Collision detection

[pollier]

Okay - I need collision detetion. I know what my formula does, but I'm not exactly sure of the equation.

Suppose you have two points, P1, and P2. Each has an X1, Y1, X2, and a Y2. Basically they're lines. I know if they're parallel or not, but if they aren't parallel, how can I find the point at which the two lines will interect? I just need the X and Y coordinates. Thanks.


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[kedaman]

Here's exactly what you need, i hope you understand the UDT's

Function ISCross(lne1 As Cline, lne2 As Cline, intersection As Coordinate) As Boolean
    Dim dx1!, dy1!, dx2!, dy2!, n!, s!
    dx1 = lne1.coord2.X - lne1.coord1.X
    dy1 = lne1.coord2.Y - lne1.coord1.Y
    dx1 = lne2.coord2.X - lne2.coord1.X
    dy2 = lne2.coord2.Y - lne2.coord1.Y
    If (dx2 * dy1 - dy2 * dx1) = 0 Then
        'they are parallell
        ISCross = False
        Exit Function
    End If
    
    n = (dx1 * (lne2.coord1.Y - lne1.coord1.Y) + dy1 * (lne1.coord1.X - lne2.coord1.X)) / (dx2 * dy1 - dy2 * dx1)
    t = (dx2 * (lne1.coord1.Y - lne2.coord1.Y) + dy2 * (lne2.coord1.X - lne1.coord1.X)) / (dy2 * dx1 - dx2 * dy1)
    ISCross = s >= 0 And s <= 1 And n >= 0 And n <= 1
    'the intersection is then:
    If ISCross Then intersection.X = X1 + t * dx: intersection.Y = Y1 + t * dy
End Function

[HarryW]

Try this:

Function ISCross(lne1 As Cline, lne2 As Cline, intersection As Coordinate) As Boolean
    Dim dx1!, dy1!, dx2!, dy2!, n!, s!
    dx1 = lne1.coord2.X - lne1.coord1.X
    dy1 = lne1.coord2.Y - lne1.coord1.Y
    dx2 = lne2.coord2.X - lne2.coord1.X
    dy2 = lne2.coord2.Y - lne2.coord1.Y
    If (dx2 * dy1 - dy2 * dx1) = 0 Then
        'they are parallell
        ISCross = False
        Exit Function
    End If
    
    n = (dx1 * (lne2.coord1.Y - lne1.coord1.Y) + dy1 * (lne1.coord1.X - lne2.coord1.X)) / (dx2 * dy1 - dy2 * dx1)
    s = (dx2 * (lne1.coord1.Y - lne2.coord1.Y) + dy2 * (lne2.coord1.X - lne1.coord1.X)) / (dy2 * dx1 - dx2 * dy1)
    ISCross = s >= 0 And s <= 1 And n >= 0 And n <= 1
    'the intersection is then:
    If ISCross Then intersection.X = X1 + s * dx: intersection.Y = Y1 + s * dy
End Function

Just a couple of things that look like they're wrong chaged. Just typos.

[kedaman]

Sorry about that.. Maybe got tired

[HarryW]

Yeah that was probably it

[Jotaf98]

Hum...I think I once saw a function like that, but it consisted of an If with 4 conditions (not sure if this works). The guy who did it said it was the fastest method. It needed the type Rect (hope you know what I'm talking about!).

public function RectInRect(rRect1 as rect, rect2 as rect) as boolean

if rect1.top < rect2.top and _
rect1.left < rect2.left and _
rect1.bottom > rect2.bottom and _
rect1.right > rect3.right then RectInRect = True

end function

Of course, I'm not sure if it works. Give it a try.

[pollier]

Thanks, but it's still not totally corrected...
There are still some different variables, for example X1 suddenly pops up in the last line, and so does Y1, dx, and dy, and none of these have been used in an earlier equation.

[YoungBuck]

I think this is the function Jotaf is talking about

Declare Function IntersectRect Lib "user32" Alias "IntersectRect" (lpDestRect As RECT, lpSrc1Rect As RECT, lpSrc2Rect As RECT) As Long

lpDestRect = Buffer to return the coordinates of the actual area of which lpSrc1Rect and lpSrc2Rect intersect, most likely not needed for what you are looking for
lpSrc1Rect and lpSrc2Rect = The rectangles which define the two images you want detect if they collide

You will have to run a loop to test all Images on the screen to see if they intersect.

[kedaman]

Sorry about that (again)
last line should be
If ISCross Then intersection.X = lne1.coord2.X + s * dx1: intersection.Y = lne1.coord2.Y + s * dy1

[pollier]

Thanks, I have yet to test it, and I'll let you know if it works.
I am not using the IntersectRect stuff because the things I am intersecting are not rectangular. So you don't need to tell me the rectangle things.

[Jotaf98]

You're right, you're not using rectangular objects, you're using square objects (joke)

YoungBuck, thanks for the function! But I once saw it without the DLL... Wait, I now remember it:

If r1.Left > r2.Right And _
   r1.Left < r2.Right And _
   r1.Top > r2.Bottom And _
   r1.Top < r2.Bottom Then
       'Return True
End If

This is the exact code, give it a try! r1 is the first rect, r2 is the second one. If you want, I can explain the RECT type too, it's not very complex

[Jotaf98]

Oops, sometimes I'm soooo stupid: you said you were using lines and I completely forgot it, this one is for testing if an object is on top of the other one

[Jotaf98]

Just another thing, and sorry for posting this much:
I'm just curious about where in agame intercecting 2 lines is needed...

[Xero]

I have another question about collision detection. I'm using IntersectRect for collision detection in a tile-based environment. Its using an array Map(#,#) = 0 or 1, 0 being water, 1 being grass. And i need to have it detect when the player is on a water(0) tile and not allow them to move over it. uhh.... help!

[Jotaf98]

That one's easy!

You have the player's coordinates: playerx and playery.

When the player presses the key to move left, use this piece of code:

If Map(playerx - 1, playery) = tileGrass Then
    playerx = playerx - 1
End If

Same thing for right...

If Map(playerx + 1, playery) = tileGrass Then
    playerx = playerx + 1
End If

This one's for moving up:

If Map(playerx, playery - 1) = tileGrass Then
    playery = playery - 1
End If

And this one's for moving down:

If Map(playerx, playery + 1) = tileGrass Then
    playery = playery + 1
End If

That should do it! All you need now is to draw the player at the new coordinates!

[kedaman]

'your detection of keys goes here, and they should only set the offsets.
if map(x+xoffset,y+yoffset)>0 then
 x=x+xoffset
 y=y+yoffset
end if

[Xero]

aaaah... mucho coolness. Thanks!