2D: SpriteCollision on layers not possible?

[Marksman]

Hi,

why does this function returns true if i shot with the mouse
on the top left corner of the blue building? It shouldn't, because the
part of the (red) enemy is behind the building, not visible and
so should not be hitable.

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If SpritePixelCollision(#Sprite_Mouse,MouseX(),MouseY(),#Sprite_Enemy,x,y) And MouseButton(1)

[MadMax]

Quite simple realy, PB checks if the sprites would collide if they were where you say, it doesn't check the screen. This is not a bad thing, as it is quite useful in many games. Of course in your case you'll need a different aproach. Don't realy know what you want to do, but I suggest checking if it's behind something before you check for collision.

[Shannara]

Before calling the collision, clip the enemy sprite's right/bottom by the building's sprite's X/Y. Then run the collision test, then "unclip" the enemy sprite

[Marksman]

Okay, but what about a complex set of some trees and 20 enemies flying around?

[MadMax]

Well, let's say you have a whole lot of blue rectangles and another of lot of yellow ones that can hide behind the blue ones.

The way I would go about it:
I would check collision with the blue rectangles.
a) If no collision then check collision with the yellow rectangles
b) If collision with blue DON'T check yellow

Hope this helps.

[netmaestro]

In this case you need to add one test before accepting collision. Each object on the screen must have a number of attributes, such as x, y, #sprite and a number of others. In your program, one of these others will be "layer". So to test collision you would do: If SpritePixelCollision(...) And Object1()\Layer = Object2()\Layer <success>.

[Marksman]

The actual Structure is this:

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Structure Enemy

  x.l
  y.l
  speed.l
  direction.l
  layer.l

EndStructure

NewList Enemy.Enemy()

And the CollisionCheck this:

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If SpritePixelCollision(#Sprite_Mouse,MouseX(),MouseY(),#Sprite_Enemy,Enemy()\x,Enemy()\y) And MouseButton(1)

What value should "layer" have? 1 if in back and 2 if in front of the building?

And how must i exactly do the check?

Why does this not work (but looks logically?):

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If
SpritePixelCollision(#Sprite_Mouse,MouseX(),MouseY(),#Sprite_Enemy,Enemy()\x,Enemy()\y)
And
SpritePixelCollision(#Sprite_Mouse,MouseX(),MouseY(),#Sprite_Building,MouseX(),MouseY())=0
And MouseButton(1)

[netmaestro]

I'd have to see more code to say why exactly it doesn't work, but if you are shooting a projectile, and the projectile is coming from the foreground of what we're considering to be a 3d-like view, then this seems logical:

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Repeat
  Display sprites for objects on layer 2
  Display sprites for objects on layer 1
  Display sprites for objects on layer 0
  Collisioncheck projectile vs all objects on layer 0
  if none
    Collisioncheck projectile vs all objects on layer 1
    if none
      Collisioncheck projectile vs all objects on layer 2
    endif
  endif
until <something>

That's just some quick pseudocode, but if you implement that logic, if your projectile hits the top left corner of the building, that's the collision you'll catch, not the object behind it. (Assuming object behind has layer=2, building has layer=1)

[Marksman]

This one should just check Enemies behind the buildings:

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Repeat

     ExamineKeyboard():ExamineMouse():ClearScreen(0,0,0)
   
     Gosub MakeEnemies
     Gosub CheckEnemies ; if they are out of screen
     Gosub MoveEnemies

     DisplayTransparentSprite(#Sprite_Building,400,300)
     DisplayTransparentSprite(#Sprite_Mouse,MouseX(),MouseY())

     Gosub CheckCollision

     FlipBuffers()

Until KeyboardReleased(#PB_Key_Escape)




CheckCollision:

If MouseButton(1)

If SpritePixelCollision(#Sprite_Mouse,MouseX(),MouseY(),#Sprite_Building,MouseX(),MouseY())=0 ; if not shot on building

ForEach Aliens()

     If SpritePixelCollision(#Sprite_Mouse,MouseX(),MouseY(),#Sprite_Enemy,Enemy()\x,Enenmy()\y)
     DeleteElement(Enemy())
     EndIf

Next

EndIf

EndIf

Return

[/size]