Get Started With The A* Project

This is the Get Started Guide for the A* Project.
To begin with, download the project.

Next, open the example scene “Terrain” in Unity.
Click on the “A*” gameObject in the hierarchy view, in the inspector you will now see a bunch of controls, click on the Show Grid button (almost at the top) to make it enabled if it isn’t already, in the scene view you can now see a grid covering the terrain.

This grid is what the units can navigate around on, each line represents one connection and each dot from where the lines are drawn are nodes, the units will navigate from node to node to reach the target.
You will notice there are holes in the grid, that’s where there are obstacles or the slope is to step for the units to climb, those nodes are marked as unwalkable.
You will also notice the big white box around the grid, that box is the boundary of our grid, outside that grid, no path’s can begin or end.
The grid’s nodes have different colors, all nodes in one area are sharing the same color, the areas are separated by unwalkable nodes, so units can’t travel from one area to another one, most often you would want your world to have only one area (and thus only one color).

Back to the controls.

You know that many controls in the editor have tooltips, so if you are unsure about the meaning of a variable, hover over it with the mouse and see if I have put a tooltip there.

The first row of controls in the editor is a toolbar with names like “Static Settings”, “Runtime Settings” etc.
First I will talk about the Static Settings.

Static Settings

Here are the settings which should only be changed in the editor when not playing, if these are changed during runtime errors might occur.

The most important control in the static settings is the button called “Scan Map”, it is placed almost at the bottom, if you click the button the grid will update with your chosen settings (you might have to rotate the camera in the scene view to see the grid updates), it will also be called on startup if you have the “Calculate Grid On Startup” toggle checked.

Grids

So these were the Grids, but what if you don’t want a flat grid, but instead would want the grid to follow your terrain, here’s the settings for that.

There are three different settings for defining the position of the nodes along the Y axis, the first is Flat, that generates a flat grid, no settings for it, the second one is the Terrain mode, this will sample the terrain height at each node position and place it at the correct height.

A warning for terrain mode though: If you use a large terrain (I think that’s the reason, it’s on the Unity side), then you might get the “vertexCount > 60000″ error, that means the script can’t sample the height values, to get rid of it you will have to change to raycast mode, place the terrain in a separate layer and assign the raycast to only hit that layer, then it will work just the same.

The last mode is called raycast mode, it uses raycasting to figure out where to place the nodes, it does also have more features than the terrain mode since it can get the normal of the point it hit too, this makes it possible to make nodes with a too high slope unwalkable, the variable for changing that is called Static Max Angle, it can also be used with terrain mode but with the drawback that the terrain mode can only use the angles between the nodes and not the real normal which makes it a bit unreliable.
I suggest that you use raycast mode in all cases if your grid shouldn’t be completely flat.

Variables

Next up comes the Neighbours variable.
If you look closely at the grid you should see that each point has eight or four lines (depending on which option is chosen) drawn from it to neighbouring nodes, in some games you might want the player/NPC/whatever to move diagonally, and in some games you might not want that, so that’s why I have given you the option to choose what you like the best, try it out and change the setting and see what happens.

The last variable is the heap size variable. This one is quite advanced and it’s tricky to know what it does but I will try to explain.
If you have a basic understanding of A* you should know that the script stores an “open” list of all possible nodes it could move to next time it iterates (one iteration for each node it searches), the next one it chooses is the node with the lowest F value, this is the distance travelled from the start added to the estimated distance to the end. So the script needs to search for the node with lowest F value in an array maybe 200, maybe 2000 times in one frame, that isn’t so fast if we are going to search through the whole list every time, so the script uses a so called Binary Heap (look up on wikipedia for more info) to make it faster.
What the heap size variable adjusts is the maximum length of the binary heap array, 1 means that the whole grid can be in the open list if required (this will never happen) but it uses more memory (if a grid is 100*100 nodes it will create an array of length 10 000), 0.1 means 10% of the grid can be in the open list and it uses a lot less memory. I recommend that you use the value 0.5, that was the default value in all pre 2.5 versions by the way. Maybe you should use higher values for small grids.

If you haven’t understood a thing of what I said about the binary heap… Don’t worry, you will likely not have to care about it anyway ; )

So now you have had a complete description of the Static Settings (sorry if it was a bit long).
I the next part I will show you how to set up a grid in a new scene, Part 2