In This installment of our 2D tutorial series, we will be adding enemy AI to our Lode Runner clone using A* Pathfinding. This post has been guest written by Adrian Seeto of Fun Mob Games who was also kind enough to write the AI scripts that are a huge part of bringing the game to life.

Adding the enemy AI required changes to many of the scripts that we added in the previous tutorials. You can download the complete project here or you can follow the steps below to update your existing project. I recommend following along with the tutorial as there’s tons of great information on implementing the AI with A*. You can also play the game as it will be at the end of this tutorial here.

Installing the A* Package

1. Import the free version of Aron Granberg’s A* Pathfinding Project into Unity3d
2. Create an empty GameObject and rename it A*. Make sure it’s positioned at 0,0,0
3. Drag the /AstarPathfindingProject/Core/AstarPath.cs file onto the A* GameObject
4. Click on the A* GameObject and disable the “Allow Javascript” option

Importing game C# scripts

Many of the scripts have been modified since Part 4 of this series, so you’ll need to update them. If you’ve made changes to your own scripts then you’ll need to merge your changes into these new scripts.

1. Copy these scripts into your Assets/Scripts/ folder:
1. Player.cs (updated)
2. PlayerAnims.cs (updated)
3. xa.cs (updated)
4. ChangeBehaviorGUI.cs (new)
5. Character.cs (new)
6. Enemy.cs (new)
2. Copy this script into your AstarPathfindingProject/Editor/GraphEditors folder:
1. MyGridGeneratorEditor.cs
3. And finally copy this script into your AstarPathfindingProject/Generators folder:
1. MyGridGraph.cs

Configure The xa.cs Script

Create a new Layer by going to Edit –> Project Settings –> Tags and then enter “Enemy” in one of the empty User Layer slots.

1. Select the Global/Scripts/xa GameObject
2. Set Ground Mask field to “Ground”
3. Set Ladder and Rope Mask field to “NoDraw”
4. Set Enemy Mask field to “Enemy”

Miscellaneous Project Changes

Note that some of these may be unnecessary or redundant depending on how your project is currently setup.

1. Expand the Global game object in the Hierarchy and then select the “border bottom” and “border top” GameObjects and set their layer to Ground.  This is so enemies can stand on it if/when we dig a trap above the bottom border and our enemies fall into it they won’t fall continue through the border into the great abyss.
2. Select the player object in the Hierarchy and then change its Tag to “Player”
3. Select the OT/View object in the Hierarchy and uncheck “Always Pixel Perfect“. Note I’m not sure why Adrian turns off pixel perfect, feel free experiment on your own.

Configure A* Pathfinding For Our Level

A* is a graph traversal algorithm widely used in computer science, and commonly used in games for path finding.  A full discussion of A* is beyond the scope of this tutorial, but there is some terminology you should be aware of.  A graph consists of nodes and the network of connections between them.  Once A* has been provided with a graph, you can query it to find the shortest path from one node to another.  A* will return a path way list of all the nodes that you must travel along to get from your start node to your end node, in the shortest path possible.  

More accurately, the path returned is the “least cost” path, because nodes have a specified penalty cost for traveling along it.  A* will attempt to create a path which incurs the least cost.  The unit of a penalty is up to the application developer.  For example, in an isometric RTS game the terrain tiles would be nodes, with 8 connections per node (except for the boundary tiles).  We could have the penalty cost units be related to the time it takes to travel on it.   A swampy area might therefore have very high penalties on the nodes in it, while the grass land nodes surrounding the swamp have smaller penalties.  A computer-controlled tank which wants to get from one side of the swamp to the other may then plot a course around the swamp if it costs less, even though it is physically further (i.e. more tiles must be travelled, but the total travel duration is less) . Of course, it is up to us as the game developer to implement all the code to actually slow the tank down if/when it travels over swamps, A* is only the pathfinder.

