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The Secrets Of Enemy AI In Uncharted 2
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The Secrets Of Enemy AI In Uncharted 2

November 3, 2010 Article Start Previous Page 4 of 5 Next

For animations we have a system that allows us the ability to remap any animations at any time so we can mix and match whatever we need. What this means is that when the AI system calls for certain actions under the hood, we can map any action to any animation we chose.

This is very powerful and flexible in that you don't have to create a whole animation set for each character. Instead you can just remap specific actions to new animations.

As an example, in Uncharted 2 we had differences between light and medium soldiers in how they would fire from cover in that the light guys would expose themselves more, and medium guys would not. We just used a remap so that when the AI system issued a command to fire from cover, it would use different animations for light and medium soldiers.

Other examples of where we used this system are the sequence of escorting the wounded cameraman Jeff through the alleyways, and when Drake was walking around holding a flaming torch in the old temple in the swamps.

Thinking Machines

So let's take a look at how the decision making processes works and how we as designers interact with them to get the behaviors we want.

Which Way Did He Go George...

The AI has to understand the environment in order for them to move around and do cool stuff. To do this, we create what we call a navigation mesh (navmesh for short) which defines the boundaries of the world for the AI. It's a polygonal mesh created in Charter using a special set of tools to make it fast and intuitive.

[You can find out more about Charter in the previous article.]

(click for full size)

This is the start of a basic navmesh when we initially place one inside of Charter. We have a special editing mode for navmeshes where you see the yellow discs on each of the vertices.

(click for full size)

These discs allow us to just drag a vertex around in the horizontal plane while maintaining its height relationship to the ground. The green arrow on each vertex allows us to move it vertically if we need to adjust the height.

(click for full size)

We can create new vertices by clicking on the red lines that connect the existing vertices. We can also delete a vertex by control clicking in the orange circle at the center of the yellow disc.

(click for full size)

We can cut out a hole by control clicking and dragging in the middle of the navmesh. The shape of the hole can then be modified by dragging its vertices, and adding or removing vertices as normal.

Using these tools we can create and manipulate navmeshes to conform to the area that we want the AI to navigate. We can also link multiple navmeshes together by snapping their vertices to one another, and when it compiles it will combine them all into a single navmesh.

While the navmesh defines the boundaries of the navigable space, the AI rely on a second system that works in conjunction with this called the navmap. The navmap is a 2D grid projection on the horizontal axis around the AI. It takes into account the navmesh as well as dynamic objects.

(click for full size)

In this image you can see the navmap of the selected AI (the one near the center with the circle around him). You can see there's a grid overlaid onto the world, and that some of the grid squares are colored in yellow. The yellow represents space that is non-navigable. It uses the navmesh to find the world boundaries (such as the area around the physical structures), plus the navblockers of dynamic objects such as other AI.

A navblocker is something the navmap specifically reads in and can be dynamically altered. It's usually derived from the collision boundaries of objects, but can also be placed manually in Charter, as well as generated with specific settings in code (such as with other AI). This gives designers the flexibility to dynamically block off areas for gameplay purposes. Also by using this kind of system we save on computational expenses as the AI does not have to continually probe the environment for dynamic obstacles.

This system handles most of the heavy lifting of allowing the AI to properly navigate through the environment, but what to do when you want the AI to be able to follow the player over more complex landscapes? For this we have what are called Traversal Action Packs (TAPs). We create animations that meet the demands of the environment (i.e. scaling walls, jumping over gaps, climbing ladders, etc...), which we then plug into the TAPs system and place them in Charter. The AI can then use these to navigate from navmesh to navmesh, being able to better follow the player.

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