Something Interesting Regarding Circle collision

[Dragon20942]

What is it you are trying to achieve?
So I was wondering if we use the Pythagorean theorem to find the exact pixels of a circle's circumference, or if there is an easier way. From my current Gr. 10 math, it is suggested that we must substitute every value of x to get y (or vice versa) using the radius as the hypotenuse. However, this would create some issues concerning the amount of computing required to run a for-loop running as many times as there are pixels in the radius.

[Tony]

Checking all the pixels is not guaranteed to work. The trivial counter-example is where one circle is completely inside a bigger circle. More interesting edgecases are those where pixels round in such a way that they don't end up in the same spot.

Try drawing some diagrams to get other ideas; http://compsci.ca/blog/super-paper-programming/

[Dragon20942]

The fact that you use the centers of both circles instead of just the center of one circle and the end of its radius provides a new concept for me. Id better think that out before coding. Thanks!

[Insectoid]

There are a number of tutorials on this from very basic concepts to advanced collision correction scattered around the forums. Have a look.

[mirhagk]

Cicle collision is the simplest, and arguably the most important collision detection.

Almost any shape can be rougly estimated with a circle, allowing you to quickly check to see if something could potentially intersect something else without a full collision detection.

Also it's a matter of how far something is from another thing, every mission objective that involves getting to a specific point uses circle collision detection to see if your close enough.


Oh and a quick tip, the mathematical algorithm is

code:

d=sqrt((x2-x1)^2+(y2-y1)^2) if (d<=r1+r2)    print (They hit)

This is the proper way to do it, but a suggested hack is to drop the square root sign, and then just square (r1+r2). The cost to multiply two numbers is a lot cheaper than the cost of doing a square root.

code:

d=(x2-x1)^2+(y2-y1)^2 if (d<=(r1+r2)^2)    print (They hit)

[DemonWasp]

Long story short: two circles (or spheres, or hyperspheres) are intersecting (have collided) if the distance between their centres is less than the sum of their radii.

[Dragon20942]

Great! I got:

Turing:

var rad1 := 25 %radius of first circle (red) var rad2 := 25 %radius of second circle (green) var circleX := Rand.Int (rad1, maxx - rad1)%red var circleY := Rand.Int (rad1, maxy - rad1) var circle2X := Rand.Int (rad2, maxx - rad2)%green var circle2Y := Rand.Int (rad2, maxy - rad2) var circleColour := red var circleColour2 := green var circleXVel : int var circle2XVel : int var circleYVel : int var circle2YVel : int loop     circleXVel := Rand.Int (-3, 3)     circle2XVel := Rand.Int (-1, 1)     circleYVel := Rand.Int (-3, 3)     circle2YVel := Rand.Int (-1, 1)     exit when circleXVel not= 0 and circle2XVel not= 0 and circleYVel not= 0 and circle2YVel not= 0 end loop var xDiff, yDiff, cDiff : real         %cDiff is the distance between the two centers of the circles (hypotenuse) View.Set ("offscreenonly") loop     circleX += circleXVel     circleY += circleYVel     circle2X += circle2XVel     circle2Y += circle2YVel     if circleX < rad1 then         circleXVel := circleXVel * (-1)     elsif circleX > (maxx - rad1) then         circleXVel := circleXVel * (-1)     end if     if circleY < rad1 then         circleYVel := circleYVel * (-1)     elsif circleY > (maxy - rad1) then         circleYVel := circleYVel * (-1)     end if     if circle2X < rad2 then         circle2XVel := circle2XVel * (-1)     elsif circle2X > (maxx - rad2) then         circle2XVel := circle2XVel * (-1)     end if     if circle2Y < rad2 then         circle2YVel := circle2YVel * (-1)     elsif circle2Y > (maxy - rad2) then         circle2YVel := circle2YVel * (-1)     end if     xDiff := (circleX - circle2X)     yDiff := (circleY - circle2Y)     cDiff := (xDiff ** 2 + yDiff ** 2) ** (1 / 2)     put "Pixels between midpoints: ",cDiff     if cDiff < rad1 + rad2 then %if the hypotenuse is less than the combined radii, then         circleColour := blue         circleColour2 := blue         delay (100)     else         circleColour := red         circleColour2 := green     end if     drawfilloval (circleX, circleY, rad1, rad1, circleColour) %green     drawfilloval (circle2X, circle2Y, rad2, rad2, circleColour2) %red     View.Update     delay (5)     cls end loop

Ok so I go and make up some code before realizing that you guys already posted it...