From 4d881e633625a8ea322d51345283fdbcc3632c10 Mon Sep 17 00:00:00 2001
From: Nico Lueg <nicolueg@yahoo.de>
Date: Mon, 17 Jul 2023 06:59:26 +0200
Subject: [PATCH] fix(eraser): solves issues with eraser (#725)

---
 src/signature_pad.ts        | 19 ++++++++++++++++---
 tests/fixtures/face.ts      |  3 +++
 tests/fixtures/square.ts    |  1 +
 tests/signature_pad.test.ts | 15 +++++++++++++++
 4 files changed, 35 insertions(+), 3 deletions(-)

diff --git a/src/signature_pad.ts b/src/signature_pad.ts
index c7a01af0..e4fd17a7 100644
--- a/src/signature_pad.ts
+++ b/src/signature_pad.ts
@@ -36,6 +36,12 @@ export interface PointGroupOptions {
   maxWidth: number;
   penColor: string;
   velocityFilterWeight: number;
+  /**
+   * This is the globalCompositeOperation for the line.
+   * *default: 'source-over'*
+   * @see https://developer.mozilla.org/en-US/docs/Web/API/CanvasRenderingContext2D/globalCompositeOperation
+   */
+  compositeOperation: GlobalCompositeOperation;
 }
 
 export interface Options extends Partial<PointGroupOptions> {
@@ -56,6 +62,7 @@ export default class SignaturePad extends SignatureEventTarget {
   public penColor: string;
   public minDistance: number;
   public velocityFilterWeight: number;
+  public compositeOperation: GlobalCompositeOperation;
   public backgroundColor: string;
   public throttle: number;
 
@@ -83,6 +90,7 @@ export default class SignaturePad extends SignatureEventTarget {
     this.dotSize = options.dotSize || 0;
     this.penColor = options.penColor || 'black';
     this.backgroundColor = options.backgroundColor || 'rgba(0,0,0,0)';
+    this.compositeOperation = options.compositeOperation || 'source-over';
 
     this._strokeMoveUpdate = this.throttle
       ? throttle(SignaturePad.prototype._strokeUpdate, this.throttle)
@@ -314,7 +322,7 @@ export default class SignaturePad extends SignatureEventTarget {
     }
   };
 
-  private _getPointGroupOptions(group?: PointGroup) {
+  private _getPointGroupOptions(group?: PointGroup): PointGroupOptions {
     return {
       penColor: group && 'penColor' in group ? group.penColor : this.penColor,
       dotSize: group && 'dotSize' in group ? group.dotSize : this.dotSize,
@@ -324,6 +332,10 @@ export default class SignaturePad extends SignatureEventTarget {
         group && 'velocityFilterWeight' in group
           ? group.velocityFilterWeight
           : this.velocityFilterWeight,
+      compositeOperation:
+        group && 'compositeOperation' in group
+          ? group.compositeOperation
+          : this.compositeOperation,
     };
   }
 
@@ -361,8 +373,8 @@ export default class SignaturePad extends SignatureEventTarget {
       (event as PointerEvent).pressure !== undefined
         ? (event as PointerEvent).pressure
         : (event as Touch).force !== undefined
-        ? (event as Touch).force
-        : 0;
+          ? (event as Touch).force
+          : 0;
 
     const point = this._createPoint(x, y, pressure);
     const lastPointGroup = this._data[this._data.length - 1];
@@ -432,6 +444,7 @@ export default class SignaturePad extends SignatureEventTarget {
     this._lastVelocity = 0;
     this._lastWidth = (options.minWidth + options.maxWidth) / 2;
     this._ctx.fillStyle = options.penColor;
+    this._ctx.globalCompositeOperation = options.compositeOperation;
   }
 
   private _createPoint(x: number, y: number, pressure: number): Point {
diff --git a/tests/fixtures/face.ts b/tests/fixtures/face.ts
index e3235d8b..bdc19b7b 100644
--- a/tests/fixtures/face.ts
+++ b/tests/fixtures/face.ts
@@ -7,6 +7,7 @@ export const face: PointGroup[] = [
     minWidth: 0.5,
     maxWidth: 2.5,
     velocityFilterWeight: 0.7,
+    compositeOperation: 'source-over',
     points: [
       {
         time: 1523730547109,
@@ -22,6 +23,7 @@ export const face: PointGroup[] = [
     minWidth: 0.5,
     maxWidth: 2.5,
     velocityFilterWeight: 0.7,
+    compositeOperation: 'source-over',
     points: [
       {
         time: 1523730547775,
@@ -37,6 +39,7 @@ export const face: PointGroup[] = [
     minWidth: 0.5,
     maxWidth: 2.5,
     velocityFilterWeight: 0.7,
+    compositeOperation: 'source-over',
     points: [
       {
         time: 1523730548448,
diff --git a/tests/fixtures/square.ts b/tests/fixtures/square.ts
index d35f4ef8..f91ce800 100644
--- a/tests/fixtures/square.ts
+++ b/tests/fixtures/square.ts
@@ -7,6 +7,7 @@ export const square: PointGroup[] = [
     minWidth: 0.5,
     maxWidth: 2.5,
     velocityFilterWeight: 0.7,
+    compositeOperation: 'source-over',
     points: [
       {
         time: 1637131731972,
diff --git a/tests/signature_pad.test.ts b/tests/signature_pad.test.ts
index 36c92c38..521046cb 100644
--- a/tests/signature_pad.test.ts
+++ b/tests/signature_pad.test.ts
@@ -53,6 +53,21 @@ describe('#clear', () => {
     expect(pad.isEmpty()).toBe(true);
     expect(pad.toData()).toEqual([]);
   });
+
+  it('clear should apply erase option to the canvas context', () => {
+    const pad = new SignaturePad(canvas);
+
+    pad.fromData(face);
+    expect(pad.isEmpty()).toBe(false);
+
+    pad.penColor = 'pink';
+    pad.compositeOperation = 'destination-out';
+
+    pad.clear();
+
+    const context = canvas.getContext('2d') as CanvasRenderingContext2D;
+    expect(context.globalCompositeOperation).toBe('destination-out');
+  })
 });
 
 describe('#isEmpty', () => {