fix: correct 16 bits image thresholding

Closes: #374

src/operations/__tests__/threshold.test.ts

@@ -1,3 +1,4 @@
+import { ImageColorModel, Image } from '../..';
 import { computeThreshold, threshold } from '../threshold';
 
 test('threshold with a fixed value of 100', () => {
@@ -94,3 +95,24 @@ test('error threshold out of range', () => {
     /threshold must be a value between 0 and 1/,
   );
 });
+
+test('16 bits image simple', () => {
+  const image = new Image(2, 2, {
+    colorModel: ImageColorModel.GREY,
+    bitDepth: 16,
+    data: new Uint16Array([0, 100, 20000, 30000]),
+  });
+  const threshold = image.threshold();
+
+  expect(threshold).toMatchImageData([
+    [0, 0],
+    [1, 1],
+  ]);
+});
+
+test('16 bits image', () => {
+  const image = testUtils.load('formats/grey16.png');
+  const threshold = image.threshold();
+
+  expect(threshold).toMatchImageSnapshot();
+});

src/operations/threshold.ts

@@ -81,7 +81,7 @@ export function computeThreshold(
       'threshold can only be computed on images with one channel',
     );
   }
-  const histogram = image.histogram();
+  const histogram = image.histogram({slots: 2**image.bitDepth});
 
   switch (algorithm) {
     case 'huang':