From cfd9a630ff8cacd17880b16b27f32781eaac6bb7 Mon Sep 17 00:00:00 2001
From: Oleksii Trekhleb <trehleb@gmail.com>
Date: Wed, 21 Apr 2021 07:39:58 +0200
Subject: [PATCH] Test that two images are identical for the Seam Carving
 algorithm. (#694)

* Test that two images are identical for the Seam Carving algorithm.

* Tune the Seam Carving tests.

* Tune the Seam Carving tests.

* Tune the Seam Carving tests.

* Tune the Seam Carving tests.
---
 .husky/pre-commit                             |  1 +
 .../__tests__/resizeImageWidth.test.js        | 44 ++++++++++++++++++-
 2 files changed, 43 insertions(+), 2 deletions(-)

diff --git a/.husky/pre-commit b/.husky/pre-commit
index bec1c08a..a29a4166 100755
--- a/.husky/pre-commit
+++ b/.husky/pre-commit
@@ -1,5 +1,6 @@
 #!/bin/sh
 . "$(dirname "$0")/_/husky.sh"
 
+# @TODO: Implement the pre-commit checks.
 # npm run lint
 # npm run test
diff --git a/src/algorithms/image-processing/seam-carving/__tests__/resizeImageWidth.test.js b/src/algorithms/image-processing/seam-carving/__tests__/resizeImageWidth.test.js
index b6400a35..9afed971 100644
--- a/src/algorithms/image-processing/seam-carving/__tests__/resizeImageWidth.test.js
+++ b/src/algorithms/image-processing/seam-carving/__tests__/resizeImageWidth.test.js
@@ -4,6 +4,40 @@ import resizeImageWidth from '../resizeImageWidth';
 const testImageBeforePath = './src/algorithms/image-processing/seam-carving/__tests__/test-image-before.jpg';
 const testImageAfterPath = './src/algorithms/image-processing/seam-carving/__tests__/test-image-after.jpg';
 
+/**
+ * Compares two images and finds the number of different pixels.
+ *
+ * @param {ImageData} imgA - ImageData for the first image.
+ * @param {ImageData} imgB - ImageData for the second image.
+ * @param {number} threshold - Color difference threshold [0..255]. Smaller - stricter.
+ * @returns {number} - Number of different pixels.
+ */
+function pixelsDiff(imgA, imgB, threshold = 0) {
+  if (imgA.width !== imgB.width || imgA.height !== imgB.height) {
+    throw new Error('Images must have the same size');
+  }
+
+  let differentPixels = 0;
+  const numColorParams = 4; // RGBA
+
+  for (let pixelIndex = 0; pixelIndex < imgA.data.length; pixelIndex += numColorParams) {
+    // Get pixel's color for each image.
+    const [aR, aG, aB] = imgA.data.subarray(pixelIndex, pixelIndex + numColorParams);
+    const [bR, bG, bB] = imgB.data.subarray(pixelIndex, pixelIndex + numColorParams);
+
+    // Get average pixel's color for each image (make them greyscale).
+    const aAvgColor = Math.floor((aR + aG + aB) / 3);
+    const bAvgColor = Math.floor((bR + bG + bB) / 3);
+
+    // Compare pixel colors.
+    if (Math.abs(aAvgColor - bAvgColor) > threshold) {
+      differentPixels += 1;
+    }
+  }
+
+  return differentPixels;
+}
+
 describe('resizeImageWidth', () => {
   it('should perform content-aware image width reduction', () => {
     // @see: https://jestjs.io/docs/asynchronous
@@ -21,6 +55,7 @@ describe('resizeImageWidth', () => {
       const canvasAfter = createCanvas(imgAfter.width, imgAfter.height);
       const ctxAfter = canvasAfter.getContext('2d');
       ctxAfter.drawImage(imgAfter, 0, 0, imgAfter.width, imgAfter.height);
+      const imgDataAfter = ctxAfter.getImageData(0, 0, imgAfter.width, imgAfter.height);
 
       const toWidth = Math.floor(imgBefore.width / 2);
 
@@ -44,8 +79,13 @@ describe('resizeImageWidth', () => {
       expect(imgDataTest.width).toBe(imgAfter.width);
       expect(imgDataTest.height).toBe(imgAfter.height);
 
-      // @TODO: Check that images are identical.
-      // expect(canvasTest.toDataURL()).toEqual(canvasAfter.toDataURL());
+      const colorThreshold = 50;
+      const differentPixels = pixelsDiff(imgDataTest, imgDataAfter, colorThreshold);
+
+      // Allow 10% of pixels to be different
+      const pixelsThreshold = Math.floor((imgAfter.width * imgAfter.height) / 10);
+
+      expect(differentPixels).toBeLessThanOrEqual(pixelsThreshold);
     });
   });
 });