Functions reference

Much of the power of the Vis-Tools Geospatial client is afforded by the ability to perform real-time functional manipulation of simulation values when applying those values to visual parameters of the visualization.

Built-in functions

The Vis-Tools Geospatial client contains a healthy set of built-in functions that cover many of the kinds of manipulations you’re likely to want to carry out. An easy way to remember these functions is to use the built-in Examples toggle in the Edit function dialog box itself.

_images/vt-function-examples.png

add()

Adds the constant value x to the input value. Note that x may be negative.

add(x)

  • value \leftarrow value + x
  • x: number, constant to be added to the input value.
  • returns: number, input value plus constant x.

Examples

  • add(4.5) returns value + 4.5.
  • add(-1) returns value - 1.

fixed()

Ignores the input value and returns x directly. This function can be used even if there is no input source selected to force a particular value for a visual parameter. For example, binding point node size to “None” with the function “fixed(10)” makes all nodes ten pixels in size.

Note that x may be a string or a number. For example, if you are using fixed in a binding to a color visual parameter, return a valid color string. For example, “fixed(‘red’)” or “fixed(‘#00ff00’)”.

fixed(x)

  • value \leftarrow x
  • x: number or string, value with which to override input value.
  • returns: same as argument, exactly x.

Examples

  • fixed(5.5) returns 5.5.
  • fixed(“yellow”) returns “yellow”.

if()

Performs an inline conditional on the input value and returns either thenValue or elseValue. This function is similar to the ternary ?: operator in many C-like languages.

if(op, compArg, thenValue, elseValue)

  • value \leftarrow \begin{cases} thenV\!alue & value\:op\:\:compArg\\ elseV\!alue & otherwise \end{cases}
  • op: string, comparison operator - one of “==”, “!=”, “<”, “<=”, “>”, or “>=”.
  • compArg: number or string, comparison argument, i.e. value <op> <compArg>.
  • thenValue: number or string, value to return if condition is true.
  • elseValue: number or string, value to return if condition is false.
  • returns: number or string (depending on type of thenValue or elseValue), thenValue if condition is true; otherwise, elseValue.

Examples

  • if(“>=”, 0.8, “red”, “green”) returns “red” if input value is >= 0.8, else “green”.
  • if(“<”, 0, -1, 1) returns -1 if value is negative, otherwise 1 (equivalent to the sgn(x) function in some languages).

inRange()

Tests whether the input value is in the range [low, high] (inclusive), and returns thenValue if so; otherwise, returns elseValue.

inRange(low, high, thenValue, elseValue)

  • value \leftarrow \begin{cases} thenV\!alue & low \le value \le high\\ elseV\!alue & otherwise \end{cases}
  • low: number, low constant for range comparison, inclusive.
  • high: number, high constant for range comparison, inclusive.
  • thenValue: number or string, value to return if input value is in range.
  • elseValue: number or string, value to return if input value is out of range.
  • returns: number or string (depending on type of thenValue or elseValue), thenValue if value is in range; otherwise, elseValue.

Examples

  • inRange(0.1, 0.4, “green”, “red”) returns “green” if input value >= 0.1 and <= 0.4, otherwise “red”.
  • inRange(-5, 0, -1, 1) returns -1 if value is in range [-5, 0], otherwise 1.

multiply()

Multiplies the input value by the constant x. Note that x may be negative or a non-integer.

multiply(x)

  • value \leftarrow value \times x
  • x: number, constant to be multiplied with the input value.
  • returns: number, value * x.

Examples

  • multiply(2) returns value x 2.
  • multiply(0.5) returns value / 2.
  • multiply(-1) returns -value.

none()

Passes the input value through untouched.

none()

  • value \leftarrow value
  • returns: same type as input value, input value untouched.

Examples

  • none() returns the input value as-is.

normalize()

Calculates a normalized form of the input value, based on the range of the input data channel for all timesteps and all nodes, returning a value in the range [0, 1].

normalize()

  • value \leftarrow norm(value)
  • returns: a normalized value [0, 1] for the input value.

Examples

  • normalize() returns the normalized input value.

sampleGradient()

Returns a color by normalizing the input value then sampling the color gradient. This function only make sense for bindings to color visual parameters.

sampleGradient()

  • value \leftarrow sampleGradient(norm(value))
  • returns: color, gradient sampled with normalized value.

