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 [Tutorial] Collision Detection
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Dan




PostPosted: Tue Sep 24, 2002 3:06 pm   Post subject: [Tutorial] Collision Detection

Collision Detection

Ok I am show you how to use a common method of collision detection which is some times called rectangle collision detection. What it dose is uses if statements to make a theoretical rectangles arrowed the objects and then dose some tings if the rectangles hit.

Well here is an example of how to do this. This example uses two ovals which we wont to bounce off everything. (sorry for the bad spelling if I get time I will spell check it Embarassed )


code:

var x, y : int %var for the 1st ovals quadents
var x2, y2 : int %var for the 2nd ovals quadents
var nx, ny, nx2, ny2 : int := 1 %vars for the chage in qroadents
var s : int := 10 %var for the size of the ovals

x := Rand.Int (10, 100) %set the x quadent for the 1st oval
y := Rand.Int (10, 100) %set the y quadent for the 1st oval
x2 := Rand.Int (10, 100) %set the x quadent for the 2nd oval
y2 := Rand.Int (10, 100) %set the y quadent for the 2nd oval

%main loop
loop
    Draw.Oval (x, y, s, s, red) %draws the 1st oval
    Draw.Oval (x2, y2, s, s, green) %draws the 2nd one

    %check to see if oval gose off the screen
    %for oval 1
   
    if x <= 0 then %check to see if it gose off the bottom
        nx := 1 %chages the direction
    elsif y <= 0 then %check to see if it gose off to the left
        ny := 1 %chages the direction
    end if

    if x >= maxx then %check to see if it gose off the top
        nx := - 1 %chages the direction
    elsif y >= maxy then %check to see if it gose off to the right
        ny := - 1 %chages the direction
    end if


    %check to see if oval gose off the screen
    %for oval 2
   
    if x2 <= 0 then %check to see if it gose off the bottom
        nx2 := 1 %chages the direction
    elsif y2 <= 0 then %check to see if it gose off to the left
        ny2 := 1 %chages the direction
    end if

    if x2 >= maxx then %check to see if it gose off the top
        nx2 := - 1 %chages the direction
    elsif y2 >= maxy then %check to see if it gose off to the right
        ny2 := - 1 %chages the direction
    end if



    %this if checks to see if the ovals have hit each other
    if x + s > x2 - s and x - s < x2 + s and y + s > y2 - s and y - s < y2 +
            s then
       
        put "the balls hit"
        delay (500)

        %chages the direction of all the ovals
        nx := nx * - 1
        ny := ny * - 1
        nx2 := nx2 * - 1
        ny2 := ny2 * - 1
   
    end if

    %this part adds the chage in qroadents to the
    %quradents and makes the ovals move
    x += nx
    y += ny
    x2 += nx2
    y2 += ny2

    delay (10)
    cls
end loop


Also i think tony has some stuff to add to this about other methods of doing this.

P.S. if there is anything you dont understand (or cant read becuse of my spelling) plz feal free to post and let us know. Wink
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Tony




PostPosted: Wed Sep 25, 2002 12:28 pm   Post subject: (No subject)

[tutorial] Oval Collision Detection

Well Dan explained how to use rectangle collision Detection... sord off...

Anyway, that is good enough for some cases, but in sertain games oval detection might prove better. Especially if you deal with circles Wink

The idea behind it is to find the distance between the centers of the circles and see if its less then sum of radiuses. If so, they have collided.

To make a collision Detection function, you need to know pythagorian theorem (C^2 = A^2 + B^2)

Here's how you use it:

code:

distance = ((x2-x1)*(x2-x1) + (y2-y1)*(y2-y1))**0.5
%you take the square of differences of X coordinates and add
%a square of differences of Y coordinates then take a square
%root of the sum
%%%%%%
%I did (x)*(x) instead of x**2 because exponents is a function
%and it takes up more resourses then *, thus its faster
%Also **0.5 is the same as sqrt() - learn your math guys

if distance < radious1 + radious2
%might be a good idea to save radious1+radious2 as const/var
then
collision = true
end if


I hope this little tutorial helped. If something is still unclear, or you want to know how to do other types of detections, please post.
Latest from compsci.ca/blog: Tony's programming blog. DWITE - a programming contest.
R.W.Z




PostPosted: Fri Oct 04, 2002 5:05 pm   Post subject: Try This

This would be a good tutorial but I didn't label much but this is almost the same thing as the other one but with this you can change the number of bouncing balls by changing BALLCOUNT but only the first two balls say if they collided with each other.

code:
const BALLCOUNT : int := 2
var ballX : array 1 .. BALLCOUNT of int
var ballY : array 1 .. BALLCOUNT of int
var directionX : array 1 .. BALLCOUNT of int
var directionY : array 1 .. BALLCOUNT of int

const RADIUS : int := 20

% Set random values for the x and y of each ball also the
% x and y direction for each ball
for i : 1 .. BALLCOUNT
    randint (ballX (i), RADIUS * 2, maxx - RADIUS * 2)
    randint (ballY (i), RADIUS * 2, maxy - RADIUS * 2)
    loop
        randint (directionX (i), -3, 3)
        randint (directionY (i), -3, 3)
        exit when directionX (i) not= 0 and directionY (i) not= 0
    end loop
end for

loop
    for i : 1 .. BALLCOUNT
        drawfilloval (ballX (i), ballY (i), RADIUS, RADIUS, 0)
        ballX (i) := ballX (i) + directionX (i)
        ballY (i) := ballY (i) + directionY (i)
        if ballX (i) < RADIUS or ballX (i) > maxx - RADIUS then
            directionX (i) := -directionX (i)
        end if
        if ballY (i) < RADIUS or ballY (i) > maxy - RADIUS then
            directionY (i) := -directionY (i)
        end if
        drawfilloval (ballX (i), ballY (i), RADIUS, RADIUS, 12)
    end for
    if ballX (1) + RADIUS > ballX (2) - RADIUS and ballX (1) - RADIUS < ballX (2) + RADIUS and ballY (1) + RADIUS > ballY (2) - RADIUS and ballY (1) - RADIUS < ballY (2) + RADIUS then
        locatexy (maxx div 2, maxy div 2)
        put "Balls Hit"
        delay(100)
        cls
    end if
    delay (10)
end loop


