# Animation states and blend trees

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In a game with a complex animation system for a player, when using an animation blend tree, would one tree be used for all possible animations?  Or would there be, for example, one blend tree for idling, one for walking/running, one for punching, kicking, or whatever moves there are?  I've created a binary animation blend tree system which is pretty flexible but I'm not sure I should be making it so complex to contain all of my animations.

And also, how does the animation state machine relate to the blend tree?

Thanks

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Generally you have a state machine that has different states. These states can be either for example a single motion, another state machine, or a blend tree.

This way you can build hierarchical systems to keep things much more clear and easy to understand visually.

Mostly you would see an idle state, which can be a state machine in which some random motion is picked for example. You can also make it a blend tree, in which you place a  a state machine. The output of that state machine that picks the idle motion can then act as input to say a lookat node, which makes the character look at a given goal again, allowing to modify the idle motions.

Then next to Idle there can also be some Moving state, which is most likely a blend tree in which you blend different animations together, based on speed, turn angle, etc.

So to answer your first question, no, in modern games you wouldn't really put everything in one blend tree or state. Technically you could, but you will have it hierarchical where your blend tree would contain state machines as well. Generally you have one root state machine, in which you place things like Idle, and Moving, Attacking, etc.

I hope that also answered your question about how state machines relate to blend trees, if not please explain what exactly you mean in further detail

Edited by Buckshag

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Thanks Buckshag.

The thinking behind my one animation blend tree is that I can conceivably have all my complex animations and blends in there.  All I would need to give my player any animation is to supply some scalar values to the blend tree including blends between such and such or choices of this clip over that one, etc.  I thought then I could have a separate state machine that contained the values I'd need to pass to the blend tree in order to get the relevant pose.

So in the blend tree you might have this (for my snowboarder character):

b1) IDLE

b1) Jump/board on/off

c1) Jump

c2) Put board on

c3) Take board off

b2) Idle

c1) Idle 1

c2) Idle 2

c3) Idle 3

b2) MOVE

c1) board off

c1) Motion

b1) Walk

b2) Run

c2) Misc

c1) Come to stop

c2) Jump

c2) board on

b1) Motion

b1) Upright stance

b2) Medium stance

b3) Crouched stance

b2) Jump

b1) Jump initiation

c1) Ollie

c2) Nollie

b2) Grab

c1) Indy

c2) Tail

c3) Nose

c4) Mute

b3) Style out (Depends on grab)

b3) Land

b4) Fall

Where an entry starting with b (e.g. b1)) is a blend node and c (e.g. c1)) is a choice node (choice node meaning out of these nodes only one clip can be played).  So by passing in around 3-5 values into the blend tree, I can pretty much come up with any animation for my character.

Storing each set of values required for a particular animation into a state machine state would allow me to just use the state machine and then depending what state i'm in, play that particular animation.

Have I over-complicated the use of blend trees?

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I just read some Unity documentation where they talk about animation states, transitions and blend trees. I think I've got a clearer picture now

So a state can be just an animation clip or the result from a blend tree and the transition between states can be a blend from one state to another. Does that sound about right? It seems like it fits my requirements at least

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Yes that sounds correct

States can be anything that output a pose without needing a real input, other than some settings you can configure for that state (like which motion to use).

So anything that 'generates' an output pose can act as state. So instead of a predefined motion node you can also have say a procedurally generated motion node that does some physics based walking motion or so.

These states can be connected with transitions. These transitions can have conditions on them, which define when transitions should be activated. For example if you have an idle state and a move state, you can make a transition from Idle to Move, and put some condition on it, which triggers when the character 'speed' parameter becomes bigger than zero.

So when your game would then pass a value of say 0.5 for speed, it would automatically make the transition from idle to move. During the transition both outputs poses are calculated of both the idle and move state, and a blend is done between them.

The move state could be a blend tree. Blend trees have some final node to which you connect. This final node then represents the output pose of the blend tree.

