Add support for changing translucency of overlays through alpha values

Implements milestone 1.3 of the [overlays implementation plan](https://docs.google.com/document/d/1EcP2GN8k8N74hHZyD0KTqm9oQo5-W1dZMqIVyqVGtlo/edit#bookmark=id.76uzcie1dg9d)

PiperOrigin-RevId: 493290147

libraries/effect/src/androidTest/java/androidx/media3/effect/OverlayTextureProcessorPixelTest.java

@@ -64,6 +64,8 @@ public class OverlayTextureProcessorPixelTest {
       "media/bitmap/sample_mp4_first_frame/electrical_colors/overlay_bitmap_default.png";
   public static final String OVERLAY_BITMAP_SCALED =
       "media/bitmap/sample_mp4_first_frame/electrical_colors/overlay_bitmap_scaled.png";
+  public static final String OVERLAY_BITMAP_TRANSLUCENT =
+      "media/bitmap/sample_mp4_first_frame/electrical_colors/overlay_bitmap_translucent.png";
   public static final String OVERLAY_TEXT_DEFAULT =
       "media/bitmap/sample_mp4_first_frame/electrical_colors/overlay_text_default.png";
   public static final String OVERLAY_TEXT_TRANSLATE =
@@ -167,6 +169,59 @@ public class OverlayTextureProcessorPixelTest {
     assertThat(averagePixelAbsoluteDifference).isAtMost(MAXIMUM_AVERAGE_PIXEL_ABSOLUTE_DIFFERENCE);
   }
 
+  @Test
+  public void drawFrame_translucentBitmapOverlay_blendsBitmapIntoFrame() throws Exception {
+    String testId = "drawFrame_translucentBitmapOverlay";
+    Bitmap bitmap = readBitmap(OVERLAY_PNG_ASSET_PATH);
+    OverlaySettings overlaySettings = new OverlaySettings.Builder().setAlpha(0.5f).build();
+    BitmapOverlay translucentBitmapOverlay =
+        BitmapOverlay.createStaticBitmapOverlay(bitmap, overlaySettings);
+    overlayTextureProcessor =
+        new OverlayEffect(ImmutableList.of(translucentBitmapOverlay))
+            .toGlTextureProcessor(context, false);
+    Pair<Integer, Integer> outputSize = overlayTextureProcessor.configure(inputWidth, inputHeight);
+    setupOutputTexture(outputSize.first, outputSize.second);
+    Bitmap expectedBitmap = readBitmap(OVERLAY_BITMAP_TRANSLUCENT);
+
+    overlayTextureProcessor.drawFrame(inputTexId, /* presentationTimeUs= */ 0);
+    Bitmap actualBitmap =
+        createArgb8888BitmapFromCurrentGlFramebuffer(outputSize.first, outputSize.second);
+
+    maybeSaveTestBitmapToCacheDirectory(testId, /* bitmapLabel= */ "actual", actualBitmap);
+    float averagePixelAbsoluteDifference =
+        getBitmapAveragePixelAbsoluteDifferenceArgb8888(expectedBitmap, actualBitmap, testId);
+    assertThat(averagePixelAbsoluteDifference).isAtMost(MAXIMUM_AVERAGE_PIXEL_ABSOLUTE_DIFFERENCE);
+  }
+
+  @Test
+  public void drawFrame_transparentTextOverlay_blendsBitmapIntoFrame() throws Exception {
+    String testId = "drawFrame_transparentTextOverlay";
+    SpannableString overlayText = new SpannableString(/* source= */ "Text styling");
+    OverlaySettings overlaySettings = new OverlaySettings.Builder().setAlpha(0f).build();
+    overlayText.setSpan(
+        new ForegroundColorSpan(Color.GRAY),
+        /* start= */ 0,
+        /* end= */ 4,
+        Spannable.SPAN_EXCLUSIVE_EXCLUSIVE);
+    TextOverlay staticTextOverlay =
+        TextOverlay.createStaticTextOverlay(overlayText, overlaySettings);
+    overlayTextureProcessor =
+        new OverlayEffect(ImmutableList.of(staticTextOverlay))
+            .toGlTextureProcessor(context, /* useHdr= */ false);
+    Pair<Integer, Integer> outputSize = overlayTextureProcessor.configure(inputWidth, inputHeight);
+    setupOutputTexture(outputSize.first, outputSize.second);
+    Bitmap expectedBitmap = readBitmap(ORIGINAL_PNG_ASSET_PATH);
+
+    overlayTextureProcessor.drawFrame(inputTexId, /* presentationTimeUs= */ 0);
+    Bitmap actualBitmap =
+        createArgb8888BitmapFromCurrentGlFramebuffer(outputSize.first, outputSize.second);
+
+    maybeSaveTestBitmapToCacheDirectory(testId, /* bitmapLabel= */ "actual", actualBitmap);
+    float averagePixelAbsoluteDifference =
+        getBitmapAveragePixelAbsoluteDifferenceArgb8888(expectedBitmap, actualBitmap, testId);
+    assertThat(averagePixelAbsoluteDifference).isAtMost(MAXIMUM_AVERAGE_PIXEL_ABSOLUTE_DIFFERENCE);
+  }
+
   @Test
   public void drawFrame_textOverlay_blendsTextIntoFrame() throws Exception {
     String testId = "drawFrame_textOverlay";

