feat(autoLevel): add uniform option

Refs: https://github.com/zakodium/profid-padif-rework/issues/21

Author: opatiny

src/filters/__tests__/autoLevel.test.ts

@@ -16,6 +16,33 @@ test('3x1 rgba image, custom output min and max', () => {
   ]);
 });
 
+test('3x1 rgba image, uniform = true, image should not change', () => {
+  const image = testUtils.createRgbaImage([
+    [0, 100, 0, 50],
+    [127, 150, 0, 50],
+    [255, 200, 0, 50],
+  ]);
+
+  const result = autoLevel(image, { uniform: true });
+
+  expect(result).toMatchImage(image);
+});
+
+test('3x1 rgb image, uniform = true', () => {
+  const image = testUtils.createRgbImage([
+    [0, 100, 0],
+    [150, 150, 0],
+    [200, 200, 0],
+  ]);
+
+  const result = autoLevel(image, { uniform: true });
+  expect(result).toMatchImageData([
+    [0, 127, 0],
+    [191, 191, 0],
+    [255, 255, 0],
+  ]);
+});
+
 test('1x3 grey image', () => {
   const image = testUtils.createGreyImage([[50, 100]]);
   expect(autoLevel(image)).toMatchImageData([[0, 255]]);

src/filters/autoLevel.ts

@@ -7,6 +7,11 @@ export interface AutoLevelOptions {
    * Image to which to output.
    */
   out?: Image;
+  /**
+   * Level all channels uniformly in order to preserve the color balance.
+   * @default false
+   */
+  uniform?: boolean;
 }
 
 /**
@@ -16,11 +21,29 @@ export interface AutoLevelOptions {
  * @returns The enhanced image.
  */
 export function autoLevel(image: Image, options: AutoLevelOptions = {}): Image {
+  const { uniform = false } = options;
   checkProcessable(image, {
     bitDepth: [8, 16],
   });
   const minMax = image.minMax();
 
+  let min: number | number[] = minMax.min;
+  let max: number | number[] = minMax.max;
+
+  if (uniform) {
+    let minDiffIndex = -1;
+    let previousDiff = -1;
+    for (let i = 0; i < minMax.max.length; i++) {
+      const difference = minMax.max[i] - minMax.min[i];
+      if (difference > previousDiff) {
+        minDiffIndex = i;
+        previousDiff = difference;
+      }
+    }
+    min = minMax.min[minDiffIndex];
+    max = minMax.max[minDiffIndex];
+  }
+
   let channels: number[] = new Array(image.components)
     .fill(0)
     .map((value, index) => index);
@@ -32,8 +55,8 @@ export function autoLevel(image: Image, options: AutoLevelOptions = {}): Image {
   }
 
   return image.level({
-    inputMin: minMax.min,
-    inputMax: minMax.max,
+    inputMin: min,
+    inputMax: max,
     outputMin: 0,
     outputMax: image.maxValue,
     channels,