A state can also be another state machine. So the output of that state would be the output of the state machine it represents.

This way you can build hierarchies. I think Unity 5 will also support that. I think its amazing they didn't/don't support hierarchies yet, as that makes it almost useless or impossible to manage.

I have seen some of our clients of our animation middleware EMotion FX have graphs of over 1500 nodes. Imagine having to place that all in one state machine. That would end up in one big spaghetti of transitions

Btw, to answer your question if you over-complicate the use of blend trees: you seem to pick between different motions quite frequently.

In theory you wouldn't need a blend tree for that, but in practise that can do that as well.

You could either make that a state machine that picks the right motion or pose based on say what kind of stance you are in, or you could make a blend tree which has some blend node which takes multiple input motions, and your weight will represent which motion to pick. The blend node can automatically blend between them. Or you make a node that picks a given motion based on an input value, without blending.

For your jump types you would most likely want a state machine rather than blend tree, as you are not likely to blend between different types of jumps.

Edited by Buckshag

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Thanks for the explanation, really appreciate it. With what you've said I think I've got enough to go forward now.

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I've been doing some work on the state machine tonight and I have a couple more questions...

Firstly, I obviously need some form of instance data with regard to my states, or at least the current state I'm in.  For example, I have my player character and as an entity, he has a component which is a state machine.  He is idling and so the current state in his state machine is the idle state.  In that state, i'll need to have some record of the local time for the idle animation that is playing.  That's all well and good, but if I have two players on the screen, would they each have their own state machine?  I'm wondering whether I keep the data seperate from the structure of the state machine although I'm not really sure how that would work.  A lot of the data in the state machine states would be common (fade time, fade type, animation clip name), it's only animation clip positions that would need to be stored, so copying the whole structure seems like a waste of memory.  Also, if a state uses a blend tree, there may be lerp positions that needs to be kept track of.

Secondly, from an animation perspective, I understand the states and transitions that can occur but what should control the state machine?  I mean if it's entirely data-driven (which I intend it to be), some game logic somewhere has to know that when the player is in the idle state, he can walk or run (for example).  I know the data-driven state machine can represent this easily, but how would the game logic control that?  My initial thought was that the current state of each state machine would handle events so,  for example, when the user presses the up button on the controller, that event is fed through to the player's state machine (it's a component after all which can receive events) and the state knows (through data) that in this instance, pressing up in the idle state transitions to the walk forward state.

So the state data might be something like (in very simplistic terms):

<state name="idle" animation="idle_anim">

<transition name="idle-walk" type="fade" timeSpan="0.5" newState="walk" actionEventType="controller_up" velocity="controller_up_distance"/>

</state>

<state name="walk" animation="walk">

<transition name="walk-idle" type="fade" timeSpan="0.5" newState="idle" actionEventType="controller_centre"/>

<transition name="walk-run" type="fade" timeSpan="0.5" newState="run" actionEventType="controller_up,controller_trigger"/>

</state>

Is this how it is generally done?  I can see it working this way and if any extra game logic is required, it can perhaps be hard-coded based on actionEventType.  An example of this might be to add some IK for feet touching the ground when a player walks or runs.

Thanks

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You indeed have shared data and unique data per character per anim graph node.

The play offset would be unique as it is different per character, while the transition time will be shared. I do not clone the actual full state machines though, although it would also be possible, but less memory efficient as you have a lot of shared data.

The game is setting parameter values, for example a "button_up_pressed" parameter could be a boolean. Then the state machine can make a transition when that button is pressed using some condition that checks for that boolean to become true. At least that is one way to do it.

Your xml would work to describe a simple state, although there can be more state types, not just with an animation.

Also you need a set of conditions on each transition, rather than just an actionEventType condition. So I would make that more flexible there.

