Sprite Arrays - Featured Article

I recently wrote a TechNet Wiki article describing how to manage arrays of sprites in a game.

For the full article see http://social.technet.microsoft.com/wiki/contents/articles/24857.small-basic-sprite-arrays.aspx.

The following is an excerpt from one section...


Recycling sprites

We often want sprites to appear and disappear.  One way to do this is to recycle the sprites from a 'pool', activating and using them as required.  This can be good for example to fire bullets or missiles, when we only ever need a limited number on screen at the same time.

The example below fires missiles when the mouse is clicked.  A new spriteData property "Status" is used to flag that missiles are active or not.  Inactive missiles are hidden and they are shown while they are active on the screen.

 Missile Sprite

gw `` = ``600

gh `` = ``600

GraphicsWindow``.`` Width `` = ``gw

GraphicsWindow``.`` Height `` = ``gh

GraphicsWindow``.`` MouseDown `` = ``OnMouseDown

 

CreateSprites``(``)

 

'Game Loop

While ``(``"True"``)

  `` start `` = ``Clock``.``ElapsedMilliseconds

  

  `` If ``(``mouseDown`` ) ``Then

    ``FireMissile``(``)

    `` mouseDown `` = ``"False"

  ``EndIf

  ``UpdateSprites``(``)

  

  `` delay `` = `` 20 - ``(``Clock``.`` ElapsedMilliseconds `` - ``start``)

  `` If ``(`` delay > ``0`` ) ``Then

    ``Program``.``Delay``(``delay``)

  ``EndIf

EndWhile

 

Sub ``CreateSprites

  `` spriteImage `` = ``ImageList``.``LoadImage``(``"http://litdev.hostoi.com/game_images/missile.png"``)

  ``'Sprite dimensions we use the half width and height

  `` spriteWidth `` = ``ImageList``.``GetWidthOfImage``(``spriteImage``)``/``2

  `` spriteHeight `` = ``ImageList``.``GetHeightOfImage``(``spriteImage``)``/``2

  

  `` numSprite `` = ``50

  `` For `` i `` = 1 `` To ``numSprite

    ``spriteData``[``"image"`` ] `` = ``Shapes``.``AddImage``(``spriteImage``)

    ``spriteData``[``"Xpos"`` ] `` = `` spriteWidth `` + ``Math``.``GetRandomNumber``(``gw``-``2``*``spriteWidth``)

    ``spriteData``[``"Ypos"`` ] `` = ``gh``-``spriteHeight

    ``spriteData``[``"Xvel"`` ] `` = ``0

    ``spriteData``[``"Yvel"`` ] `` = ``-``5

    ``spriteData``[``"Status"`` ] `` = ``0

    ``Shapes``.``HideShape``(``spriteData``[``"image"``]``)

    ``sprites``[``i`` ] `` = ``spriteData

  ``EndFor

EndSub

 

Sub ``UpdateSprites

  `` For `` i `` = 1 `` To ``numSprite

    `` spriteData `` = ``sprites``[``i`` ] ``'get current sprite array

    

    `` If ``(``spriteData``[``"Status"`` ] `` = ``1`` ) ``Then

      ``'Reposition sprite center

      ``spriteData``[``"Xpos"`` ] `` = ``spriteData``[``"Xpos"`` ] `` + ``spriteData``[``"Xvel"``]

      ``spriteData``[``"Ypos"`` ] `` = ``spriteData``[``"Ypos"`` ] `` + ``spriteData``[``"Yvel"``]

      

      ``'Move sprite center

      ``Shapes``.``Move``(``spriteData``[``"image"``]``,``spriteData``[``"Xpos"``]``-``spriteWidth``,``spriteData``[``"Ypos"``]``-``spriteHeight``)

      

      ``'Sprite finished with

      `` If ``(``spriteData``[``"Ypos"`` ] `` < ``-``spriteHeight`` ) ``Then

        ``spriteData``[``"Status"`` ] `` = ``0

        ``Shapes``.``HideShape``(``spriteData``[``"image"``]``)

      ``EndIf

      

      ``sprites``[``i`` ] `` = `` spriteData ``'save updated sprite array (it may have been modified)

    ``EndIf

  ``EndFor

EndSub

 

Sub ``FireMissile

  `` For `` i `` = 1 `` To ``numSprite

    `` spriteData `` = ``sprites``[``i`` ] ``'get current sprite array

    `` If ``(``spriteData``[``"Status"`` ] `` = ``0`` ) ``Then

      ``spriteData``[``"Status"`` ] `` = ``1

      ``Shapes``.``ShowShape``(``spriteData``[``"image"``]``)

      ``spriteData``[``"Xpos"`` ] `` = ``GraphicsWindow``.``MouseX

      ``spriteData``[``"Ypos"`` ] `` = ``gh``-``spriteHeight

      

      ``sprites``[``i`` ] `` = `` spriteData ``'save updated sprite array (it may have been modified)

      `` i `` = `` numSprite ``'End loop

    ``EndIf

  ``EndFor

EndSub

 

Sub ``OnMouseDown

  `` mouseDown `` = ``"True"

EndSub