Preallocate buffer and use byte for Luminance in SSIM.

PiperOrigin-RevId: 439855702

library/transformer/src/androidTest/java/com/google/android/exoplayer2/transformer/SsimHelper.java

@@ -50,6 +50,9 @@ import java.nio.ByteBuffer;
  * images like MSE (mean squared error), but rather outputs the human perceptual difference. A
  * higher SSIM score signifies higher similarity, while a SSIM score of 1 means the two images are
  * exactly the same.
+ *
+ * <p>SSIM is traditionally computed with the luminance channel (Y), this class uses the luma
+ * channel (Y') because the {@linkplain MediaCodec decoder} decodes to luma.
  */
 public final class SsimHelper {
 
@@ -76,6 +79,8 @@ public final class SsimHelper {
         new VideoDecodingWrapper(context, expectedVideoPath, DEFAULT_COMPARISON_INTERVAL);
     VideoDecodingWrapper actualDecodingWrapper =
         new VideoDecodingWrapper(context, actualVideoPath, DEFAULT_COMPARISON_INTERVAL);
+    @Nullable byte[] expectedLumaBuffer = null;
+    @Nullable byte[] actualLumaBuffer = null;
     double accumulatedSsim = 0.0;
     int comparedImagesCount = 0;
     try {
@@ -90,13 +95,21 @@ public final class SsimHelper {
 
         int width = expectedImage.getWidth();
         int height = expectedImage.getHeight();
+
         assertThat(actualImage.getWidth()).isEqualTo(width);
         assertThat(actualImage.getHeight()).isEqualTo(height);
+
+        if (expectedLumaBuffer == null || expectedLumaBuffer.length != width * height) {
+          expectedLumaBuffer = new byte[width * height];
+        }
+        if (actualLumaBuffer == null || actualLumaBuffer.length != width * height) {
+          actualLumaBuffer = new byte[width * height];
+        }
         try {
           accumulatedSsim +=
               SsimCalculator.calculate(
-                  extractLumaChannelBuffer(expectedImage),
-                  extractLumaChannelBuffer(actualImage),
+                  extractLumaChannelBuffer(expectedImage, expectedLumaBuffer),
+                  extractLumaChannelBuffer(actualImage, actualLumaBuffer),
                   /* offset= */ 0,
                   /* stride= */ width,
                   width,
@@ -116,11 +129,13 @@ public final class SsimHelper {
   }
 
   /**
-   * Returns the buffer of the luma (Y) channel of the image.
+   * Extracts, sets and returns the buffer of the luma (Y') channel of the image.
    *
    * @param image The {@link Image} in YUV format.
+   * @param lumaChannelBuffer The buffer where the extracted luma values are stored.
+   * @return The {@code lumaChannelBuffer} for convenience.
    */
-  private static int[] extractLumaChannelBuffer(Image image) {
+  private static byte[] extractLumaChannelBuffer(Image image, byte[] lumaChannelBuffer) {
     // This method is invoked on the main thread.
     // `image` should contain YUV channels.
     Image.Plane[] imagePlanes = image.getPlanes();
@@ -131,7 +146,6 @@ public final class SsimHelper {
     int width = image.getWidth();
     int height = image.getHeight();
     ByteBuffer lumaByteBuffer = lumaPlane.getBuffer();
-    int[] lumaChannelBuffer = new int[width * height];
     for (int y = 0; y < height; y++) {
       for (int x = 0; x < width; x++) {
         lumaChannelBuffer[y * width + x] = lumaByteBuffer.get(y * rowStride + x * pixelStride);
@@ -148,7 +162,7 @@ public final class SsimHelper {
     // Use ExoPlayer's 10ms timeout setting. In practise, the test durations from using timeouts of
     // 1/10/100ms don't differ significantly.
     private static final long DEQUEUE_TIMEOUT_US = 10_000;
-    // SSIM should be calculated using the luma (Y) channel, thus using the YUV color space.
+    // SSIM should be calculated using the luma (Y') channel, thus using the YUV color space.
     private static final int IMAGE_READER_COLOR_SPACE = ImageFormat.YUV_420_888;
     private static final int MEDIA_CODEC_COLOR_SPACE =
         MediaCodecInfo.CodecCapabilities.COLOR_FormatYUV420Flexible;
@@ -333,8 +347,8 @@ public final class SsimHelper {
     /**
      * Calculates the Structural Similarity Index (SSIM) between two images.
      *
-     * @param expected The luminance channel (Y) bitmap of the expected image.
-     * @param actual The luminance channel (Y) bitmap of the actual image.
+     * @param expected The luma channel (Y) bitmap of the expected image.
+     * @param actual The luma channel (Y) bitmap of the actual image.
      * @param offset The offset.
      * @param stride The stride of the bitmap.
      * @param width The image width in pixels.
@@ -342,7 +356,7 @@ public final class SsimHelper {
      * @return The SSIM score between the input images.
      */
     public static double calculate(
-        int[] expected, int[] actual, int offset, int stride, int width, int height) {
+        byte[] expected, byte[] actual, int offset, int stride, int width, int height) {
       double totalSsim = 0;
       int windowsCount = 0;
 
@@ -398,7 +412,7 @@ public final class SsimHelper {
 
     /** Returns the means of the pixels in the two provided windows, in order. */
     private static double getMean(
-        int[] pixels, int start, int stride, int windowWidth, int windowHeight) {
+        byte[] pixels, int start, int stride, int windowWidth, int windowHeight) {
       double total = 0;
       for (int y = 0; y < windowHeight; y++) {
         for (int x = 0; x < windowWidth; x++) {
@@ -410,8 +424,8 @@ public final class SsimHelper {
 
     /** Returns the two variances and the covariance of the two windows. */
     private static double[] getVariancesAndCovariance(
-        int[] pixelsX,
-        int[] pixelsY,
+        byte[] pixelsX,
+        byte[] pixelsY,
         double meanX,
         double meanY,
         int start,