Add support for skipping windows in MssimCalculator.

By skipping every other row and column, SSIM calculation time reduces by 10-30%.

PiperOrigin-RevId: 474286702

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/MssimCalculator.java

@@ -44,23 +44,47 @@ import static java.lang.Math.pow;
 
   private MssimCalculator() {}
 
+  /**
+   * Calculates the Mean Structural Similarity (MSSIM) between two images with window skipping.
+   *
+   * @see #calculate(byte[], byte[], int, int, boolean).
+   */
+  public static double calculate(
+      byte[] referenceBuffer, byte[] distortedBuffer, int width, int height) {
+    return calculate(
+        referenceBuffer, distortedBuffer, width, height, /* enableWindowSkipping= */ true);
+  }
+
   /**
    * Calculates the Mean Structural Similarity (MSSIM) between two images.
    *
+   * <p>The images are split into a grid of windows. For each window, the structural similarity
+   * (SSIM) is calculated. The MSSIM returned from this method is the mean of these SSIM values. If
+   * window skipping is enabled, only every other row and column are considered, thereby only one in
+   * four windows are evaluated.
+   *
    * @param referenceBuffer The luma channel (Y) buffer of the reference image.
    * @param distortedBuffer The luma channel (Y) buffer of the distorted image.
    * @param width The image width in pixels.
    * @param height The image height in pixels.
+   * @param enableWindowSkipping Whether to skip every other row and column when evaluating windows
+   *     for SSIM calculation.
    * @return The MSSIM score between the input images.
    */
   public static double calculate(
-      byte[] referenceBuffer, byte[] distortedBuffer, int width, int height) {
+      byte[] referenceBuffer,
+      byte[] distortedBuffer,
+      int width,
+      int height,
+      boolean enableWindowSkipping) {
     double totalSsim = 0;
     int windowsCount = 0;
 
-    for (int currentWindowY = 0; currentWindowY < height; currentWindowY += WINDOW_SIZE) {
+    int dimensionIncrement = WINDOW_SIZE * (enableWindowSkipping ? 2 : 1);
+
+    for (int currentWindowY = 0; currentWindowY < height; currentWindowY += dimensionIncrement) {
       int windowHeight = computeWindowSize(currentWindowY, height);
-      for (int currentWindowX = 0; currentWindowX < width; currentWindowX += WINDOW_SIZE) {
+      for (int currentWindowX = 0; currentWindowX < width; currentWindowX += dimensionIncrement) {
         windowsCount++;
         int windowWidth = computeWindowSize(currentWindowX, width);
         int bufferIndexOffset =

library/transformer/src/test/java/com/google/android/exoplayer2/transformer/MssimCalculatorTest.java

@@ -62,6 +62,34 @@ public class MssimCalculatorTest {
         .isEqualTo(63);
   }
 
+  @Test
+  public void calculateSsim_withWindowSkipping_similarToWithout() throws Exception {
+    Bitmap referenceBitmap = readBitmap("media/bitmap/sample_mp4_first_frame/original.png");
+    Bitmap distortedBitmap =
+        readBitmap("media/bitmap/sample_mp4_first_frame/increase_brightness.png");
+    byte[] referenceLuminosity = bitmapToLuminosityArray(referenceBitmap);
+    byte[] distortedLuminosity = bitmapToLuminosityArray(distortedBitmap);
+
+    assertThat(
+            (int)
+                (MssimCalculator.calculate(
+                        referenceLuminosity,
+                        distortedLuminosity,
+                        referenceBitmap.getWidth(),
+                        referenceBitmap.getHeight(),
+                        /* enableWindowSkipping= */ false)
+                    * 100))
+        .isEqualTo(
+            (int)
+                (MssimCalculator.calculate(
+                        referenceLuminosity,
+                        distortedLuminosity,
+                        referenceBitmap.getWidth(),
+                        referenceBitmap.getHeight(),
+                        /* enableWindowSkipping= */ true)
+                    * 100));
+  }
+
   private static Bitmap readBitmap(String assetString) throws IOException {
     try (InputStream inputStream = getApplicationContext().getAssets().open(assetString)) {
       return BitmapFactory.decodeStream(inputStream);