Examples

  • sampleGradient() returns a color based on the normalized input value.

scale()

Returns a value mapped into the given range. The range is treated inclusively.

scale(min, max)

  • value \leftarrow min + norm(value) \times (max - min)
  • min: number, minimum value in output range inclusive
  • max: number, maximum value in output range inclusive
  • returns: number, value mapped into [min, max]

Examples

  • scale(3, 20) returns a value between 3 and 20 inclusive that represents the input value within that range.
  • scale(-1, 1) returns a value between -1 and 1 inclusive that represents the input value within that range (i.e 0 \rightarrow -1, 0.5 \rightarrow 0, 1 \rightarrow 1).

stepwise()

Returns a stepwise value. It can be used to create custom stepwise gradients or specialized quantizations.

stepwise(step1, firstValue, step2, secondValue, …, “else”, elseValue)

  • value \leftarrow \begin{cases} firstV\!alue & value < step1\\ secondV\!alue & value < step2\\ ... & ...\\ elseV\!alue & otherwise \end{cases}
  • step1: number, first step argument, where firsValue will be returned if input value is less than step1.
  • firstValue: number or string, result to be returned if input value is less than step1.
  • step2: number, second step argument, where secondValue will be returned if input value is less than step2.
  • secondValue: number or string, result to be returned if input value is less than step2.
  • …: more step/value pairs.
  • “else”: string, the explicit string “else”.
  • elseValue: number or string, result to be returned if input value is greater than or equal to the last step.
  • returns: number or string, value for matching step; otherwise, elseValue.

Examples

  • stepwise(0.2, ‘red’, 0.75, ‘yellow’, ‘else’, ‘green’) returns the string red for input values less than 0.2, the string “yellow” for values between 0.2 and 0.75, and “green” otherwise.
  • stepwise(0.25, 0, ‘else’ 1) quantizes input values < 0.25 to 0, otherwise 1.

Custom functions

While for most cases the Vis-Tools built-in functions will suffice, it is possible to use custom user-defined functions for special cases. This section examines that capability in detail.

Simple example

What if we wanted to invert a normalized value? The value naturally ranges from 0 to 1, but we want it to be the other way around, i.e. 1 - value. None of the built-in functions can do that. However, that result and many more can be achieved using a custom function which is essentially a specialized JavaScript function body. Consider this custom function:

{ return 1.0 - binding.value; }

That function carries out the desired function. It is simple enough, except for the binding reference, which is explained below.

Basic syntax

As might be inferred from the previous example, the basic syntax of a custom function is:

{ return <expression>; }

A custom function is the text of a JavaScript function body that will be executed for each node, for each timestep, before a value is applied to the bound visual parameter. The expression can be anything, but typically it is some manipulation of the input value or other available data. The input data and other information are provided to the custom function as an implicit argument called binding.

Note

Your custom function must be valid JavaScript. If it is not, or the code in the function throws an exception, you’ll see an error message in the message area of the client window.

Note

The code for your custom function may be arbitrarily complex. You can declare and use local variables, and use as many statements as you want. The custom function is evaluated as a real JavaScript function, and as long as it ultimately returns a valid return value, it will work. But be warned that your custom function is going to be executed for every node, on every timestep, so complex custom functions will detrimentally affect animation framerate.

The binding object

The binding object is passed to a custom function and provides access to the raw bound simulation data channel value, along with other potentially useful data.

Here are the most commonly used data members of the binding object.

Field Type Description
value number The raw data value of the bound data source for the current node and timestep.
min number The minimum value for this data channel over all nodes and timesteps. Along with max, can be used to calculate a normalized value.
max number The maximum value for this data channel over all nodes and timesteps. Along with min, can be used to calculate a normalized value.

The binding object is implicitly available to your custom function, so to use min you simply type binding.min within your custom function. For example, the following function normalizes the input value, which is functionally equivalent to the built-in normalize() function:

{ return (binding.value - binding.min) / (binding.max - binding.min); }

The above fields are enough to do most numerical manipulations of the input value. There are, however, some additional data members that may be of use for other specialized functions.

Field Type Description
gradient Gradient

The color gradient object for this binding. To use this object, calculate a normalized value, then:

return gradient.sampleHashCss(normValue);

which returns a color string like “#00ff00”.

gradientLow number The low value of the gradient range slider, which is a number in the range [0, gradientHigh). You may choose to incorporate this into your function if you are sampling the gradient directly.
gradientHigh number The high value of the gradient range slider, which is a number in the range (gradientLow, 1]. You may choose to incorporate this into your function if you are sampling the gradient directly.
node Node

An object describing the current node. This object has the following potentially valuable fields.

  • latitude: number, latitude in degrees.
  • longitude: number, longitude in degrees.
  • altitude: number: altitude in meters.
  • nodeId: number or string: node ID from the demographics file.
  • other NodeAttribute fields added during the preprocessing step, such as InitialPopulation, BirthRate, etc.
timestep number Current timestep, in the range [0, timestepCount).
timestepCount number Number of timesteps in the present simulation.

More examples

Here are some more examples of things you can do with custom functions.

  • For point nodes, to outline a particular node of interest (id 340, for example), bind:

    • Outline color to:

      fixed("red")
      
    • Outline thickness to:

      { return (binding.node.nodeId == 340) ? 3 : 0; }
      
  • To invert a normalized value, use the function:

    { return 1.0 - binding.value; }
    
  • To apply a log scale to a value, use the function:

    { return Math.log10(binding.value); }
    
  • To apply a square root to a value, use the function:

    { return Math.sqrt(binding.value); }
    
  • To apply a random color to each node, bind node Color to the function:

    { return Gradient.presets["Hue"].sampleHashCss(Math.random()); }