R.W.Z
Martin was here. Use code tags
JayLo




PostPosted: Tue Nov 26, 2002 11:46 pm   Post subject: for the rectangle collision...

I'm trying out a pool program... thanks for the rectangle collision program... However, I would like to make the collision more realistic as in a "real" pool bounce... I tried out the oval collision, but it seems as if the program only tells when it hits! I would like to find out if the balls can realistically bounce off each other in a real way. Please post any sort of help.
Tony




PostPosted: Wed Nov 27, 2002 12:35 am   Post subject: (No subject)

ya, you can do that... you need to apply some physics and trig for it though... you should have no problem with grade11 knowledge of math and physics.

I will shortly post a new tutorial on this topic. Check out tutorial section for [trig]
Latest from compsci.ca/blog: Tony's programming blog. DWITE - a programming contest.
void




PostPosted: Mon Apr 07, 2003 8:00 pm   Post subject: collison detection

when i click on a certain spot i want it to check if there is something within a certain radius of the point, say i click on (x,y)...i want to check if there is something within a 40 pixel radius of the spot Confused ...any ideas are welcome....can u try to explain it to me because i dont wanna just see code and copy it (im lazy Embarassed so if its there, i'll copy it Twisted Evil ), if you hafta tho ..i dont mind... Very Happy thanks alot.....this site is really helpful...and its a good oppurtunity for me to test my own skills by debuggin and helping others and receive help from you crazzy programmmers.......if i had a job i might give you pplz money for such an amazing site...its one of the very few on the net that have a point to them...alongside google and shockwave.com Wink GOOD JOB!!!! i know its commentss like this that make your day j/k Razz
im out...pz
Blade




PostPosted: Mon Apr 07, 2003 8:05 pm   Post subject: (No subject)

use an if statement... lets say if you have a ball.... xy will be the mouse... so you make the forign object enter the radius of the mouse click.... then use an if statement to see if its there.. .such as
if forignobject < xy of ball + 40 then
the object has entered the radius

understand?
void




PostPosted: Mon Apr 07, 2003 8:18 pm   Post subject: (No subject)

thanks alot ....i think i get it...but the thing is...what if its at like a 45 degree angle from the center (using a horizontal plane as 0 degreees and 180)...then how do you check it? lets say its a line headed at a circle at a 45 degree angle from the center of the circle....how would you check that?...because the xy only check the horizontal and vertical values does it not? the diagonals are completely ignored
Blade




PostPosted: Mon Apr 07, 2003 8:38 pm   Post subject: (No subject)

what happens if you add 40 to x, and y, then find the distance between the both then get the midpoint from that.... would that work?
Catalyst




PostPosted: Tue Apr 08, 2003 6:08 am   Post subject: (No subject)

use this function to find out if its in the circle

code:
function distance (x1, y1, x2, y2 : real) : real
    result ((x1 - x2) ** 2 + (y1 - y2) ** 2) ** 0.5
end distance
void




PostPosted: Tue Apr 08, 2003 6:45 am   Post subject: (No subject)

thanks man...that helped alot...im just wondering how come everyone uses **0.5 instead of sqrt...is it betta proggramming? or is it just kinda what ur used ta doing?
Catalyst




PostPosted: Tue Apr 08, 2003 7:25 am   Post subject: (No subject)

might me a little faster since its not a function like sqrt, but i mainly just use by habit and that i can do cube roots and such by doing **0.3333,etc.
void




PostPosted: Tue Apr 08, 2003 6:33 pm   Post subject: (No subject)

code:

var x, y, button : int

function distance (x1, y1, x2, y2 : real) : real
    result ( (x1 - x2) ** 2 - ( y1 - y1) ** 2) ** 0.5
end distance
loop
    Mouse.Where (x, y, button)
    locate (13, 13)
    put distance (6, 4, x, y)
    delay (5)
end loop

okay....heres what im attempting to do....i wanna see what the distance is....by radius from the mouse and the point that i randomly set in there.....but the thing is....i get a value of 0 (which should mean im directly over the point) far too many...how many different (6,4)'s can there be.....i've tried switching the (x,y) to x1 and y1 positions istead of their current x2,y2 possition...but the result is still the same..... Evil or Very Mad
Catalyst




PostPosted: Tue Apr 08, 2003 9:14 pm   Post subject: (No subject)

u messed the code up..

Quote:

result ( (x1 - x2) ** 2 - ( y1 - y1) ** 2) ** 0.5


should be


Quote:

result ( (x1 - x2) ** 2 - ( y1 - y2 )** 2) ** 0.5
void




PostPosted: Wed Apr 09, 2003 6:56 am   Post subject: (No subject)

hehehehehehe Embarassed ooopsssiiies......dont i feel like an idiot.... Laughing
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