Animating the player model: a complete guide to Modify Model Parts (Power Type) • Handbook Blog

Modify Model Parts (Power Type) is the power that lets a data pack pose and animate the player. It is also the one people bounce off hardest, because the field that looks like “how long the pose lasts” is not that at all.

This is the long version. If you only read one section, read Duration is not how long the pose lasts.

The shape of the power

The power is a list. Each entry targets one part and one property:

{
  "type": "apoli:modify_model_parts",
  "transformations": [
    {
      "model_part": "head",
      "type": "pitch",
      "value": 0.45
    }
  ]
}

That is the whole power. A pose is just a lot of those entries — one per part per axis. The “Hero Pose” at the end of this post is twenty of them.

There is no duration on the power, no timeline, no clip. Every entry is independent, and every entry is live for exactly as long as the power itself is active.

The parts

head, hat, body, right_arm, left_arm, right_leg, left_leg.

Names are matched loosely — underscores, spaces, hyphens and case are all ignored, so right_arm, rightArm and RIGHT ARM are the same part. A handful of aliases are also accepted: torso and jacket mean body, headwear means hat, right_sleeve means right_arm, left_pants means left_leg, and so on.

On a player, editing a base part also moves its skin-overlay layer. You do not need a second entry for the jacket or the sleeve.

The properties

`type`	Unit	What it does
`pitch`	radians	Rotation around X — nodding, arms swinging forward and back.
`yaw`	radians	Rotation around Y — turning left and right.
`roll`	radians	Rotation around Z — tilting sideways.
`pivot_x` / `pivot_y` / `pivot_z`	model units	Moves the part. 16 units = one block.
`x_scale` / `y_scale` / `z_scale`	multiplier offset	Added to the part’s normal scale of `1.0`, so `0.5` means 1.5× and `-0.5` means half size.
`visible`	`0` or `1`	Shows or hides the part outright.
`hidden`	`0` or `1`	Sets the part’s skip-draw flag.

Rotations are radians, not degrees. π ≈ 3.14159 is a half turn, so a right angle is about 1.5708 and a gentle 25° tilt is about 0.45. Multiply degrees by 0.01745 if you think in degrees.

Which way is which

Player-model axes are not world axes, and this is where most of the trial-and-error goes. From vanilla’s own HumanoidModel:

Axis	Positive direction
`pivot_x`	toward the player’s left (the right arm sits at x = −5)
`pivot_y`	down (the legs sit at y = 12, below the body at y = 0)
`pivot_z`	backward (crouching pushes the legs to z = 4)

So "pivot_y": 9 drops a part nine units — over half a block — and "pivot_z": -4 pushes it four units forward. If something moves the wrong way, negate it; that is not a bug, it is the model’s coordinate system.

Duration is not how long the pose lasts

This is the one that trips everyone.

{
  "model_part": "head",
  "type": "pitch",
  "value": 0.45,
  "duration": 5
}

duration: 5 does not mean “hold this pose for 5 ticks”. It means “take 5 ticks to blend into this pose”. The pose then stays until the power stops being active. fade_out_duration is the mirror: how many ticks to blend back out afterwards, defaulting to whatever duration is.

What controls how long the pose lasts is the power’s condition. While the condition passes, the pose holds. When it fails, the fade-out starts.

That is exactly the pattern behind the Hero Pose below: a resource counts down, the power’s condition is resource > 0, and an Action Over Time (Power Type) ticks the resource down. duration: 5 and fade_out_duration: 5 only make the entry and exit smooth. The length of the pose is however long the resource stays above zero.

Leave duration at its default of 0 and the pose snaps on instantly. That is right for things like visible, and wrong for almost every rotation.

The two blend modes

Every entry blends with vanilla’s own animation, and override_animation decides how:

override_animation: false (the default) — your value is added to whatever vanilla is doing. The arm keeps swinging as you walk; your rotation rides on top.
override_animation: true — your value replaces vanilla’s. The arm stops swinging and goes exactly where you put it.

For a deliberate pose you almost always want true, otherwise walking will smear it. For an effect that should read as “on top of” normal movement — a permanent slight head tilt, a limp — you want false.

override_animation only affects pitch, yaw and roll. Pivots and scales are always relative: a pivot is added to the part’s position and a scale is added to 1.0, whatever you set override_animation to. visible and hidden ignore blending entirely and snap once the fade is more than halfway.

Easing

easing shapes the fade in and out, and doubles as the default curve between keyframes. The full set:

linear, step, catmullrom, smoothstep, smootherstep, and ease_in_* / ease_out_* / ease_in_out_* for sine, quad, cubic, quart, quint, expo, circ, back, elastic and bounce.

In practice:

Want	Use
A pose that arrives naturally	`ease_out_quad` or `smoothstep`
A pose that winds up then snaps	`ease_in_cubic`
An impact that overshoots slightly	`ease_out_back`
A bouncy, cartoonish landing	`ease_out_bounce`
A snap with no blending at all	`step`
Smooth motion through many keyframes	`catmullrom`

catmullrom is special: instead of interpolating between two keyframes it fits a curve through the surrounding four, so a chain of keyframes flows instead of hitching at each one. It is what you want for an idle loop.

