xpFlowField

xpFlowField enables you to create custom velocity fields to affect your particles.
xpFlowField 使您能够创建自定义速度场来影响您的粒子。

A flow field uses an array of vectors to control the movement of particles within.
流场使用矢量阵列来控制其中粒子的运动。

It is especially useful for such things as chaotic movement, fluid movement, movement controlled by procedural formula and noise-controlled movement.
它对于混乱的运动、流体的运动、由程序公式控制的运动和噪声控制的运动等特别有用。

A flow field is an area of 3D space, divided up into a grid of smaller cubes or cells.
流场是 3D 空间的一个区域，被划分为由较小立方体或单元组成的网格。

Each cell contains a parameter which is a velocity vector: a direction and speed of movement.
每个单元包含一个参数，即速度矢量：运动的方向和速度。

When a particle enters a cell, its direction is transformed to match that of the vector in the cell.
当粒子进入细胞时，其方向会转变为与细胞中的矢量方向相匹配。

As it moves through the field it will enter other cells containing velocity vectors, that change its movement again; this will continue until the particle leaves the field.
当它穿过场时，它将进入包含速度矢量的其他单元，这些单元再次改变其运动；这将持续直到粒子离开场。

The crucial factor in a velocity field is the arrangement of the vectors; these could be completely random or they could force the particle to follow a path.
速度场中的关键因素是矢量的排列；这些矢量可能是完全随机的，也可能迫使粒子遵循一条路径。

xpFlowfield offers a number of ways to set these vectors.
xpFlowfield 提供了多种设置这些向量的方法。

General tab 常规选项卡

General tab settings menu.
常规选项卡设置菜单。

Enabled 已启用

On, by default, this box can be unchecked to disable xpFlowField.
默认情况下，可以取消选中此框以禁用 xpFlowField。

xpFlowField enabled, with the Layers setting of Random
启用 xpFlowField，并将 Layers 设置为 Random.

Layers 图层

To add layers to this list, select the layer type from the Add menu button below the list; you can add as many layers as you wish.
要将图层添加到此列表，请从列表下方的添加菜单按钮中选择图层类型；您可以根据需要添加任意数量的图层。

At any point you can temporarily disable a layer by unchecking the box on its left.
您可以在任何时候通过取消选中左侧的框来暂时禁用某个图层。

Each layer has a different set of options.
每一层都有一组不同的选项。

To edit these options, click a layer to select it, and then they will appear in the Layer Options section.
要编辑这些选项，请单击一个图层以选择它，然后它们将出现在 “图层选项” 部分。

You can also reorder the layers by dragging them above or below other layers; for blending purposes the layer order is important.
您还可以通过将图层拖到其他图层上方或下方来重新排序图层；对于混合目的而言，图层顺序很重要。

Add 添加

Use this menu button to add layers to the Layers link field.
使用此菜单按钮将图层添加到图层链接字段。

The options are: Random, Shader, Target, Along Spline, To Spline, Surface Tangent, Particles, Turbulence, Curl, ExplosiaFX, Import, Fields and Surface Flow.
选项包括：随机、着色器、目标、沿样条线、到样条线、表面切线、粒子、湍流、卷曲、 ExplosiaFX 、导入、场和表面流。

Strength 力量

The overall strength of the flow field.
流场的整体强度。

The higher this value, the greater the effect on the particle.
该值越高，对粒子的影响越大。

When a particle enters a cell, its direction will change to that of the vector in the cell but this will result in an abrupt change of direction, which may be undesirable.
当粒子进入细胞时，其方向将改变为细胞中的矢量方向，但这会导致方向的突然改变，这可能是不希望的。

This parameter controls how quickly the direction changes.
此参数控制方向变化的速度。

Low values cause a slow change and a smoother curve, but with low values the particle may leave the cell and enter another before the change in direction is completed.
较低的值会导致缓慢的变化和更平滑的曲线，但如果值较低，粒子可能会在方向改变完成之前离开单元格并进入另一个单元格。

The value in this parameter will depend on a number of things including particle speed, cell size and how desirable it is that the particle fully completes its change of direction before leaving the cell.
此参数的值取决于许多因素，包括粒子速度、细胞大小以及粒子在离开细胞之前完全完成方向改变的程度。

Animation to demonstrate the effect of increasing the Strength parameter.
动画演示了增加强度参数的效果。

Cell Size 像元大小

The size in scene units of the individual cells.
各个单元的场景单位大小。

Each cell is a cube, which does not change, whatever the size or resolution; the cell has the same dimensions on each side.
每个单元都是一个立方体，无论其大小或分辨率如何，它都不会改变；单元的每一边都有相同的尺寸。

This animation shows how decreasing the Cell Size value affects the overall scene.

