# How can I generate a lightning bolt effect?

Is there an algorithm to generate a lightning bolt?

I would like an algorithm that generates a list of segment or point objects specifying where the bolt will land. The method would need a start point parameter, along with an endpoint. The bolt should have random branches coming off it, and zig-zag at random intervals. The result will be a random lightning effect that would look somewhat like this

(source: wikimedia.org)

If anyone knows of an algorithm that this may work for, help would be greatly appreciated!

## 2 Answers

There's a fairly straightforward algorithm you can use to generate lighting bolts.

Start with a line segment between the bolt's origin (O) and end point (E)

Choose a point on that line (approximately or exactly in the middle), called S and split the segment into two line segments (O->S and S->E). Displace S away from the original line segment (along the segment's normal) by some small random amount. This gives you a single "bend" of lightning.

After you compute the bend, based on a small random chance you'll want to add a third line segment (usually an extension of the O->S segment). This is how you produce the "forks" in the lightning. You'll usually want to track information about the intensity of the bolt during this generation process, because you'll want the forks to be dimmer or have a more subtle blur:

Then, repeat the above process for all of the new line segments you have; you'll need to choose a repetition amount that produces shapes you like:

There's a fairly clear explanation of this technique at my friend's blog here (it's where I shamelessly stole the pictures from); it goes into additional depth about adding the glow effect as well.

Finally, there's also this NVIDIA paper that describes the same basic algorithm (also with more details).

I would recommend an alternative approach: the rapidly exploring random tree (RRT). One cool thing about it is you can get it to go around corners, or explode in all directions.

The algorithm is really basic:

// Returns a random tree containing the start and the goal.
// Grows the tree for a maximum number of iterations.
Tree RRT(Node start, Node goal, int maxIters)
{
// Initialize a tree with a root as the start node.
Tree t = new Tree();
t.Root = start;

bool reachedGoal = false;
int iter = 0;

// Keep growing the tree until it contains the goal and we've
// grown for the required number of iterations.
while (!reachedGoal || iter < maxIters)
{
// Get a random node somewhere near the goal
Node random = RandomSample(goal);
// Get the closest node in the tree to the sample.
Node closest = t.GetClosestNode(random);
// Create a new node between the closest node and the sample.
Node extension = ExtendToward(closest, random);
// If we managed to create a new node, add it to the tree.
if (extension)
{
closest.AddChild(extension);

// If we haven't yet reached the goal, and the new node
// is very near the goal, add the goal to the tree.
if(!reachedGoal && extension.IsNear(goal))
{
extension.AddChild(goal);
reachedGoal = true;
}
}
iter++;
}
return t;
}


By modifying the RandomSample and ExtendToward functions, you can get very different trees. If RandomSample just uniformly samples everywhere, the tree will grow uniformly in all directions. If its biased toward the goal, the tree will tend to grow towards the goal. If it always samples the goal, the tree will be a straight line from the start to the goal.

ExtendToward can allow you to do interesting things to the tree as well. For one thing, if you have obstacles (such as walls), you can get the tree to grow around them simply by rejecting extensions that collide with walls.

This is what it looks like when you don't bias the sampling toward the goal:

(source: uiuc.edu)

And here's what it looks like with walls

Some cool properties of the RRT once its finished:

• The RRT will never cross itself
• The RRT will eventually cover the entire space with smaller and smaller branches
• The path from the start to the goal can be completely random and weird.
• I just used this algorithm to generate a lightning bolt animation. I must say it worked really well! There's a major typo in the code, such as iter not being incremented every loop. Other than that this is almost ready to use as posted – applejacks01 Feb 12 '16 at 23:54