Keyframes

Give a transformation a keyframes list and it animates on its own instead of holding one value.

{
  "model_part": "right_arm",
  "type": "pitch",
  "override_animation": true,
  "keyframes": [
    {
      "time": 0,
      "value": 0
    },
    {
      "time": 4,
      "value": -2.4,
      "easing": "ease_out_quad"
    },
    {
      "time": 10,
      "value": 0.3,
      "easing": "ease_in_cubic"
    },
    {
      "time": 14,
      "value": 0,
      "easing": "ease_out_back"
    }
  ]
}

time is in ticks, counted from the moment the transformation became active. 20 ticks = one second.
value is that keyframe’s target, in the same unit as the type.
easing on a keyframe applies to the segment ending at it. So the curve on the time: 4 frame governs the 0 → 4 stretch. Without one, the transformation’s own easing is used.
Keyframes are sorted by time for you, so you can write them out of order.
Before the first keyframe the animation sits on the first value; after the last it holds the last value.

With keyframes you do not need value, a transformation is valid with either a value or a non-empty keyframes list. duration and fade_out_duration still apply, and still mean fade in and fade out, layered on top of whatever the keyframes are doing.

Looping

"loop": true wraps time back around between the first and last keyframe. A three-second idle sway is a loop from time: 0 to time: 60.

Make the first and last values identical or the loop will jump. This is the single most common keyframe mistake.

Five worked examples

1. A permanent tweak

Half-size head. No animation, no blending — it just is.

{
  "type": "apoli:modify_model_parts",
  "transformations": [
    {
      "model_part": "head",
      "type": "x_scale",
      "value": -0.5
    },
    {
      "model_part": "head",
      "type": "y_scale",
      "value": -0.5
    },
    {
      "model_part": "head",
      "type": "z_scale",
      "value": -0.5
    }
  ]
}

Remember scales are offsets from 1.0, so -0.5 is half size and 1.0 would be double.

2. A conditional pose that eases in and out

Arms out to the sides whenever the holder is sprinting.

{
  "type": "apoli:modify_model_parts",
  "condition": {
    "type": "apoli:sprinting"
  },
  "transformations": [
    {
      "model_part": "right_arm",
      "type": "roll",
      "value": 1.4,
      "override_animation": true,
      "duration": 6,
      "fade_out_duration": 8,
      "easing": "ease_out_quad"
    },
    {
      "model_part": "left_arm",
      "type": "roll",
      "value": -1.4,
      "override_animation": true,
      "duration": 6,
      "fade_out_duration": 8,
      "easing": "ease_out_quad"
    }
  ]
}

Start sprinting and the arms take 6 ticks to swing out; stop and they take 8 to come back. The pose lasts as long as you sprint — duration had nothing to do with it.

3. A looping idle

A slow sway, running forever while the power is active.

{
  "type": "apoli:modify_model_parts",
  "transformations": [
    {
      "model_part": "body",
      "type": "roll",
      "loop": true,
      "easing": "catmullrom",
      "duration": 10,
      "keyframes": [
        {
          "time": 0,
          "value": 0
        },
        {
          "time": 20,
          "value": 0.06
        },
        {
          "time": 40,
          "value": -0.06
        },
        {
          "time": 60,
          "value": 0
        }
      ]
    }
  ]
}

First and last values are both 0.0, so it cycles cleanly. catmullrom keeps the motion flowing through the middle keyframes instead of easing to a stop at each one. override_animation is left off, so the sway rides on top of walking rather than replacing it.

4. A one-shot swing

Fire this with a resource that counts down, the same way the Hero Pose works — the keyframes handle the motion, the resource decides when the power switches off.

{
  "type": "apoli:modify_model_parts",
  "condition": {
    "type": "apoli:resource",
    "resource": "example:swing_timer",
    "comparison": ">",
    "compare_to": 0
  },
  "transformations": [
    {
      "model_part": "right_arm",
      "type": "pitch",
      "override_animation": true,
      "fade_out_duration": 4,
      "keyframes": [
        {
          "time": 0,
          "value": 0
        },
        {
          "time": 3,
          "value": -2.6,
          "easing": "ease_out_quad"
        },
        {
          "time": 9,
          "value": 0.6,
          "easing": "ease_in_cubic"
        },
        {
          "time": 13,
          "value": 0,
          "easing": "ease_out_back"
        }
      ]
    },
    {
      "model_part": "body",
      "type": "yaw",
      "override_animation": true,
      "fade_out_duration": 4,
      "keyframes": [
        {
          "time": 0,
          "value": 0
        },
        {
          "time": 3,
          "value": 0.4,
          "easing": "ease_out_quad"
        },
        {
          "time": 9,
          "value": -0.2,
          "easing": "ease_in_cubic"
        },
        {
          "time": 13,
          "value": 0
        }
      ]
    }
  ]
}

The arm winds up fast, throws through, and settles with a slight overshoot from ease_out_back. Rotating the body slightly on the same beats is what makes it read as a real swing rather than a detached arm.

