6.4. Expression — Using expressions to drive animations

An Expression outputs a single value that is driven by a user defined expression. The expression is specified by a string and can use an arbitrary number of parameters. The parameters and their default values have to be provided to the constructor via keyword arguments. An exception is the special variable t which will always hold the current time (unless you declare it explicitly). For each parameter a slot is created (<name>_slot), so it is also possible to animate the parameters. The output value can be accessed via the output and output_slot attributes.

class Expression(expr = "", exprtype = None, name = "Expression", **parameters)

expr is the expression (in Python syntax) that is used to compute the output values. It may use mathematical functions defined in the math and sl modules.

exprtype is the return type of the expression (float, vec3, ...). If this parameter is not given, the component tries to figure out the return type itself by evaluating the expression using the default values and inspecting the return type. If the expression returns a tuple the output type is automatically set to vec3, vec4, mat3 or mat4 (depending on the number of elements).

name is the component’s name.

Any additional keyword argument is a parameter used in the expression. Every parameter used in the expression has to be declared except the variable t which will automatically receive the current time value. If you declare t yourself, it will be just an ordinary variable.


s = Sphere()
e = Expression("1.0 + amp*sin(freq*t)", amp=0.2, freq=2.0)

When the above expression is created, it will have the following slots:

  • output_slot
  • amp_slot
  • freq_slot
  • t_slot

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