chore: improve alignMinDifference (#389)

Original code: 1700 ms
Avoiding the creation of images: 1200 ms
Changing Math.abs to if / else : 200 ms

Author: lpatiny

src/align/__tests__/alignMinDifference.test.ts

@@ -96,6 +96,5 @@ test('other id crops', () => {
   const result = alignMinDifference(source, destination);
 
   const overlap = overlapImages(source, destination, { origin: result });
-
   expect(overlap).toMatchImageSnapshot();
 });

src/align/alignMinDifference.ts

@@ -1,5 +1,4 @@
 import { Image, ImageColorModel, Point } from '..';
-import { subtract } from '../compare/subtract';
 
 export interface AlignMinDifferenceOptions {
   /**
@@ -46,7 +45,7 @@ export function alignMinDifference(
     destination = destination.grey();
   }
 
-  let bestMean = Number.POSITIVE_INFINITY;
+  let bestDifference = Number.POSITIVE_INFINITY;
   let bestShiftX = 0;
   let bestShiftY = 0;
 
@@ -60,18 +59,29 @@ export function alignMinDifference(
 
     for (let shiftX = startX; shiftX <= endX; shiftX += step) {
       for (let shiftY = startY; shiftY <= endY; shiftY += step) {
-        const destinationCropped = destination.crop({
-          origin: { row: shiftY, column: shiftX },
-          width: source.width,
-          height: source.height,
-        });
-
-        const imagesDiff = subtract(source, destinationCropped, {
-          absolute: true,
-        });
-        const mean = imagesDiff.mean()[0];
-        if (mean < bestMean) {
-          bestMean = mean;
+        let currentDifference = 0;
+        next: for (let column = 0; column < source.width; column++) {
+          for (let row = 0; row < source.height; row++) {
+            const sourceValue = source.getValue(column, row, 0);
+            const destinationValue = destination.getValue(
+              column + shiftX,
+              row + shiftY,
+              0,
+            );
+            const difference = sourceValue - destinationValue;
+            if (difference < 0) {
+              // Math.abs is super slow, this simple trick is 5x faster
+              currentDifference -= difference;
+            } else {
+              currentDifference += difference;
+            }
+            if (currentDifference > bestDifference) {
+              break next;
+            }
+          }
+        }
+        if (currentDifference < bestDifference) {
+          bestDifference = currentDifference;
           bestShiftX = shiftX;
           bestShiftY = shiftY;
         }

src/compare/subtract.ts

@@ -57,9 +57,15 @@ export function subtract(
           image.getValueByIndex(index, channel) -
           otherImage.getValueByIndex(index, channel);
         if (absolute) {
-          newImage.setValueByIndex(index, channel, Math.abs(value));
+          if (value < 0) {
+            newImage.setValueByIndex(index, channel, -value);
+          } else {
+            newImage.setValueByIndex(index, channel, value);
+          }
+        } else if (value < 0) {
+          newImage.setValueByIndex(index, channel, 0);
         } else {
-          newImage.setValueByIndex(index, channel, Math.max(value, 0));
+          newImage.setValueByIndex(index, channel, value);
         }
       }
     }
@@ -68,9 +74,9 @@ export function subtract(
       const value =
         image.getBitByIndex(index) - otherImage.getBitByIndex(index);
       if (absolute) {
-        newImage.setBitByIndex(index, Math.abs(value) ? 1 : 0);
+        newImage.setBitByIndex(index, value ? 1 : 0);
       } else {
-        newImage.setBitByIndex(index, Math.max(value, 0) ? 1 : 0);
+        newImage.setBitByIndex(index, value > 0 ? 1 : 0);
       }
     }
   }