5. The Hero Pose

This one is real — it came from a pack author who built it with no keyframes at all, using nothing but duration, fade_out_duration and a resource to hold it. It is a good demonstration that you do not need the animation system to get something that looks animated: twenty static entries, all easing in over 5 ticks, is a pose.

{
  "type": "apoli:modify_model_parts",
  "condition": {
    "type": "apoli:resource",
    "resource": "example:pose_timer",
    "comparison": ">",
    "compare_to": 0
  },
  "transformations": [
    {
      "model_part": "head",
      "type": "pitch",
      "value": 0.45,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "head",
      "type": "pivot_y",
      "value": 9,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "head",
      "type": "pivot_z",
      "value": -4,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "body",
      "type": "pitch",
      "value": 0.9,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "body",
      "type": "pivot_y",
      "value": 9,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "body",
      "type": "pivot_z",
      "value": -4,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_arm",
      "type": "pitch",
      "value": -0.4,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_arm",
      "type": "roll",
      "value": 1.1,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_arm",
      "type": "pivot_x",
      "value": -0.3,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_arm",
      "type": "pivot_y",
      "value": 11,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_arm",
      "type": "pivot_z",
      "value": -4,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "left_arm",
      "type": "roll",
      "value": -0.25,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "left_arm",
      "type": "pivot_y",
      "value": 11,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "left_arm",
      "type": "pivot_z",
      "value": -4,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_leg",
      "type": "pitch",
      "value": 1.1,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_leg",
      "type": "yaw",
      "value": -0.1,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "right_leg",
      "type": "pivot_y",
      "value": 5,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "left_leg",
      "type": "pitch",
      "value": 0.1,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "left_leg",
      "type": "yaw",
      "value": 0.2,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    },
    {
      "model_part": "left_leg",
      "type": "pivot_z",
      "value": 0.4,
      "override_animation": true,
      "duration": 5,
      "fade_out_duration": 5
    }
  ]
}

Notice the structure, because it generalises to any pose:

Every entry has the same duration and fade_out_duration. That is what makes twenty separate edits arrive as one movement instead of twenty. If you give different parts different durations you get a ripple — which is a great effect, deliberately, and a mess by accident.
Every entry sets override_animation: true. A pose has to beat walking, or it comes apart the moment you move.
Body and head share pivot_y: 9 and pivot_z: -4. Moving a parent-ish group together keeps the silhouette intact; moving the body without the head detaches them.
The arms sit at pivot_y: 11, lower than the torso’s 9. Small offsets between parts are what stop a pose from looking like a mannequin.
Nothing here is a keyframe. The entire animation is the 5-tick ease in and out.

To drive it, pair it with a resource and an action over time:

{
  "type": "apoli:action_over_time",
  "interval": 1,
  "entity_action": {
    "type": "apoli:change_resource",
    "resource": "example:pose_timer",
    "change": -1
  }
}

Set example:pose_timer to 40 and you get a two-second pose that eases in, holds, and eases out.

Things that go wrong

The pose flickers or fights the walk cycle. Add override_animation: true. Without it your value is added to vanilla’s animation, so walking drags the pose around.

The pose snaps instead of easing. duration is 0 by default. Give it 4–8 ticks.

The pose never goes away. Whatever holds the power active is still true. duration will not end it — check the condition.

A loop jumps every cycle. The first and last keyframe values differ. Make them equal.

The part moves the wrong way. Negate the value. pivot_y is down, pivot_z is backward, pivot_x is toward the player’s left.

A scale of 1.0 made it double-size, not normal-size. Scales are offsets added to 1.0. Use 0 for normal.

override_animation did nothing on a pivot or scale. It only applies to pitch, yaw and roll. Pivots and scales are always relative.

Nothing renders at all. This is a client-side power on biped models only. It does nothing on non-humanoid mobs, and the holder needs to actually be rendered — check third person.

Performance

Each animated entity is sampled once per frame, not once per part, and the result is shared by every part that frame. A pose with twenty entries costs about as much as a pose with one.

The part that does cost you is the number of entities wearing one. A looping animation on a mob that thirty players can see is thirty sample sets. Prefer conditions that are false most of the time — a pose that only exists while a resource is above zero is free the rest of the time, because the power’s slots are dropped entirely once they finish fading out.

The shape of the power#

The parts#

The properties#

Which way is which#

Duration is not how long the pose lasts#

The two blend modes#

Easing#

Keyframes#

Looping#

Five worked examples#

1. A permanent tweak#

2. A conditional pose that eases in and out#

3. A looping idle#

4. A one-shot swing#

5. The Hero Pose#

Things that go wrong#

Performance#