The Surface Force operator allows you to add forces to particles using the surface properties of scene geometry.

**Object list**: the list of geometry input objects.**Sample type**: controls which sampler will be used to determine closest-object proximities for particles.**Mesh selection**: controls which meshes that forces will be derived from.

All: the closest mesh from the list will be used.Random: a random mesh from the list will be used.Custom Float as Index: a custom channel value will be used as a list index to determine which object’s mesh to use.

**Channel**: the custom float data channel to use.**Relative to mass**: the amount of force applied will be relative to a particle’s mass.**Simulate substeps**: forces will be interpolated in a way that simulates the addition of forces at smaller simulation substeps.

**Seed**: the seed value for all varied parameters.

Attraction forces are forces which are directed from the particle location to the closest point on the target surface.

**Force:**: the amount of attraction force.**Variation %**: the per-particle percentage of variation to apply.**Divergence**: the degrees of random divergence to apply to the resulting force.**Accel %**: controls how quickly particle forces will accelerate from their old values to the newly applied values.**Above/Below**: controls whether forces will be applied above/below surface normals.**Clamp to Distance**: if the distance between a particle and the surface is less than the magnitude of the attraction force, the attraction force magnitude will be clamped to that distance.

The distance affect extends outwards from the affecting surface.

**Distance**: particles within this distance will be fully affected.**Falloff**: the effect on particles beyond the base distance, but within this falloff distance, will diminish according to the inverse-square law.

Follow forces are forces which are parallel to the target surface.

**Force:**: the amount of follow force.**Variation %**: the per-particle percentage of variation to apply.**Divergence**: the degrees of random divergence to apply to the resulting force.**Accel %**: controls how quickly particle forces will accelerate from their old values to the newly applied values.

The distance affect extends outwards from the affecting surface.

**Distance**: particles within this distance will be fully affected.**Falloff**: the effect on particles beyond the base distance, but within this falloff distance, will diminish according to the inverse-square law.

Normal forces are forces which extend along the normal of the closest point on the target surface.

**Force**: the amount of normal force.**Variation %**: the per-particle percentage of variation to apply.**Divergence**: the degrees of random divergence to apply to the resulting force.**Accel %**: controls how quickly particle forces will accelerate from their old values to the newly applied values.

The distance affect extends outwards from the affecting surface.

**Distance**: particles within this distance will be fully affected.**Falloff**: the effect on particles beyond the base distance, but within this falloff distance, will diminish according to the inverse-square law.

Motion forces are forces which are derived from the nearest surface velocity of the target surface.

**Force**: the amount of motion force.**Variation %**: the per-particle percentage of variation to apply.**Divergence**: the degrees of random divergence to apply to the resulting force.**Accel %**: controls how quickly particle forces will accelerate from their old values to the newly applied values.

The distance affect extends outwards from the affecting surface.

**Distance**: particles within this distance will be fully affected.**Falloff**: the effect on particles beyond the base distance, but within this falloff distance, will diminish according to the inverse-square law.

Texture forces are forces which are derived from texture map values sampled from the closest point on the target surface.

**Texmap**: the texmap to sample colors from.**Surface offset as W**: the distance from the particle to the closest surface will be used as the W coordinate in the UVW value which is used to sample the texture.**UVW Radius**: the samples radius (in UVW coordinates) to derive the gradient from.**Accuracy**: the number of points to sampled in order to derive the gradient.

RGB values sampled at the closest point on the surface to the particle will be used a world-space force vector.

**Force**: the amount of RGB force.**UV-to-World space conversion**: when enabled, converts RGB values from UV space to world space. Disable this setting if RGB values already represent world-space force vectors.

Monochrome intensity values sampled around the closest point on the surface to the particle will be used to derive a directional gradient, transformed from UVW-space into world-space as a force vector.

The direction of a gradient ascent force is from darker parts of a texture map to lighter parts of a texture map. If the monochrome intensity values in a particular area of the texture map are perfectly uniform, no force will be derived from that area.

**Force**: the amount of ascent force.

Monochrome intensity values sampled around the closest point on the surface to the particle will be used to derive a directional gradient. A perpendicular vector to that gradient will then be transformed from UVW-space into world-space as a force vector.

The direction of a gradient trace force is around the borders of light/dark features of a texture map. If the monochrome intensity values in a particular area of the texture map are perfectly uniform, no force will be derived from that area.

**Force**: the amount of trace force.

Monochrome intensity values sampled around the closest point on the surface to the particle will be used to dampen accumulated Surface Force velocities. For example, a velocity of [1,0,0] will be changed to [0.75, 0, 0] if the sampled monochrome value is 0.75 and “affect” is set to 100%.

**Affect %:**the amount of influence that the grayscale force multiplier will have on accumulated Surface Force velocities.

**Variation %**: the per-particle percentage of variation to apply.**Divergence**: the degrees of random divergence to apply to the resulting force.**Accel %**: controls how quickly particle forces will accelerate from their old values to the newly applied values.

The distance affect extends outwards from the affecting surface.

**Distance**: particles within this distance will be fully affected.**Falloff**: the effect on particles beyond the base distance, but within this falloff distance, will diminish according to the inverse-square law.

**Face/Vertex normals**: controls which surface normals will be used to compose the UV-to-World conversion matrix. Face normals are better for segmented/flat surfaces, Vertex normals are better for smooth/curved surfaces.

Steering forces are forces which steering particles towards the cross product between the particle’s velocity and the nearest surface normal.

**Angle**: the amount of steering force to apply to a particle’s direction of travel.**Variation %**: the per-particle percentage of variation to apply.**Randomize direction**: controls whether the steering force can be on either side of the particle’s velocity vector, relative to the nearest surface normal.**Seed by time**: steering force variation will be reseeded at each time step.

The distance affect extends outwards from the affecting surface.

**Distance**: particles within this distance will be fully affected.**Falloff**: the effect on particles beyond the base distance, but within this falloff distance, will diminish according to the inverse-square law.