Hand Signature

A Flutter plugin providing a Signature Pad for drawing smooth signatures. This library is written in a pure Dart/Flutter environment to provide support for all platforms. It's easy to use, with a variety of drawing and export settings, and also supports SVG files.

The signature pad drawing is based on Cubic Bézier curves and offers a choice between performance and 'beauty' modes.

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Features

• Cross-Platform: Works on any platform supported by Flutter.
• Customizable: Adjust stroke width, color, and smoothing.
• Multiple Drawing Types: Choose between line, shape, and arc presets for different visual styles. It's also possible to implement a custom drawing experience.
• Velocity and Pressure Based Stroke: The thickness of the line can vary based on the drawing speed and pressure.
• Export Options: Export signatures as PNG, SVG, or raw data (JSON/Map).
• Import/Export: Save and load signature data state.

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Usage

Here is a simple example of how to use the signature pad.

import 'package:hand_signature/signature.dart';

final control = HandSignatureControl(
  initialSetup: SignaturePathSetup(
    threshold: 5.0,
    smoothRatio: 0.65,
    velocityRange: 2.0,
    pressureRatio: 0.0,
  ),
);

final widget = HandSignature(
  control: control,
  drawer: ShapeSignatureDrawer(
    color: Colors.blueGrey,
    width: 2.0,
    maxWidth: 10.0,
  ),
);

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HandSignatureControl

This class handles the logic behind the signature, controlling input touches and managing the control points of the signature curve. SignaturePathSetup defines how input data will be handled.

• threshold: Controls the minimal distance between two points. A higher distance creates a smoother, but less precise, curve.
• smoothRatio: (0 - 1) Controls how smooth the curve will be. A higher ratio creates a smoother, but less precise, curve. The best results are typically between 0.5 and 0.75.
• velocityRange: Controls the curve size based on the drawing speed. A thin line corresponds to a fast movement, while a thick line corresponds to a slow movement.
• presureRatio: (0 - 1) Ratio between pressure sensitivity and velocity. 0.0 = only velocity, 1.0 = only pressure. Some platforms/devices (like mouse pointer) doesn't support sensitivity'
• args: Custom variables - mostly used for custom HandSignatureDrawer.

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HandSignature

This widget handles touch gestures and draws the curve on the canvas.

• control: The HandSignatureControl that processes input and stores the raw data.
• drawer: The HandSignatureDrawer that handles the painting of the signature.
• backgroundColor: The background color of the signature canvas.
• placeholder: A widget to display when the signature is empty.

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Drawers

Drawers are responsible for rendering the signature on the canvas. You can use one of the pre-built drawers or create your own.

Pre-built Drawers

• LineSignatureDrawer: Basic line with a single line width.
• ShapeSignatureDrawer: Default drawer, mimics an ink pen. It varies line width based on velocity and/or pressure. This is a good option for exporting to SVG.
• ArcSignatureDrawer: Also mimics an ink pen, but is less performant than ShapeSignatureDrawer. This is the default drawer when exporting to an image.
• DynamicSignatureDrawer: Dynamically selects the drawing type based on provided arguments.
• MultiSignatureDrawer: Combines multiple drawers, allowing for complex effects and layering.

Custom Drawer

You can create a custom drawer by extending HandSignatureDrawer and implementing the paint method. This allows full control over how the signature is rendered.

class MyCustomDrawer extends HandSignatureDrawer {
  @override
  void paint(Canvas canvas, Size size, List<CubicPath> paths) {
    // Custom painting logic here
  }
}

Check custom_sample.dart for dynamic setup and drawing.

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Exporting

You can modify properties like canvas size, stroke width, and color during export. You can also provide a custom drawer when exporting to an image. The most common export formats are SVG and PNG. During export, data are re-rendered with the preferred setup and output dimensions.

State can be saved with toMap() and restored with the import() method.

final control = HandSignatureControl();

// Export to SVG
final svg = control.toSvg();

// Export to PNG
final png = await control.toImage(
  width: 512,
  height: 512,
  drawer: ShapeSignatureDrawer(
    color: Colors.black,
  ),
);

// Export to raw data
final data = control.toMap();

// Import from raw data
control.import(data);

• SVG:
  • shape: Generates a reasonably small file that is well-supported by most programs.
  • arc: Generates a larger SVG file, which some programs may struggle to handle.
  • line: A simple Bezier curve.
• Image: Export to an Image or Picture object, which can then be converted to PNG bytes.
  • arc: Default drawer when exporting image data.
  • drawer: Specifies HandSignatureDrawer to use during canvas export.
• JSON/Map: Export the current state as raw data, which can be used later to restore the signature.
  • Data is exported with timestamp and pressure information for each point.

Displaying an exported SVG file

You can display an exported SVG file using a library like flutter_svg.