pDirectionalForce

pDirectionalForce is Fusion's basic force node for applying one consistent directional influence. A typical chain is pEmitter → pDirectionalForce → pRender: the emitter creates particle data, the force changes motion, and pRender converts the stream into visible pixels.

The primary controls are Strength, Direction and Direction Z. Strength determines force magnitude. Direction specifies the heading in X/Y space, while Direction Z orients the force through depth, so a 3D particle setup should be checked from more than the front Viewer.

Positive Strength moves particles in the direction defined by Direction/Direction Z. Negative Strength is documented to reverse the influence and move particles in the opposite direction. A negative value therefore does more than merely reduce a positive force.

Blackmagic uses gravity as the canonical pDirectionalForce example. The default XY direction points down the Y axis at about -90 degrees. Strength still has to be tuned to the emitter velocity, particle lifespan, scene scale and composition duration.

A force develops through the simulation over time. On an early still frame, a small Strength may look almost identical to Strength = 0, while later frames diverge clearly. Judge force nodes across playback or several fixed frames rather than one still.

pDirectionalForce is not the same as Velocity in pEmitter. Velocity gives a particle its initial motion at birth; Directional Force continues influencing existing particles on subsequent frames for as long as Conditions and Region allow it.

Conditions filter the particle stream before the force is applied. Probability = 1 gives every eligible particle a 100 percent chance. Lower values are evaluated for each particle on each frame rather than selecting one fixed subset for the entire lifespan.

Age Start/End are normalized fractions of particle lifespan, not timeline frame numbers. A range of 0.5–1.0 applies the force only during the second half of life whether a particle lives for 20 frames or 200.

Set Mode and Set 1–32 target particle groups that were assigned upstream. The checkboxes in pDirectionalForce do not create new Sets; sets are assigned by particle-producing or set-changing tools such as pEmitter, pImageEmitter, pChangeStyle or pSpawn.

Region Mode defines the spatial/event condition relative to the selected Region: the force can ignore the region, act while intersecting or inside/outside it, or react when entering/leaving. Region separately selects the geometry used for that condition.

The Region menu contains built-in shapes and external-source modes. All, Bezier, Cube, Line, Rectangle and Sphere are generated inside the node, while Bitmap and Mesh require external geometry and therefore create a separate dynamic Region input in the Flow.

Only the orange Particles input exists by default. Bitmap exposes a Region input for a 2D image/selectable channel; Mesh exposes the Region input for a 3D mesh. The reference notes that the socket can appear green or magenta depending on which mode was selected first.

Line Region is useful for diagnosis because Start/End endpoints expose X/Y/Z offsets and can be linked to Paths or Trackers. This makes it possible to build a moving boundary inside which, or on crossing which, directional force becomes active.

Rectangle is a flat region without Z depth, while Sphere and Cube are volumetric. In a 3D particle scene, matching X/Y projection does not guarantee that a particle is actually inside the Region; inspect Z and multiple Viewer orientations.

The Resolve 21 reference contains an editorial inconsistency: the common section says there are seven Region types but then lists All, Bezier, Bitmap, Cube, Line, Mesh, Rectangle and Sphere—eight entries. This article keeps the explicitly listed inventory and still treats live target-build verification as the production gate.

pDirectionalForce combines well with pTurbulence: Directional Force establishes the large-scale flow while Turbulence breaks up mechanical uniformity. During debugging, disable other forces first so the source of trajectory changes stays obvious.

For wind, gravity and simple drift, begin with a minimal chain, Probability = 1, full Age, Ignore Sets and Ignore region. Once the baseline is understood, add Sets, Age gating, Region and turbulence one at a time.

pDirectionalForce output remains particle data. Even when Bitmap supplies a Region image, that does not turn the output into RGBA and the Bitmap is not an ordinary Effect Mask. Visible pixels appear only downstream at pRender.