We don’t make use of penalties in this game because traveling along ropes, ladders, the ground, and falling is all done at the same speed.  If gravity in our game was a useful non-lethal force (i.e. falling is faster than climbing and just as safe) then we can set node penalties so that A* would return a path where jumping off a platform would be a preferable shortcut to climbing down the ladder. If your game enforces lethal falls from great heights, then you would need additional logic.

A game could have multiple graphs, for use by the various units (ships have different travel behavior to an infantry man for example).  For this tutorial, we only need to create one graph.  In the graph one node will be created for every tile in a 2d grid-like layout.  Each node may have up to 4 connections (up, down, left, right).  Aron’s A* Project includes several different path graph generators, such as GridGraph, ListGraph, and NavMeshGraph.  The GridGraph is a suitable starting point for our game.

The node connections generated by the base GridGraph are always bidirectional (if it creates a connection from from node1 to node2 then it will also create a connection from node2 to node1) which is not necessarily what we want.  Bidirectional paths are fine for ladders, ropes, and the ground because you can go up and down a ladder, and both left-to-right and right-to-left on rope and ground.  The problem arises when you want to include fall lanes. A fall lane is a term I’ve made up for a vertical one-way path that you can only travel on by falling.  Fall lanes exist under the ropes (you can let go of a rope and fall) and off of cliff edges (you can run off a cliff and fall down to a lower platform).  Once on a fall-lane you can’t travel in any direction but down until you’ve hit bottom.  This is a uni-directional connection so the nodes on a fall lane only have one connection each: a connection to the node directly below it.  Other nodes in the level will have between 2 and 4 connections each.  

We could use the base GridGraph and not bother about having any fall lanes, it would just mean that our AI would not plan any paths that involve letting go of ropes or running off platforms.  In this scenario you could still have your AI let go of ropes in a “reactionary” manner i.e. if it’s already on the rope and see a player below it, it could let go.  You would do this will “special case” code in your enemy AI loop.  However, its important to understand this is different from forward planning a route which takes into account the ability to jump off ropes.  In order to that kind of path planning with A*, you need to provide that information in the graph, via the fall lane concept.  

I’ve created a new generator class called “MyGridGraph” which inherits from GridGraph and overrides the Scan() and IsValidConnection() functions.  Our new MyGridGraph class implements the fall lanes idea so let’s add it the scene.

1. Select the A* GameObject
2. Click the Graphs -> Add Graph button
1. Add a new “My Grid Graph”
2. Width: 26 (these dimensions match our level)
3. Depth: 20
4. Node Size: 1
5. Aspect Ratio: 1
6. Center: 0, 0.3, 0.01 (y is 0.3 so that our nodes are centered in the middle of each tile (which we already previously aligned at 0.3), the z is 0.01 because the integer coordinates used for Aron’s A* node positions have a fixed precision of 0.01 and anything smaller than that would cause troubles.)
7. Rotation: -90, 0, 0 (rotate x because our grid is for a side-on view rather than the expected top-down view)
8. Disable Cut Corners checkbox
9. Connections: Six (we only need 4 connections per node up/down/left/right, but we don’t have that option)
10. Disable Collision testing checkbox
11. Disable Height testing checkbox
12. Level Layers: Ground and NoDraw
13. Scroll to the bottom and enable the Show Graphs checkbox then hit Scan

If it worked right you should now see the A* graph layout in your game/scene window (enable Gizmos) similar to the following image (click to see a larger version).

The red squares represent nodes that the AI can not travel on.  All the bricks should have a red node centered on them.  Much of the black “air” tiles should also have a red square gizmo.  The tiles without the red gizmos are the nodes the AI can travel on.  The blue line represents the connections between nodes. You should therefore see that the AI can travel along the surface of ground tiles, along ropes, and up and down ladders.  You should also see the vertical fall lanes under ropes and over the edge of a platform.  If you are very perceptive you will see that the blue lines come in 2 thicknesses.  A thick line means that the connection is bidirectional (the line gizmo was rendered twice, once each way, making it thicker).  A thin line represents a unidirectional connection.

