----------------------------------- Cervantes Sun May 30, 2004 5:57 pm [Tutorial]Circular Collision Data ----------------------------------- One of the greatest challanges I have faced in my programming life is getting two circles to bounce off each other as they would in real life: obeying the laws of physics. There have been many attempts at solving this riddle. Cervantes' Approach A while back tony taught me how to create gravity fields. When you have a gravity field (a spot that sucks whatever objects towards it) it is easy to reverse its effect, creating a repulsing field. :wink: Today I had a revelation: this could be used as a form of circular collision. I was using it in a slime volleyball setting, so I'm going to be talking in terms of players and balls. If the ball has collided with the player (determine using the distance formula) then make the player a repulsing gravity field. Also, have a maximum speed for the ball's velocity, so as to not let it get out of control. setscreen ("offscreenonly") var player : record x, y, vy, GFstrength, GFdist, GFforce, GFangle : real %GF = Gravity Field -- Collisions between ball and players in this radius, score : int %uses repulsing gravity fields jump : boolean end record player.x := maxx div 2 player.y := maxy div 2 player.vy := 0 player.GFstrength := 3 player.radius := 40 player.score := 0 player.jump := false var ball : record x, y, vx, vy : real radius : int end record ball.x := 100 ball.y := 100 ball.vx := 0 ball.vy := 0 ball.radius := 12 var keys : array char of boolean const max_speed := 7 loop ball.x += ball.vx ball.y += ball.vy if ball.x = maxx - ball.radius then ball.vx *= -1 end if if ball.y = maxy - ball.radius then ball.vy *= -1 end if if ball.vx > max_speed then ball.vx := max_speed elsif ball.vx < -max_speed then ball.vx := -max_speed end if if ball.vy > max_speed then ball.vy := max_speed elsif ball.vy < -max_speed then ball.vy := -max_speed end if Input.KeyDown (keys) if keys (KEY_LEFT_ARROW) then player.x -= 2 end if if keys (KEY_RIGHT_ARROW) then player.x += 2 end if if keys (KEY_UP_ARROW) then player.y += 2 end if if keys (KEY_DOWN_ARROW) then player.y -= 2 end if if Math.Distance (player.x, player.y, ball.x, ball.y) < player.radius + ball.radius then player.GFdist := Math.Distance (player.x, player.y, ball.x, ball.y) %figure out the distance between the player and the ball player.GFforce := player.GFstrength * (1 / player.GFdist * player.GFdist) %inverse square. it's a gravity field thing. if player.x = ball.x then %this must be here or a division by 0 error could occur. if ball.y > player.y then player.GFangle := 90 else player.GFangle := 270 end if else player.GFangle := arctand ((player.x - ball.x) / (player.y - ball.y)) %determine the angle between the player and the ball end if if ball.y < player.y then %without this, if the ball hits the underside of the player, it will player.GFangle += 180 %go into the player and do some wild turns, then shoot out the top somewhere. end if ball.vx += player.GFforce * sind (player.GFangle) %finally, apply the force of the gravity field to the x vector of the ball ball.vy += player.GFforce * cosd (player.GFangle) %apply the force of the gravity field to the y vector of the ball end if cls drawfilloval (round (player.x), round (player.y), player.radius, player.radius, 42) drawfilloval (round (ball.x), round (ball.y), ball.radius, ball.radius, blue) View.Update delay (10) end loop Please, let me know of any errors in the collision data in this program. In this program, if the ball is moving really slowly, and it hits you (you are still) it will fire off with decent velocity away. I haven't gotten around to trying to fix this, but I have an idea of how to. The methods of determining the colision data between two circles that I have seen so far all have their varying pros and cons. I hope this new method has very few cons. :wink: ----------------------------------- Tony Sun May 30, 2004 7:11 pm ----------------------------------- not bad Cervantes - I see you advancing in your programming skillz. I'm glad that you're able to take the consept from one program and modify it to apply properly to another, but there's one major flaw. when two balls collide, an inpulse force is created that acts only once during the brief moment two balls are in physical contact. a force field (such as gravity, or negative gravity as in the example) on the other hand act continuely (all the time), but are just not as effective at the distance. and while the code works since the field's activation is limited to the actual contact moment, the reasoning behind the physics model is wrong :? ----------------------------------- Cervantes Sun May 30, 2004 9:46 pm ----------------------------------- true, its not an actual collision, but it works well enough. Since the gravity field has no effect until there is a collision between the player and the ball (Math.Distance < sum of 2 radii) the ball gets repulsed quickly and it doesn't really look like its going into the player at all. and while the code works since the field's activation is limited to the actual contact moment it's very interesting playing slime volleyball if you take out the two if Math.Distance lines and have the repulsing gravity fields working non-stop :wink: ----------------------------------- TheZsterBunny Tue Feb 08, 2005 8:27 pm ----------------------------------- Hey, Nice effect. How 'bout a tutorial on gravity fields? this looks like something worth learning. well, the concept anyways. -Z ----------------------------------- evildaddy911 Sun Oct 16, 2011 12:11 pm Re: [Tutorial]Circular Collision Data ----------------------------------- umm... you should make it so your ball cant go off the screen, i find a simple if statement works well, [code] if player.y < (maxy - player.radius) then if keys ('w') then player.y := player.y + 5 end if end if [/code] ----------------------------------- smool Thu Dec 08, 2011 10:16 am RE:[Tutorial]Circular Collision Data ----------------------------------- Why not calculate tangent of cirlce at point of collision and use angle of incidence = angle of reflection to calculate a bounce?