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# Lightning

By Ian McDougall | Published Oct 24 2001 11:17 AM in Graphics Programming and Theory

single sub bolt set public curr end byval inext
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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.

All the demo code is in Visual Basic with DirectX8. There is a demo with code and exe that you can via the attached article resource.

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

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

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.telusplan...public/javaman/

E-Mail: javaman@telusplanet.net