Draw perfect pressure-sensitive freehand lines.

🔗 A port of the perfect-freehand JavaScript library. Try out the demo!

💕 Love this library? Consider becoming a sponsor for steveruizok (the author of the original JavaScript library) or adil192 (the maintainer of the Dart package).

Table of Contents

• Installation
• Usage
• Community

Introduction

This package exports a function named getStroke that will generate the points for a polygon based on an array of points.

To do this work, getStroke first creates a set of spline points (red) based on the input points (grey) and then creates outline points (blue). You can render the result any way you like, using whichever technology you prefer.

Installation

This package is available on pub.dev and requires Flutter. If you're using Dart without Flutter, check out versions 1.x.x of this package.

flutter pub add perfect_freehand

See here for more.

Usage

This package exports a function named getStroke that:

• accepts an array of points and several options
• returns a stroke outline as an array of points

import 'package:perfect_freehand/perfect_freehand.dart';

List<PointVector> myPoints = [
  PointVector(0, 0),
  PointVector(1, 2),
  // etc...
];

final stroke = getStroke(myPoints);

You may also provide options to getStroke. You'll most likely store the StrokeOptions object in a variable, but you can also pass it directly to getStroke.

final stroke = getStroke(
  myPoints,
  options: StrokeOptions(
    size: 16,
    thinning: 0.7,
    smoothing: 0.5,
    streamline: 0.5,
    simulatePressure: true,
    start: StrokeEndOptions(
      cap: true,
      taperEnabled: true,
    ),
    end: StrokeEndOptions(
      cap: true,
      taperEnabled: true,
    ),
    isComplete: false,
  ),
);

To use real pressure, provide each point's pressure as a third parameter.

List<PointVector> myPoints = [
  PointVector(0, 0, 0.2),
  PointVector(1, 2, 0.3),
  PointVector(2, 4, 0.4),
  // etc...
];

final stroke = getStroke(
  myPoints,
  options: StrokeOptions(
    simulatePressure: false,
  ),
);

Options

You can customize the stroke by passing a StrokeOptions object as the second parameter to getStroke. This object accepts the following properties:

Property	Type	Default	Description
size	double	16	The base size (diameter) of the stroke.
thinning	double	.5	The effect of pressure on the stroke's size.
smoothing	double	.5	How much to soften the stroke's edges.
streamline	double	.5	How much to remove variation from the input points.
simulatePressure	bool	true	Whether to simulate pressure based on distance between points, or else use the provided PointVectors' pressures.
isComplete	bool	true	Whether the stroke is complete.
start	StrokeEndOptions		How far to taper the start of the line.
end	StrokeEndOptions		How far to taper the end of the line.
start.cap	bool	true	Whether to cap the start of the line.
start.taperEnabled	bool	false	Whether to taper the start of the line.
start.customTaper	double	null	A custom taper value for the start of the line, defaults to the total running length.

Notes:

• When the isComplete property is true, the line's end will be drawn at the last input point, rather than slightly behind it.
• The StrokeEndOptions.cap property has no effect when StrokeEndOptions.taperEnabled is true.
• To create a stroke with a constant width, set the thinning option to 0.
• To create a stroke that gets thinner with pressure instead of thicker, use a negative number for the thinning option.
• You can change the defaults by setting e.g. StrokeOptions.defaultThinning or StrokeEndOptions.defaultCap to a different value.

Rendering

While getStroke returns an array of points representing the outline of a stroke, it's up to you to decide how you will render these points.

See the StrokePainter class in the example project to see how you might draw these points in Flutter with a CustomPainter.

Advanced Usage

For advanced usage, the library also exports smaller functions that getStroke uses to generate its outline points.

getStrokePoints

A function that accepts an array of PointVectors and returns a set of StrokePoints. The path's total length will be the runningLength of the last point in the array. Like getStroke, this function also accepts any of the optional named parameters listed above.

List<PointVector> myPoints = [
  PointVector(0, 0),
  PointVector(1, 2),
  // etc...
];
final options = StrokeOptions(
  size: 16,
);

final strokePoints = getStrokePoints(myPoints, options: options);

getOutlinePoints

A function that accepts an array of StrokePoints (i.e. the output of getStrokePoint) and returns an array of Points defining the outline of a stroke. Like getStroke, this function also accepts any of the optional named parameters listed above.

List<PointVector> myPoints = [
  PointVector(0, 0),
  PointVector(1, 2),
  // etc...
];
final options = StrokeOptions(
  size: 16,
);

final myStrokePoints = getStrokePoints(myPoints, options: options);

final myOutlinePoints = getStrokeOutlinePoints(myStrokePoints, options: options);

Note: Internally, the getStroke function passes the result of getStrokePoints to getStrokeOutlinePoints, just as shown in this example. This means that, in this example, the result of myOutlinePoints will be the same as if the myPoints List had been passed to getStroke.

rememberSimulatedPressure

Pressure simulation relies on the distance between points to determine the pressure at each point. But in some scenarios, it would be wasteful to store all of these points.

The rememberSimulatedPressure argument solves this problem by calculating the pressure once and then storing it in the original array of points.

/// Input points without pressure values
List<PointVector> myPoints = [/* ... */];

getStroke(
  myPoints,
  options: StrokeOptions(
    // isComplete and simulatePressure must be true for rememberSimulatedPressure
    simulatePressure: true,
    isComplete: true,
  ),
  rememberSimulatedPressure: true,
);

// myPoints now have pressure values (except for duplicate points)
print(myPoints[0].pressure); // (some number, not null)

// We are now free to compress myPoints however we want, e.g.
// this function removes duplicate points and points that are too close together:
void optimisePoints({double thresholdMultiplier = 0.1}) {
  if (points.length <= 3) return;

  final minDistance = strokeOptions.size * thresholdMultiplier;

  // Duplicate points have null pressure values, so we can remove them
  myPoints.removeWhere((point) => point.pressure == null);

  for (int i = 1; i < points.length - 1; i++) {
    final point = points[i];
    final prev = points[i - 1];
    final next = points[i + 1];

    if (prev.distanceSquaredTo(point) < minDistance * minDistance &&
        next.distanceSquaredTo(point) < minDistance * minDistance) {
      points.removeAt(i);
      i--;
    }
  }
}

Community

Support

Need help? Please open an issue for support.

Discussion

Have an idea or casual question? Visit the discussion page.

License

• MIT
• ...but if you're using perfect-freehand in a commercial product, consider becoming a sponsor for steveruizok (the author of the original JavaScript library) or adil192 (the maintainer of the Dart package).. 💰