Collision Detection

[Mr. T]

Given the following code, can you show me how to do collision detection with the 1) whatdotcolour method 2) regular method.
And can you explain why the ball looks so jittery.

code:

View.Set ("offscreenonly") const RADIUS : int := 5 const NUM_BALLS : int := 1 var x, y, dx, dy, clr : array 1 .. NUM_BALLS of int  for i : 1 .. NUM_BALLS             x (i) := Rand.Int (RADIUS, maxx - RADIUS)             y (i) := Rand.Int (RADIUS, maxy - RADIUS)             dx (i) := Rand.Int (-3, 3)             dy (i) := Rand.Int (-3, 3)             clr (i) := Rand.Int (1, 15)         end for loop     for i : 1 .. NUM_BALLS         if x (i) + dx (i) < RADIUS or                 x (i) + dx (i) > maxx - RADIUS then             dx (i) := -dx (i)         end if         if y (i) + dy (i) < RADIUS or                 y (i) + dy (i) > maxy - RADIUS then             dy (i) := -dy (i)         end if         x (i) := x (i) + dx (i)         y (i) := y (i) + dy (i)         Draw.FillOval (x (i), y (i), RADIUS, RADIUS, red)         Draw.FillBox (300, 200, 200, 300, blue)     end for         View.Update     cls end loop

[Cervantes]

1) Don't go there. There are two reasons I say this. First, it's complicated for this sort of thing. Ideally, you would check a grid of pixels in front of each ball with a width equal to that of the ball and a depth equal to the speed (vector addition of x and y components of velocity) of the ball. This is complicated by the fact that the ball can be moving in 24 different directions.

2) Use the distance formula.

code:

if Math.Distance (  x (j), y (j), x (k), y (k)  ) < radius (j) + radius (k) then     %Handle Collision end if

(This is assuming you are checking for collisions between different, not with that blue square you've got in the middle (?) )

On that note, the blue box drawing method should be outside the for loop. There's no point in drawing a box multiple times. (Yes, your for loop only executes once, but that can be changed easily enough.)

Lastly, it doesn't look jittery to me. It actually runs very fast, as there is no delay.

If you are looking to do collisions with the box, use some if statements.

code:

if x (i) + RADIUS >= box_x1 and y (i) + RADIUS >= box_y1 and x (i) - RADIUS <= box_x2 and y (i) - RADIUS <= box_y2 then     % collision end if

[Mr. T]

Cervantes wrote:

(This is assuming you are checking for collisions between different, not with that blue square you've got in the middle (?) )

leave something out

[Cervantes]

Oh my gosh! Thank you for catching that! +allmybits

[Mr. T]

i was being serious...whatd u wanna say?

[Cervantes]

Between different balls. Hence the circular collision detection approach.

[Mr. T]

for this method can you break down what each part represents

code:

if Math.Distance (  x (j), y (j), x (k), y (k)  ) < radius (j) + radius (k) then     %Handle Collision end if

for this method, do i havta break up each part into its own if statement? because each side of the box represents a different collision outcome.

code:

if x (i) + RADIUS >= box_x1 and y (i) + RADIUS >= box_y1 and x (i) - RADIUS <= box_x2 and y (i) - RADIUS <= box_y2 then     % collision end if

[Cervantes]

1) If the distance between the centre's of two balls is less than the sum of their radii, a collision occurs.

2) Sort of. You need to split up the x and the y conditions. Hitting the box from the left or right both result in reversing the x velocity. (vx *= -1) Same goes for hitting the box from the top or bottom.

[Mr. T]

but if i break it up like this

code:

if x (i) + dx(i) = 300  then         dx (i) := -dx (i)         end if

it covers x = 300 on the entire y axis

[EDIT]
ok, i figured out. you just add restrictions to the what y should equal

code:

if x (i) + dx(i) = 300 and y(i)>=200 and y(i)<=300 then         dx (i) := -dx (i)         end if

[Cervantes]

Pwned wrote:

code:

if x (i) + dx(i) = 300 and y(i)>=200 and y(i)<=300 then         dx (i) := -dx (i)         end if

Don't use equals. Use >= or <=. Reason being that the ball is moving at 3 pixels/loop iteration. Thus, at one time it's at 298, the next it's at 301, and the condition has never been met, though it should have.

code:

if x (i) + dx (i) >= 200 and x (i) + dx (i) <= 300 and y (i) >= 200 and y (i) <= 300 then       dx (i) *= -1 end if %Similarly for the y.

[Mr. T]

could you explain what the * is supposed to represent?

[Mr. T]

Ok, i am now gonna implement wat I have learned here into my jezzball game. I wanna avoid using processes, so do you have any idea how I can make the movement of the balls and the creation of the walls run in unison?

[Cervantes]

Put everything on a counter. Or a timer. Really, I don't know how you COULD put these things into processes!

dx *= -1
is the same as
dx := dx * -1

Just as
x += 1
is the same as
x := x + 1

[Mr. T]

Sorry, maybe I was being unclear. Here is my old version of JezzBall.
http://www.compsci.ca/v2/viewtopic.php?t=9749&postdays=0&postorder=asc&start=0
I wanna basically do the same thing, minus the process.

[Cervantes]

Here's a small example, illustrating how to move and create pseudo-objects simultaneously, without the use of processes.

Turing:

View.Set ("offscreenonly") var counter := 0 type obj :     record         x, y, vx, vy : real         radius : int     end record var ball : flexible array 1 .. 0 of obj fcn Init_Obj : obj     var object : obj     object.radius := Rand.Int (3, 10)     object.x := Rand.Int (object.radius, maxx - object.radius)     object.y := Rand.Int (object.radius, maxy - object.radius)     object.vx := Rand.Int (-3, 3)     object.vy := Rand.Int (-3, 3)     result object end Init_Obj proc Create_New_Ball     new ball, upper (ball) + 1     ball (upper (ball)) := Init_Obj end Create_New_Ball loop     counter := (counter + 1) mod 100     if counter = 0 then         Create_New_Ball     end if     for i : lower (ball) .. upper (ball)         if ball (i).x <= ball (i).radius | ball (i).x >= maxx - ball (i).radius then             ball (i).vx *= -1         end if         if ball (i).y <= ball (i).radius | ball (i).y >= maxy - ball (i).radius then             ball (i).vy *= -1         end if         ball (i).x += ball (i).vx         ball (i).y += ball (i).vy     end for     cls     for i : lower (ball) .. upper (ball)         Draw.FillOval (round (ball (i).x), round (ball (i).y), ball (i).radius, ball (i).radius, black)     end for     View.Update     delay (10) end loop