fix: scale threshold when slots number is changed (#450)

* fix: scale threshold when slots number is changed

* fix: fix threshold test with custom slots

* refactor: use slots instead of options.slots

Author: moonayyur

src/operations/__tests__/threshold.test.ts

@@ -124,9 +124,9 @@ test('16 bits image simple with default number of slots 2**16', () => {
     data: new Uint16Array([0, 100, 20000, 30000]),
   });
   const threshold = image.threshold({ slots: 2 ** image.bitDepth });
-  const defaultThreshold = image.threshold();
+  const defaultSlotsThreshold = image.threshold();
 
-  expect(threshold).toEqual(defaultThreshold);
+  expect(threshold).toEqual(defaultSlotsThreshold);
 });
 
 test('16 bits image simple with custom number of slots 2**8', () => {
@@ -136,7 +136,7 @@ test('16 bits image simple with custom number of slots 2**8', () => {
     data: new Uint16Array([0, 100, 20000, 30000]),
   });
   const threshold = image.threshold({ slots: 2 ** 8 });
-  const defaultThreshold = image.threshold();
+  const defaultSlotsThreshold = image.threshold();
 
-  expect(threshold).not.toEqual(defaultThreshold);
+  expect(threshold).toEqual(defaultSlotsThreshold);
 });

src/operations/threshold.ts

@@ -87,38 +87,39 @@ export function computeThreshold(
     );
   }
   const histogram = image.histogram({ slots });
+  const scale = slots ? 2 ** image.bitDepth / slots : 1;
 
   switch (algorithm) {
     case 'huang':
-      return huang(histogram);
+      return huang(histogram) * scale;
     case 'intermodes':
-      return intermodes(histogram);
+      return intermodes(histogram) * scale;
     case 'isodata':
-      return isodata(histogram);
+      return isodata(histogram) * scale;
     case 'li':
-      return li(histogram, image.size);
+      return li(histogram, image.size) * scale;
     case 'maxEntropy':
-      return maxEntropy(histogram, image.size);
+      return maxEntropy(histogram, image.size) * scale;
     case 'mean':
-      return mean(histogram, image.size);
+      return mean(histogram, image.size) * scale;
     case 'minimum':
-      return minimum(histogram);
+      return minimum(histogram) * scale;
     case 'minError':
-      return minError(histogram, image.size);
+      return minError(histogram, image.size) * scale;
     case 'moments':
-      return moments(histogram, image.size);
+      return moments(histogram, image.size) * scale;
     case 'otsu':
-      return otsu(histogram, image.size);
+      return otsu(histogram, image.size) * scale;
     case 'percentile':
-      return percentile(histogram);
+      return percentile(histogram) * scale;
     case 'renyiEntropy':
-      return renyiEntropy(histogram, image.size);
+      return renyiEntropy(histogram, image.size) * scale;
     case 'shanbhag':
-      return shanbhag(histogram, image.size);
+      return shanbhag(histogram, image.size) * scale;
     case 'triangle':
-      return triangle(histogram);
+      return triangle(histogram) * scale;
     case 'yen':
-      return yen(histogram, image.size);
+      return yen(histogram, image.size) * scale;
     default:
       throw new RangeError(`invalid threshold algorithm: ${algorithm}`);
   }