Size

The size of the flow field box in screen units.

You can also change the size by dragging one of the resizing handles of the object in the viewport.

Note: As you increase or decrease the size, the number of cells along that axis will change.

Field

Set as Direction, by default.

This setting determines what the flow field does to the particle.

The alternatives are: Velocity and Acceleration.

Direction

The flow field will alter the particle direction but not its speed.

The rate of changing direction will depend on the Strength parameter.

Velocity

The flow field will alter the particle direction and its speed, up to a maximum speed, which is set in the Speed parameter.

The rate of changing direction and speed will also depend on the Strength parameter.

Acceleration

The flow field will alter the particle speed and the speed will continue to increase while the particle remains in the flow field box.

The rate of speed increase is set in the Acceleration parameter and the direction will only change if the Acceleration value is greater than zero.

The rate of changing direction and speed will, once again, depend on the Strength parameter.

Invert

If enabled, this will invert any direction change caused by the flow field.

Speed

If Field is set to Velocity, this is the maximum speed the particle will reach.

If this value is lower than the particle's current speed, it will slow down to that speed.

Acceleration

If Field is set to Acceleration, this is the rate of speed change while the particle remains within the flow field box.

Negative values are possible and have the effect that the particle speed will slow to zero then accelerate again, but with a reversed direction.

Layer Options

All layers have this set of common properties:

Flow

This menu is the same as the Add menu below the Layers list.

It enables you to change from one type of layer to another without having to delete the layer and add a new one.

Invert

If enabled, the vectors generated by this layer are inverted.

Blend

This is a powerful option, which lets you blend layers together.

Each layer is automatically blended when it is added.

When two layers are blended they use the algorithm selected from this menu, which can be different for each layer; the blend is weighted by the Strength setting.

Normal

The default setting, when more than one layer is in the scene; vectors are simply using the Strength setting for the upper layer, which you can find in Layer Options and is 100%, by default.

This means there is no blending, the flow field is using the new, upper layer in the Layers link field for instruction.

Min

The smaller of the vectors from the layers is blended with the other layers.

Subtract

The upper layer is subtracted from the lower and the result is blended.

Multiply

The two vectors are multiplied together and the result is blended.

Max

The larger of the vectors is blended with the other layers.

Add

The two vectors are added together and the result is blended.

Strength

This is the blend weight used to blend a layer with the layer(s) beneath it.

Note: A value of 0 (zero) % will always result in the upper layer making no contribution to the final vector.

However, a value of 100% does not mean that only the upper layer is used for the vectors, except when Blend is set to Normal.

Additional Parameter Settings

There are additional parameters which are specific to certain layers only.

These are detailed below.

Random layer

This layer simply generates random vectors; there are no additional settings.

Animation to demonstrate the Random layer settings.

Shader layer

In this layer, a shader is used to generate vectors.

There are six additional settings.

This animation shows the effect of a Gradient shader in the Shader layer settings.

Shader

Use this link to specify a shader to use here.

By far the most useful are shaders that can be animated, such as Noise, but you can use any shader.

Note: If you set Color to Direction to Gradient, you must enter a gradient shader here or the layer will have no effect.

The parameters of the shader can have very significant effects on the result.

For a Noise shader, for example, changing the noise type, octaves, space, scale, animation speed, cycles, high and low clipping, brightness and contrast will all markedly affect the flow.

Note: The shader only affects the flow field vectors.

What direction the particle moves in will be influenced by the vector, but also by other factors, not least of which is the direction the particle was moving in when it entered the flow field.

Color To Direction

The default setting is Gradient.

This will use the brightness of the gradient knots to determine the direction.

The vectors will be oriented to the brightest color, but the actual color is not relevant.

If there is no brightness gradient, it will have no effect.

The other options are: Red, Green and Blue, Red Only, Green Only, Blue Only, Red and Green, Red and Blue and, finally, Green and Blue.

Zero Non-Mapped Colors

If this is enabled, there will be no contribution to the vector from any color which is not mapped, as determined by the Color To Direction parameter.

