Concepts

MotionFX has a handful of moving parts. Once you know them, every preset reads like a recipe.

Mental model

Effect=named list of Tracks+timing / stacking rules

Track=one Primitive+one Driver

Driver=produces a value over time

Handle=what Play() returns: fade, retarget, stop

Primitives are the eight things an effect can change: FOV, Position, Rotation, Blur, Color, Vignette, Sound and Overlay. See Primitives & Tracks.
Drivers are factories. Driver.Start(context) returns a fresh sampler (t, dt) -> value, so stateful drivers like noise and springs get new state on every Play(). One definition can run many times at once. See Drive.
Effects never touch the camera. Each primitive has a channel. The channel collects every active contribution, combines them, and applies the result once per frame.

Frame pipeline

The runtime binds two RenderStep callbacks around Roblox's own camera script:

Priority	Step	What happens
`Camera - 1`	`CameraRig.Strip()`	Restores the CFrame the camera script produced last frame. Scripts that read `camera.CFrame` back never see or compound the shake. Also adopts FOV changes made by other code.
`Camera`	Roblox camera script	Runs normally. Mouse, gamepad and touch input all still drive it.
`Camera + 1`	`step(dt)`	Samples every driver, then channels combine and apply. The view picks the base placement and this frame's offset goes on top.

The loop is bound only while an effect is running, or while a view or smoothing is active. When everything finishes, channels release their instances (BlurEffect, ColorCorrectionEffect, vignette GUI), the camera is reset, and the loop unbinds.

How channels combine

Primitive	Combine rule	Applied to
`FOV`	Sum of degree deltas	Added to the base `FieldOfView`
`Position`	Sum of `Vector3`	Camera-space offset (studs)
`Rotation`	Sum of `Vector3`	Euler offset in degrees (pitch, yaw, roll)
`Blur`	Max, so the strongest wins	Its own `BlurEffect` in Lighting
`Color`	Tints multiply. Saturation, Contrast and Brightness add.	Its own `ColorCorrectionEffect`
`Vignette`	Grouped into layers by Inset and Image. The strongest wins within a layer, and layers stack.	A `ScreenGui` in PlayerGui
`Sound`	Not combined. Each track spawns its own `Sound`.	`SoundService`
`Overlay`	Not combined. Each track shows its own flipbook image.	A ScreenGui in `PlayerGui`

Intensity and fades

Each frame, every non-sound track's value is scaled toward neutral before it reaches the channel:

contribution = Scale(value, handle.Intensity * fade * stopMultiplier * track.Weight)

handle.Intensity is the master scale. It is set by PlayOptions.Intensity and changed by handle:SetIntensity(value, fadeTime).
fade ramps in over FadeIn seconds. For effects with a fixed duration, it also ramps out over the last FadeOut seconds.
stopMultiplier ramps from 1 to 0 over the fade time passed to Stop(fadeTime).
Scaling is type-aware. Numbers and vectors multiply. A ColorValue tint fades toward white. A VignetteLayer fades only its opacity.

Sounds fade on their own.

The effect-level FadeIn/FadeOut don't apply to sound tracks. A sound uses its own FadeIn and FadeOut, scaled by intensity and the stop fade.

Retrigger policies

EffectDefinition.Retrigger decides what happens when you play an effect while an instance of it is already running:

Policy	Behaviour	Returns
`"Stack"` default	Runs both, and their contributions add up. When there are already `MaxInstances` (default 8), the oldest fade out over 0.1 s.	New handle
`"Refresh"`	Restarts the existing instance in place with the new options. Its sounds are cut with a 0.05 s fade and restart.	The existing handle
`"Replace"`	Fades out the old instances with their default fade, then starts a new one.	New handle
`"Ignore"`	Does nothing.	The running handle

Duration inference

An effect's length is resolved in this order:

PlayOptions.Duration
EffectDefinition.Duration
Inferred: the longest driver.Duration + track.Delay.

If any driver is open-ended (Constant, Noise, Bind, Cycle, or a combination that contains one), or a sound is Looped, the inferred duration is nil. The effect then runs until you stop it with handle:Stop(), MotionFX:Stop(name) or MotionFX:StopAll().

Finite effects end cleanly.

When a driver finishes, it holds its neutral or end value. Effects fade because their values return to zero, so drivers need no special release logic.

Validation

MotionFX:Create validates definitions up front, so mistakes fail early with a readable message instead of surfacing mid-frame. Examples are a misspelled primitive, a raw number where a driver belongs, a negative Delay, or an unknown Retrigger. Tracks returned by Build are validated each time Build runs.

MotionFX "Heartbeat", track #2: Driver must be built with MotionFX.Drive (got number)