Lightning
by Ian McDougall

1.0 Abstract

One night when there was a lightning storm (2 or 3 nights ago) I decided I would write an article on lightning just for fun. This is the first time I have ever tried to simulate lightning before. If there are any spelling mistakes or similar please E-Mail then to me at javaman@telusplanet.net.

1.1 About this Article

All the demo code is in Visual Basic with DirectX8. There is a demo with code and exe that you can get here.

2.0 Getting down to work

First off, you just set up a normal Direct3D scene. I added a bit of fog to make it look like it's raining, but you don’t have to do this if you don’t want to.

Enough of that on with the fun. I use 2 classes in this method: the first you only need on ref to its name is "clsLighteningBoltMain". It's more or less just a container class for the second class called "clsLighteningBolt".

2.1 clsLighteningBoltMain

Class variables

Private BoltArray            as clsLighteningBolt

This is the head of are singly linked list

Functions

Public Function Get_YfromXZ(ByVal X as single, ByVal Z as single) as Single

This function returns the height of the ground at X, Z. (Note: This function only returns 0 because our ground is flat)

Public Sub AddBolt(ByVal X as single, ByVal Y as single, _
                   ByVal Z as single, ByVal lColor as single)

This sub adds a bolt to the list at the starting pos of X, Y, Z with the color lColor (Note: for some unknown reason color is not working; if any one finds out why please e-mail me)

Public Sub DrawBolts()

This sub draws the bolt list and removes any bolts that say they can be removed.

2.2 clsLighteningBolt

Variables

Public iNext      as clsLighteningBolt

Pointer to the next bolt in the list

Public Parent     as clsLighteningBoltMain

Pointer to the container class (so we can call Get_YFromXZ)

Functions

Public Sub Init_Bolt(ByVal X as single, ByVal Y as single, _
                     ByVal Z as Single, iColor as Long)

Sets the bolt startup values

Public Function Draw_Bolt() as Boolean

Draws the bolt and returns false if it's done (e.g. hit the ground)

2.3 How the code works

2.3.1 clsLighteningMain

Get_YFromXZ

This function returns the height of the ground/strike plane at position (x, z).

Public Function Get_YFromXZ(ByVal x As Single, _ ByVal z As Single) As Single Get_YFromXZ = 0

End Function

AddBolt

This sub adds a bolt to the start of the bolt list with the specified position and color.

Public Sub AddBolt(ByVal x As Single, ByVal y As Single, _
                   ByVal z As Single, lColor As Long)
  Dim NewBolt As New clsLighteningBolt
  NewBolt.Init_Bolt x, y, z, lColor
  Set NewBolt.iNext = BoltArray
  Set BoltArray = NewBolt
  Set NewBolt.Parent = Me
End Sub

DrawBolts

This sub draws all of the bolts in the bolt list and removes bolts that have hit the ground.

Public Sub DrawBolts() Dim Curr As clsLighteningBolt Dim Last As clsLighteningBolt Device.SetVertexShader FVF_LineVertex Set Curr = BoltArray Device.SetTexture 0, Nothing

Do If Curr Is Nothing Then Exit Do If Curr.Draw_Bolt() = False Then If Not Last Is Nothing Then Set Last.iNext = Curr.iNext Set Curr = Curr.iNext Else Set BoltArray = Curr.iNext Set Curr = BoltArray End If Else Set Last = Curr Set Curr = Curr.iNext End If

Loop End Sub

2.3.2 clsLighteningBolt

InitBolt

This sub just sets the first line segment up.

Public Sub Init_Bolt(ByVal x As Single, ByVal y As Single, ByVal z As Single, ByVal iColor As Long)
  ReDim Lines(1)
  NumberLines = 1
  With Lines(0)
    .x = x
    .y = y
    .z = z
    .Color = iColor
  End With
  
  With Lines(1)
    .x = x
    .y = y - 0.06
    .z = z
    .Color = iColor
  End With
End Sub

DrawBolt

This sub draws and adds one more vertex to the line strip and returns true if its not done yet and false if it is.

Public Function Draw_Bolt() As Boolean
  NumberLines = NumberLines + 1
  ReDim Preserve Lines(NumberLines)
  With Lines(NumberLines)
    .x = Lines(NumberLines - 1).x + (1 - Rnd * 2) * 0.3
    .y = Lines(NumberLines - 1).y - (Rnd + 0.55)
    .z = Lines(NumberLines - 1).z + (1 - Rnd * 2) * 0.3
    .Color = Lines(0).Color
  End With

  Device.DrawPrimitiveUP
  D3DPT_LINESTRIP,NumberLines, Lines(0),_
  Length_LineVertex

  Draw_Bolt = True
  If Lines(NumberLines).y <= Parent.Get_YFromXZ(Lines(1).x, Lines(1).z) Then
    If Rnd > 0.95 Then
      Draw_Bolt = False
    End If
  End If
End Function

3.0 Theory

3.1 How it works

The way this works is really simple when you think about it. There is a list of all the bolts that this container needs to think about. You add and remove bolts from there. Here's a picture to help out with this one:

Adding a new bolt

Step One:

Create a new bolt class and set its variables.

Step 2:

Insert the new bolt into the list right after the head.

Removing a bolt

Steps:

Set 1.iNext = 2.iNext
Set 2.iNext = Nothing
Set 2 = Nothing

Figuring out where to put the next vertex for a bolt is even easier then that. Here's how:

Resize the Vertex array so you can fit one more vertex in it
Set the X to X(index-1) + 1-(rnd * 2) * 0.3 and same for the z coordint
Set y to Y(index-1) - (rnd + .55)

Now last but not least: drawing the bolt. I do it with DirectX using a line strip.

Device.DrawPrimitiveUP D3DPT_LINESTRIP, Ubound(Lines()), _
                       Lines(0),16

4.0 Conclusion

Well that’s all there is to it. Not hard, is it?

WepPage: http://www.telusplanet.net/public/javaman/

E-Mail: javaman@telusplanet.net

Discuss this article in the forums

Date this article was posted to GameDev.net: 10/24/2001
(Note that this date does not necessarily correspond to the date the article was written)

See Also:
Lightning
Sweet Snippets