemybulletx,enemybullety,0,enemybulletvelx,enemybulletvely,0,playerx,playery,0,playervelx,playervely,0,0,300,(maxbulletvel+maxvel+2))
If(enemybulletlife<0)
enemybulletlife = 0
Else
;enemybulletlife = 60 ;(uncomment this and the bullet will only travel until it reaches where it should have hit the player at...)
EndIf
EndIf
EndIf
EndIf
Cls
Color 0,255,0
Rect playerx-1,playery-1,3,3,1
Color 255,0,255
Rect enemyx-1,enemyy-1,3,3,1
Color 255,255,0
If(enemybulletlife>0) Then
Rect enemybulletx-1,enemybullety-1,3,3,1
EndIf
Flip
Until KeyDown(1)
End
End Function