If it is unchecked, any non-mapped color will contribute to the direction vector using its raw value.

Note: Such values, if non-zero, will always be positive.

Brightness to Strength

If enabled, this causes each vector in the flow field to be assigned a strength, depending on the overall brightness of the sampled color.

Animate Flow

If you are using an animated shader, such as animated noise, or have keyframed a parameter of a shader and you want the flow vectors to change as the shader changes, check this box.

If it is unchecked, the flow vectors will not change even if the shader does.

The setting has no effect if the shader is not animated, but it will cause the flow vectors to be re-calculated each frame, which may slow the simulation considerably.

For this reason the box is unchecked by default.

Resample Layer

It can sometimes happen that making changes to the shader won't alter the flow vectors until you rewind the scene.

If it is already at the start, you would need to advance at least one frame, then rewind, to update the vectors.

This is inconvenient so, if this happens, you can click this button at any time to force the flow field object to update the vectors from the shader.

Target layer

The vectors point towards a target object.

There are four additional settings.

In this animation, a Capsule has been dropped in the Target field, to illustrate the Target layer settings.

Target

Drag the object to be used as the target into this field.

As soon as you do, you will see the vectors change to point to the object and if you move it around the vectors re-orientate to point to it.

The object can be anything - a null object is fine - and once you have arranged the vectors to your liking you can delete the object.

Alternatively, you can leave it in place and animate its position and the vectors will constantly re-orientate themselves.

Cell Strength

Target mode offers several ways to alter the strength of the effect applied by each vector.

The main Strength setting will affect all cells in the object in the same way, but you can vary the individual cell strength with this drop-down menu.

Constant

The default setting; the cell strength is at maximum in all cases, regardless of distance from the target object.

By Distance

With this option, the maximum distance of any cell from the target object is calculated and then each cell is assigned an individual strength value, depending on how far the cell (not the particle) is from the target.

The closest cells to the target then have the greatest strength, while those which are farthest away have the weakest.

The value obtained for each cell is then adjusted by multiplying by the Strength Falloff setting and then again by the Strength setting to give the final result.

Strength Falloff

With this drop-down the strength of each vector can be altered depending on how far the particle (not the cell in this case) is from the target.

The default setting is None.

Other options are: Boundary, Linear and Inverse Square.

None

The strength of effect is not altered by the particle distance from the target.

Boundary

In this case, the effect strength is zero if the particle is outside the boundary (as determined by the Falloff Boundary setting) and at maximum if it is inside the boundary.

Note: A strength of 0 (zero) will mean that the particle direction will not be affected by the vector at all.

Linear

The strength of effect varies linearly with the distance of the particle to the object, being 0 (zero) outside the boundary and at maximum when the particle is at the target position.

Inverse Square

The same as Linear, except that the strength decreases by the square of the distance.

This causes a very rapid falloff and leads to little or no effect until the particle is very close to the target.

Falloff Boundary

The boundary of the effect if Strength Falloff is set to anything other than None.

It is shown as a yellow sphere in the viewport.

Along Spline, To Spline, Surface Tangent and Surface Normal layers

There are six additional identical settings for these layer types.

Along Spline

This option requires a spline object.

Here the flow vectors will point in a direction, which is at a tangent to the nearest point in the spline.

Animation to show the effect of a Helix spline, used as the object in the Along Spline layer settings.

To Spline

This option also requires a spline object but differs from Along Spline in that the vectors point towards the nearest point in the spline.

A Flower spline has been dropped in the Objects window, to illustrate the To Spline layer settings.

Surface Tangent

This option requires a polygon object.

The vectors point along a tangent to the surface normal.

This animation demonstrates the Surface Tangent layer settings, with a Capsule dropped in the Objects window.

Surface Normal

This option requires a polygon object.

The vectors point in the same direction as the normal of the nearest polygon.

Similar settings to the above animation, using a Capsule, except this animation illustrates the effect of the Surface Normal layer settings.

Objects

Drag the splines or polygon objects (depending on the layer type) into this list.

You can use multiple objects.

Falloff

If checked, the falloff Distance and Spline settings become available.

If it unchecked, the falloff spline has no effect.

Distance

Grid cells which are further away than this distance from the object(s) have a strength value derived from the right-hand end of the spline.

The closer the cell is to the object, the more the strength value is shifted to the left of the spline.

Spline

The spline used to derive the cell strength, depending on how far a cell is from the object.