Add threshold to MagickImage (#113)

src/magick-image.ts

@@ -257,6 +257,8 @@ export interface IMagickImage extends IDisposable {
     statistics(): IStatistics;
     statistics(channels: Channels): IStatistics;
     strip(): void;
+    threshold(percentage: Percentage): void;
+    threshold(percentage: Percentage, channels: Channels): void;
     toString(): string;
     transparent(color: MagickColor): void;
     trim(): void;
@@ -1386,6 +1388,15 @@ export class MagickImage extends NativeInstance implements IMagickImage {
         });
     }
 
+    threshold(percentage: Percentage): void
+    threshold(percentage: Percentage, channels: Channels): void
+    threshold(percentage: Percentage, channelsOrUndefined?: Channels): void {
+        const channels = this.valueOrDefault(channelsOrUndefined, Channels.Undefined);
+        Exception.usePointer(exception => {
+            ImageMagick._api._MagickImage_Threshold(this._instance, percentage.toQuantum(), channels, exception);
+        });
+    }
+
     toString = (): string => `${this.format} ${this.width}x${this.height} ${this.depth}-bit ${ColorSpace[this.colorSpace]}`
 
     transparent(color: MagickColor): void {

tests/magick-image/threshold.spec.ts

@@ -0,0 +1,32 @@
+// Copyright Dirk Lemstra https://github.com/dlemstra/magick-wasm.
+// Licensed under the Apache License, Version 2.0.
+
+import { Channels } from '../../src/channels';
+import { ErrorMetric } from '../../src/error-metric';
+import { Percentage } from '../../src/percentage';
+import { TestImages } from '../test-images';
+
+describe('MagickImage#threshold', () => {
+    it('should threshold the image with the correct default values', () => {
+        TestImages.Builtin.logo.use((image) => {
+            image.clone((other) => {
+                image.threshold(new Percentage(80));
+                other.threshold(new Percentage(80), Channels.Undefined);
+
+                const difference = other.compare(image, ErrorMetric.RootMeanSquared);
+                expect(difference).toBe(0);
+            });
+        });
+    });
+
+    it('should threshold the image', () => {
+        TestImages.Builtin.logo.use((image) => {
+            image.clone((other) => {
+                image.threshold(new Percentage(80));
+
+                const difference = other.compare(image, ErrorMetric.RootMeanSquared);
+                expect(difference).toBeCloseTo(0.165);
+            });
+        });
+    });
+});