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Collision Detection

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  • So i got this collision detection via events, goes like this:

    + System: [negated] Sprite.Right Lower or equal Sprite2.Left

    + System: [negated] Sprite2.Right Lower or equal Sprite.Left

    + System: [negated] Sprite.Bottom Lower or equal Sprite2.Top

    + System: [negated] Sprite2.Bottom Lower or equal Sprite.Top

    -> System: Set global variable 'collision' to 1

    + System: Else

    -> System: Set global variable 'collision' to 0

    works fine, problem is though it has a 1px error,

    for example, if i set move to stop it if detects collision,

    it does but overlapping 1px into the other object.

    any ideas on how to optimize this?

    or invert it, so that it checks for non-collision, and if so it returns 1.

    thanks

  • I used "is not overlapping at offset".

    Posting your file always helps.

  • here it is, pretty much what i posted earlier. dropbox

    anyways, i want to be able to do it via events.

    i'm thinking that the problem might be that i'm checking for collision, instead of non-collision

  • I might overlook something, but from a quick look at the events, Construct does exactly what you order it to do.

    Sprite.Right Lower or equal Sprite2.Left

    If Sprite2.Left is 100, then this is true for 0 to 100, if you negate it, it becomes true as soon as Sprite.Right is 101, and that's 1 Pixel overlapping.

    It should work, when you change all conditions to just 'lower than'

    Sprite.Right Lower than Sprite2.Left

    Is true for 0-99, if Sprite2.Left is 100. Negated it is true as soon as Sprite.Right is 100 == no overlapping

    But in general pixel perfect condition testing can only be done reliable, when not using v-sync (have a look at TimeDelta on the wiki, if you are not sure if this is important to you)

  • Thanks Tulamide, i did try that, but the same happened nevertheless.

    I tried adding +1/-1 offsets to see if it fixed it, but it went from about 2px overlap without the offset, to it stopping 1px before collision with the offset,

    the only one i got to work fine was this one:

    + System: Sprite2.Left Lower or equal Sprite.Right +1

    but without the offset, it sometimes overlaps, and sometimes works pefect.

    i'm leaning towards the thought that i'm missing some delta time element,

    i've included dt in movement which basically goes like this:

    if key down > set x = self.x + 120 + timedelta

    how could i include delta in the coll detect?

    i've tried every ticks, but didnt change anything

  • ok, seriously, someone help me, couse i'm either missing something or construct is doing something wrong XD

    no matter what i try, i always get like one offset to the left, and one to the bottom,

    like it would look like this

       0

    1     0

       1

    i'm going nuts here

  • ok, seriously, can someone please

    <img src="http://dl.dropbox.com/u/45834687/funny-3aa982af7b0b1d181cc2c75e1306ecea_h%5B1%5D.jpg" border="0" />

    how come

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set X to Sprite.x - (120 * TimeDelta)

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set X to Sprite.x + (120 * TimeDelta)

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set Y to Sprite.y - (120 * TimeDelta)

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set Y to Sprite.y + (120 * TimeDelta)

    does NOT work as intended?

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  • I really can't understand what you intended to do with this:

    Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set X to Sprite.x - (120 * TimeDelta)

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set X to Sprite.x + (120 * TimeDelta)

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set Y to Sprite.y - (120 * TimeDelta)

    + Sprite: [negated] Sprite: overlaps obj : offset (0,0)

    -> Sprite: Set Y to Sprite.y + (120 * TimeDelta)

    In these conditions you set the Sprite object to move equally to right, left,top and down. Of course nothing will happen...

    For the collision problem you have to use "For Loop" and make like this:

    Sprite is moving

       + For 0 to ceil(120*TimeDelta)-1

          + If Sprite is moving right

            + [negated]Sprite is overlapping at offset ((ceil(120*TimeDelta)-LoopIndex),0)

              -> Sprite move (120*TimeDelta)-LoopIndex at 0 Degrees

              -> Break

          + If Sprite is moving top

            + [negated]Sprite is overlapping at offset (0,((ceil(120*TimeDelta)-LoopIndex)*-1))

              -> Sprite move (120*TimeDelta)-LoopIndex at 270 Degrees

              -> Break

          + If Sprite is moving Down

            ....

          + If Sprite is moving Left

            ....

           

    Do it to the four sides.

    To make conditions for diagonal angles is a bit more complex.