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Firstly, I obviously need some form of instance data with regard to my states, or at least the current state I'm in.  For example, I have my player character and as an entity, he has a component which is a state machine.  He is idling and so the current state in his state machine is the idle state.  In that state, i'll need to have some record of the local time for the idle animation that is playing.  That's all well and good, but if I have two players on the screen, would they each have their own state machine?  I'm wondering whether I keep the data seperate from the structure of the state machine although I'm not really sure how that would work.  A lot of the data in the state machine states would be common (fade time, fade type, animation clip name), it's only animation clip positions that would need to be stored, so copying the whole structure seems like a waste of memory.  Also, if a state uses a blend tree, there may be lerp positions that needs to be kept track of.

I also separate static and dynamic data. That is useful not only here but in general (i.e. also for skeletons).

Secondly, from an animation perspective, I understand the states and transitions that can occur but what should control the state machine?  I mean if it's entirely data-driven (which I intend it to be), some game logic somewhere has to know that when the player is in the idle state, he can walk or run (for example).  I know the data-driven state machine can represent this easily, but how would the game logic control that?  My initial thought was that the current state of each state machine would handle events so,  for example, when the user presses the up button on the controller, that event is fed through to the player's state machine (it's a component after all which can receive events) and the state knows (through data) that in this instance, pressing up in the idle state transitions to the walk forward state.

As mentioned by Buckshag, the state machine as well as the various blend trees are controlled by a set of variables (of course, these variables count to the dynamic data). Nothing inside the animation system should directly investigate input or observe events. Words like "button_up" in animation transition conditions hint at a design flaw.

Input should be decoupled because the animation system by itself is not necessarily player driven. The same animation system can perhaps be used to animate a NPC. What often is called a player controller investigates input and reacts by setting variables if a specific input situation is detected. These variables are an abstraction and have a meaning w.r.t. the character, e.g. they mean "walk forward with speed x", "jump", "pick up that object", and so on, but never "button x is pressed". This helps not only to keep the animation system clean, but also to have a dedicated place (outside the animation system, just to emphasize this once more) to deal with input configurations.

Regarding event observation (although not explicitly mentioned in the question): The animation system is called at one or more specific moments during looping though the game loop. When the animation system's update() is called, its the natural moment for state transitions to be checked. There is not only no need in pushing events into the state machine, it is even counterproductive to do so.

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Thanks again guys.

I was using "controller_up " as an example, I already have a decoupled architecture for input, but it's a very valid point.

Now the thing is, for my player animation design, pressing up on, say, the left thumbstick does a different movement or action depending on which state you're in. For example, when you're stood on your snowboard idling, pressing the left thumbstick forward makes the player shuffle along to start moving. When the player is jumping, pressing the left thumbstick makes him tilt forward, and so on.

So I have to have some game logic somewhere that has this information, whether it be data-loaded or script-based, but isn't a data-driven state machine the perfect place for this? So instead of passing an event as you've mentioned, the current state looks at its loaded data and checks the state of the [decoupled] input flags. (Let's assume that the player can configure their controller as they see fit and that I have decoupled the physical sticks and buttons from the internal representation)

So for example, in the idle state, we may have loaded up the following simplistic data:

<state name="idle_board_on">
<transition name="idle_board_on-shuffle_forward" triggeredBy="leftstick_up"/>
</state>
<state name="jump_board_on">
<transition name="jump_board_on-tilt_forward" triggeredBy="leftstick_up"/>
</state>

I've missed out the rest of the attributes for brevity but hopefully that makes sense?

If I need to decouple this from the state machine completely then it feels like it may be duplicating logic elsewhere?

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Now the thing is, for my player animation design, pressing up on, say, the left thumbstick does a different movement or action depending on which state you're in. For example, when you're stood on your snowboard idling, pressing the left thumbstick forward makes the player shuffle along to start moving. When the player is jumping, pressing the left thumbstick makes him tilt forward, and so on.

A state machine works this way: One of its states is active. On each update of the state machine all outgoing transitions of the active state are investigated whether their conditions are true. At most one transition has to be chosen. If none is chosen then the state remains active, otherwise the state which is referred by the transition is the next active state. The condition is a boolean expression.

