Setting a value in millimeter as a percentage option

Percentage options are great for parametric design, but not always very intuitive for the user. For example: A user may desire 13 centimeters (5 inches) of chest ease. But what percentage should they set the chestEase option to to accomplish this?

To address this common grievance, FreeSewing allows you to add a fromAbs method that should take a value in millimeter and return the percentage the option should be set to to result in this value.

Structure

The fromAbs property should hold a function with the following signature:

function fromAbs(millimeter, settings
) {
  // return a percentage here (0.5 is 50%)
}

The first parameter is the desired value in millimeter (for example 130 for 13 cm). The second parameter is the pattern’s settings object which holds — among other things — the measurements provided by the user.

Example

In our example above, let’s say that the chestEase option is a simple percentage of the chest measurement. Our option configuration could like like this:

chestEase: {
  pct: 8,
  min: 0,
  max: 20,
  fromAbs: function(millimeter, settings
) {
    return millimeter / settings.measurements.chest
  }
}

With object destructuring and fat-arrow notation, you can write it a bit terser like this:

fromAbs: (val, { measurements }
) => val /measurements.chest

Using pctBasedOn for simple measurement fractions

Many percentage options represent a simple fraction of a measurement (chest circumference in the example above).

As this scenario is so common, @freesewing/core exports a pctBasedOn method that will do the work for you:

// First import the method
import { pctBasedOn } from '@freesewing/core'

options: {
  chestEase: {
    pct: 8,
    min: 0,
    max: 20,
    // Pass the measurement name as parameter
    // and spread the return value into your option
    ...pctBasedOn('chest')
  }
}

This will not only add an fromAbs() method to your option — one that will return the percentage of any millimeter value passed into it — it will also add a toAbs() method that does the inverse: return the value in millimeter of whatever percentage the option is set to.

See Reporting a percentage option value in millimeter for details.