Making Enemies

Our enemies and the player classes both derive from the Character class which implements the movement code.  In this way we can ensure that the enemies and player move with the same constraints to help prevent giving either an unfair advantage, and just general time-saving and efficiency.  The character class does have a movement speed variable, so we can give the enemies different movement speeds than the player, if desired.

Below we create 2 enemy game objects.  I’ve just reused the player sprite for them, and I’ve given each one a different tint so I can tell them apart (helpful during the development stage). If you create your own different enemy sprite go ahead and use it.

1. Create an empty GameObject called Enemies to use as an organizational holder.
2. Clone the player GameObject in the hierarchy and rename it to enemy-1.
3. Drag the enemy-1 to child it under your Enemies GameObject.
4. Drag the /AstarPathfindingProject/Core/Seeker.cs script onto your enemy-1.  The script’s defaults are fine.  Go ahead and break enemy-1’s connection to the player prefab when prompted.  The Seeker provides the enemy with the ability to use A* path finding.  It has gizmos which paint green lines representing the found path.
5. Expand the enemy-1 GameObject to see it’s children.
6. Delete the “shoot parent” child.
7. Add an empty GameObject child under enemy-1 named “path target”.  Local Position 0,0,0 Local Scale 1,1,1. This will be a transform that the enemy script will move around to place on the desired target for our path finding AI.  Useful during debugging to see what it is targeting.
8. Duplicate “path target” and rename to “tile bounds”.  Add a box collider to it, set to Trigger and centered at 0,0,1 and scale 1,1,1.  This is so that when we trap an enemy we can walk smoothly on top of it’s head as if it was ground.
9. Select the enemy-1 game object and set its Layer to Enemy and apply to children.  Set its Tag to Enemy (leave children untagged).
10. Expand the enemy-1 Player script component.  Drag the /Scripts/Enemy.cs script onto the Player Script field to replace it with the Enemy script.
11. The Enemy script has 3 slots that need to be filled: Player and Player Tr (drag the player gameobject to those slots), and Target (drag the path target gameobject to this slot).
12. Expand the enemy-1 OTAnimatingSprite script.  Set the Material Reference slot to “tint”.  Set the Tint Color to red (or your preference) to visually differentiate it from the player.
13. Clone enemy-1 as enemy-2 and give it a green tint. (You may have to redo the Red tint on the other enemy due to an OT quirk).
14. Drag the enemies to suitable spawn points in the scene.
15. Optional: Select the Enemies root GameObject.  Drag the ChangeBehaviors.cs script onto it.  Set the Enemies array size to 2, and drag enemy-1 and enemy-2 into the slots.  This script will allow you to modify an enemy’s behavior on-the-fly via GUI between a “plain A* to Player” and a “Lode Runner-esque” behavior.  The A* to Player behavior simply follows the shortest path toward the player’s exact position.  The Lode Runner-esque behavior still uses A* but also tries to reimplement some of the reactionary quirks of the original game, especially with regards to ladder use.  See the Enemy.cs file for details.  You can disable/remove the ChangeBehaviors.cs script when you are done testing it out.

Conclusion

While the enemy AI is not a completely faithful recreation of the original, it is hoped that it will provide you with enough of an idea on how you can use and implement A* path finding in your own creations.  

The AI scripts and this tutorial were contributed by Adrian Seeto of Fun Mob Games, creators of Pocket BMX game for iOS, a free-roaming 2D bike tricking game with physics.  You can check them out at FunMobGames.com or on twitter @FunMobGames.

If you have any questions about the tutorial, please let us know in the comments below. Your support helps to keep these tutorial coming so be sure to follow us on Twitter and Facebook. This blog post is part of iDevBlogADay, a collection of indie developers writing about their development experiences.

More Tutorials In This Series

Make A 2D Game in Unity3D Using Only Free Tools Part 1
Make A 2D Game with Unity3D Using Only Free Tools Part 2
Make A 2D Game With Unity3D Using Only Free Tools Part 3
Make A 2D Game With Unity3D Using Only Free Tools Part 4

Hi everyone, this is just a quick update to tell you about our new circus themed web game called “Jet-Skate Jo-Jo” which you can play right here on our website.