With this in mind, each of the states "idling" and "jumping" (taken from your post above) have their own transitions, and hence their own conditions, and hence may have different following states. Moreover, due to the separate states, the input configuration can bind one input trigger to different variables.

So a state definition like your example would be:

<state name="idle_board_on" initial="true">
<transition name="idle_board_on-shuffle_forward" triggeredBy="shuffle" follower="shuffle_forward" />
<!-- here more transitions outgoing from idle_board_on" -->
</state>
<state name="jump_board_on">
<transition name="jump_board_on-tilt_forward" triggeredBy="tilt" follower="tilt_forward" />
<!-- here more transitions outgoing from jump_board_on" -->
</state>
<!-- here more states -->

I don't see a problem here.

Edited by haegarr

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Sorry, yes, I had missed out the next state in each transition node for brevity.

I can't really use a triggeredBy of "shuffle" though, because I'd need some other logic completely separate from the state machine that says, pressing left stick up only results in "shuffle" being set to true if you're idling, otherwise it results in "tilt_forward" being set. My point is, this logic could be in the state machine for free, otherwise I'd need some other similar logic to do that.

Sorry if I'm labouring a point a bit, I appreciate your input

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You misunderstand my posts, I think.

You have an input configuration that provides (besides others) the abstract inputs shuffle and tilt. The both are shown in the input configuration dialog as triggers. The player picks "gamepad" as input device and from that "controller up". He does so for both abstract inputs. This is possible because both abstract inputs are in distinct spaces. That means it is logically not possible to trigger both actions shuffle and tilt as the same time. The input configuration is aware of this and hence allows this duplicate binding.

When the game runs and the player triggers the "controller up" input, both the variables for shuffle and tilt are set to true by the player controller. Now comes the clue. If the state machine is currently in the state idle_board_on, there is a transition with the condition considering shuffle. Since shuffle is true also the condition is true and the transition is chosen. There is also a state jump_board_on with a condition considering tilt, but because jump_board_on is not current, it plays no role even its condition is true, too!

You see, the same physical input is used for both abstract inputs, but things work well. What exactly is meant when "controller up" is triggered is "the player asks for shuffle or tilt". What actually happens depends on the current state of the state machine. It may even happen that the trigger causes nothing because the transitions of the current state do consider neither shuffle nor tilt.

EDIT:Maybe the word "transition" is unclear. In a state machine a transition leads from one state to another. This has nothing to do with a time based blending between two animations, i.e. a kind of visual transition.

Edited by haegarr

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No I understand, my point is that I'm not sure i see the benefit of having the player controller set both "shuffle" and "tilt" to true. You get the same result if you just set "leftstick_up" to true. "Leftstick_up" here is abstract, so in the player's controller configuration it could actually physically equate to pushing the right stick up or pressing the trigger, etc.

Using only "leftstick_up" should still work because both states check this flag and as obviously only one state can be active, it would initiate the appropriate transition from that state.

Using both "shuffle" and "tilt" means that somewhere else in the code (perhaps the player controller) I need to set both of those when the [abstracted] left stick is pushed. For that to be hard-coded seems wrong and if it's to be data-driven, it would mean we've effectively got duplicated logic.

I understand the transition part of state machines, been doing a lot of research :-)

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It doesn't matter how you name those parameters really. It comes down to the same, just like you say :)

It is just important to keep in mind that your game code will set the parameter values, which the conditions will check.

I would in your case just have one parameter "forward" or so.

And depending on the control scheme in your game you can set that value to either true or false. If you use a game controller check the stick. If you use keyboard, check the cursor up key for example. The state machine doesn't know about the keyboard or game controller, just about the parameter "forward".

Both the outgoing transition condition from idle to shuffle and from jump to lean forward can check the same "forward" parameter.

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Making some pretty good progress with this now, thanks again guys

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