Create a Rule to combine properties

Compose a Rule to manipulate two properties of your model at the same time: a slider that combines the trunk height and radius of an existing tree model.

Note

To get proper and meaningful results from this example Rule, you need to use a tree with a first level branch generator named "Trunk" that can afford a height value between 20 and 80 and a trunk radius value between 0.2 and 1.2. For example, use the Conifer available in the Samples provided with the Modeler.

Create a slider with a normalized range

  1. From the menu, select Window > Rules.

  2. In the script editor of the Rules window, compose the following statement:

    GrowthValue = rule("Growth", 0.5, 0.0, 1.0)
  3. Click anywhere outside the Rules window.

The UI section of the Rule window displays a slider named Growth in a section named Shape. The slider allows you to change the value of a variable named GrowthValue between 0 and 1 and the default value is 0.5.

Make the slider proportionally control the trunk height

  1. Create and isolate a local TrunkHeight variable and interpolate its value in a suitable range according to the slider's range.

    GrowthValue = rule("Growth", 0.5, 0.0, 1.0)

do
  local TrunkHeight = math.lerp(20, 80, GrowthValue)
end

    In this example, the math.lerp function makes the TrunkHeight variable value linearly change between 20 and 80 when you move the slider between between 0 and 1.

  2. Select the Trunk generator of the tree.

  3. In the Property Bar, select the Spine tab.

  4. Under Spine > Length, right-click on the Absolute property and select Copy edit line for script.

  5. Under the local variable statement in the Rule editor, paste the copied information.

  6. Update the VALUE parameter of the set_property_value function to match the Rule's variable name (TrunkHeight):

    GrowthValue = rule("Growth", 0.5, 0.0, 1.0)

do
  local TrunkHeight = math.lerp(20, 80, GrowthValue)
  set_property_value("=Trunk", "Spine:Length:Absolute", TrunkHeight)
end
  7. Click anywhere outside the Rules window.

When you adjust the slider, the trunk of the tree gets longer or shorter.

Combine the trunk radius with the trunk height

  1. Create a local TrunkRadius variable and interpolate its value in a suitable range according to the slider's range.

    GrowthValue = rule("Growth", 0.5, 0.0, 1.0)

do
  local TrunkHeight = math.lerp(20, 80, GrowthValue)
  local TrunkRadius = math.lerp(0.2, 1.2, GrowthValue)
  set_property_value("=Trunk", "Spine:Length:Absolute", TrunkHeight)
end

    In this example, the math.lerp function makes the TrunkRadius variable value linearly change between 0.2 and 1.2 when you move the slider between between 0 and 1.

  2. Select the Trunk generator of the tree.

  3. In the Property Bar, select the Skin tab.

  4. Under Skin > Radius, right-click on the Absolute property and select Copy edit line for script.

  5. Under the first set_property_value statement in the Rule editor, paste the copied information.

  6. Update the VALUE parameter of the new set_property_value function to match the Rule's variable name (TrunkRadius):

    GrowthValue = rule("Growth", 0.5, 0.0, 1.0)

do
  local TrunkHeight = math.lerp(20, 80, GrowthValue)
  local TrunkRadius = math.lerp(0.2, 1.2, GrowthValue)
  set_property_value("=Trunk", "Spine:Length:Absolute", TrunkHeight)
  set_property_value("=Trunk", "Skin:Radius:Absolute", TrunkRadius)
end

When you adjust the slider, the trunk height and radius values get higher or lower at the same time.

Additional resources