The game came about because over the holidays, Unity3D hosted a game creation contest titled “Flash in a Flash” where developers could submit a Flash game using the new Flash exporter in the Unity3D 3.5 Developer Preview. With a $20k first prize and a bunch of really good 2nd and 3rd place prizes, Cathy and I decided to dive head first into the competition.

For our submission we wanted to make a new game from scratch that would take advantage of as many of the great features that Unity3D brings to Flash web game publishing as possible like physics, real time lighting and of course a 3D character and environment. Since the schedule was super tight, we knew we had to make a game that was small enough to finish in a short amount of time while still being visually impressive and fun. The contest time ran from December 22nd and ended on January 5th which of course conflicted with all kinds of holiday plans. In the end were able to spend about 9 days working on the game. We submitted the game at 2:30 AM Eastern time on January 5th which gave us about 30 minutes of padding before the contest submission deadline! We ended up having to cut a few things from the game like sound, high scores and there are a few little bugs that I wish we could have fixed, but overall we made a really great little game in a super short amount of time that we’re all really happy with.

A quick shout out to our team: Cathy Feraday Miller did all of the character animations, designed the logo and contributed to much of the game design. Paul Capon did all of the level and prop modeling and texturing, monkey textures and the skate model. David Huynh rigged and weighted the monkey character which was originally modeled and rigged by Shaun Budhram and Roger Liu. Tim Miller did the programming, game and level design and the UI design and art.

Play Jet-Skate Jo-Jo here and be sure to let us know what you think of the game in the comments. And as always be sure to follow us on Facebook and Twitter to keep up to date on all the latest news from Rocket 5.

Happy holidays, everyone and here’s looking forward to a great 2012! Thank you to all of our friends and partners for making 2011 such a great year!

If you haven’t had a chance to play our holiday themed web game yet, be sure to check it out! Click to play A Very Angry Christmas!

Sincerely,
Tim and Cathy and Rocket

This past weekend we took part in the 10th installment of the Game Prototype Challenge, a monthly game jam billed as “a spur of the moment motivator for making games”. Not that we need any motivation since we’re always working on games here. But what we like about jams is that they get us to think about games differently since they usually involve including some kind of theme (“dreams” and “collectibles” for this jam) and they always have a very short deadline which gets us to try new things quickly without worrying about making the code necessarily clean or efficient.

Rocket 5 participated in GPC v.2 earlier this year with our game Snowball Jones And The Last Crusade, which I co-developed with Kevin Feraday.

This time around our Art Director (AKA my wife Cathy) and I took a stab at the challenge with a holiday themed, tongue-in-cheek spoof of the wildly popular game Angry Birds.  In our game, titled A Very Angry Christmas, you control the hero from our upcoming game XS Force as he steps up to defend our hungry friends the pigs from those incessant birds.  For us the game actually serves two purposes:  for the Prototype Challenge, and as an interactive holiday card that we’ll send around to all our friends and business partners.

This was Cathy’s first time participating in a game jam and this was my third time.  We used Unity3D to make the game and we used pre-existing gameplay code from XS Force for the player controls.  We revised the input from touch screen to keyboard and mouse so that the game could be played in a web browser (and it works great on the iPhone too, using our existing touch controls).  I also relied on several excellent Unity plugins including:  iTween for various text and effect animations, 2D Toolkit for the animated sprites and RageSpline for the level background, which, next to iTween, is easily the best Unity plugin I’ve ever used.  Cathy did all of the character art and animations in Flash which were exported as .png’s and then added as sprites to Unity using 2D Toolkit.

If you happen to live in the Toronto area, you can play our games at the #GPC Arcade: Year-One Roundup presented by the Toronto Skillswap community at George Brown College on Friday December 16th. And if you’re not going to be there, you can play A Very Angry Christmas right here on our website!

Thanks for stopping by and please let us know in the comments if you have any thoughts on the game, and especially if you find any bugs . This blog post is part of iDevBlogADay, a collection of indie developers writing about their development experiences.