  • I really can't understand what you intended to do with this:

    In these conditions you set the Sprite object to move equally to right, left,top and down. Of course nothing will happen...

    lol, i know, it's meant to represent individual movement on key presses,

    i just skipped over the key press events when posting it.

    the reasoning was something like:

    • if key down

    + sprite is not overlapping object

    -> move in given direction

    however there's going to be a moment, where it isnt overlapping, but will move once more, and end up overlapping,

    so i tried with an offset, but still sometimes stop before colliding, and at random occasions really, against the same wall it may or may not collide perfectly XD

    For the collision problem you have to use "For Loop" and make like this:

    Sprite is moving

    >    + For 0 to ceil(120*TimeDelta)-1

    >       + If Sprite is moving right

    >         + [negated]Sprite is overlapping at offset ((ceil(120*TimeDelta)-LoopIndex),0)

    >           -> Sprite move (120*TimeDelta)-LoopIndex at 0 Degrees

    >           -> Break

    >       + If Sprite is moving top

    >         + [negated]Sprite is overlapping at offset (0,((ceil(120*TimeDelta)-LoopIndex)*-1))

    >           -> Sprite move (120*TimeDelta)-LoopIndex at 270 Degrees

    >           -> Break

    >       + If Sprite is moving Down

    >         ....

    >       + If Sprite is moving Left

    >         ....

    >        

    Do it to the four sides.

    To make conditions for diagonal angles is a bit more complex.

    cool, i'll try it out, however i do not see any "is moving" events,

    thanks.

  • It will not collide perfectly because your sprite will move more than 1 pixel per frame, ex. if your sprite is 1 pixel from the object to collide and your FPS is 60 than, 120*1/60 = 2 pixels, it will move 1 pixel inside the collision object, with lower FPS is even wrost try to put it fixed aroud 15 fps and test, try it on unlimited too and you will see it work perfectly because it will move lower than 1 pixel per frame.

    The way i discribed, using loops, will always move the exact pixels before collide with the object.

    The "is moving" i mean to be your way to detect this, in your case you have to do "On keyboard: Left button is down", move to left, and so on...

  • It will not collide perfectly because your sprite will move more than 1 pixel per frame, ex. if your sprite is 1 pixel from the object to collide and your FPS is 60 than, 120*1/60 = 2 pixels, it will move 1 pixel inside the collision object, with lower FPS is even wrost try to put it fixed aroud 15 fps and test, try it on unlimited too and you will see it work perfectly because it will move lower than 1 pixel per frame.

    The way i discribed, using loops, will always move the exact pixels before collide with the object.

    The "is moving" i mean to be your way to detect this, in your case you have to do "On keyboard: Left button is down", move to left, and so on...

    Hehehe, i thought so, i felt dumb thinking there was an "is moving" event that i'd skipped over! anyways that question HAD to be asked <img src="smileys/smiley36.gif" border="0" align="middle" />

    anyways, are either fixed or unlimited FPS recommended? i thought v-sync was optimal.

    I had tried it on unlimited, and collision-wise it worked great, but i had that sprite tearing almost, that i reference before, like for example, if the sprite is 32x32, it'll show up as that size, but within its own 32x32 bounding box, it'd be moved say to the right, and the left border would be drawn into the X0 and X1 columns, weird.

    and i thought fixed FPS was supposed to hinder your TimeDelta

    i'm looking forward to implementing a loop as you suggested, it looks like i'll have to clean up the game structure first, in order to replace it neatly hahahaha <img src="smileys/smiley4.gif" border="0" align="middle" />

    as for now, while i'm restructuring it, i got around the collision enough to work with it with something like this:

    B pick closest to A.left\right\top\bottom

    + if A.left\right\top\bottom >\< B.left\right\top\bottom -\+ speed*TimeDelta

    -> then move at speed*TimeDelta

    etc

    extremely summarized but in essence that's what i got around to enough for it to work until applying loops, if it helps anyone in the future.

    XD

  • Metal_X described a lot more detailed, what I meant in my previous post by "when not working v-synced".

    And, yes, you have to decide wether to go pixel based or time based. A mixture will almost always fail. Using a fixed rate may assure you to work pixel based, but will have a jerking output (as it isn't in sync). v-sync and deltatime, on the other hand, will assure you of the same game speed at all rates and a smooth output.

    For problems like yours, when people want to work time based, they use a technique like "push out". Whenever a collision happens, the object will be moved out of the other object within one frame. In result, it looks like it collides pixel accurate. There also is "overlap at offset" which helps a lot, too, when working time based.

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