libraries/effect/src/main/assets/shaders/fragment_shader_overlay_es2.glsl

@@ -20,6 +20,9 @@ precision mediump float;
 uniform sampler2D uVideoTexSampler0;
 // Texture containing the overlay bitmap.
 uniform sampler2D uOverlayTexSampler1;
+// The alpha values for the texture.
+uniform float uOverlayAlpha1;
+
 varying vec2 vVideoTexSamplingCoord;
 varying vec2 vOverlayTexSamplingCoord1;
 
@@ -27,15 +30,18 @@ varying vec2 vOverlayTexSamplingCoord1;
 // (https://open.gl/textures) since it's not implemented until OpenGL ES 3.2.
 vec4 getClampToBorderOverlayColor() {
   if (vOverlayTexSamplingCoord1.x > 1.0 || vOverlayTexSamplingCoord1.x < 0.0
-    || vOverlayTexSamplingCoord1.y > 1.0 || vOverlayTexSamplingCoord1.y < 0.0){
+    || vOverlayTexSamplingCoord1.y > 1.0 || vOverlayTexSamplingCoord1.y < 0.0) {
     return vec4(0.0, 0.0, 0.0, 0.0);
   } else {
-    return vec4(texture2D(uOverlayTexSampler1, vOverlayTexSamplingCoord1));
+    vec4 overlayColor = vec4(
+      texture2D(uOverlayTexSampler1, vOverlayTexSamplingCoord1));
+    overlayColor.a = uOverlayAlpha1 * overlayColor.a;
+    return overlayColor;
   }
 }
 
 float getMixAlpha(float videoAlpha, float overlayAlpha) {
-  if (videoAlpha == 0.0){
+  if (videoAlpha == 0.0) {
     return 1.0;
   } else {
     return clamp(overlayAlpha/videoAlpha, 0.0, 1.0);

libraries/effect/src/main/java/androidx/media3/effect/OverlaySettings.java

@@ -14,6 +14,9 @@ package androidx.media3.effect;
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+import static androidx.media3.common.util.Assertions.checkArgument;
+
+import androidx.annotation.FloatRange;
 import androidx.media3.common.util.GlUtil;
 import androidx.media3.common.util.UnstableApi;
 import com.google.errorprone.annotations.CanIgnoreReturnValue;
@@ -22,16 +25,19 @@ import com.google.errorprone.annotations.CanIgnoreReturnValue;
 @UnstableApi
 public final class OverlaySettings {
   public final boolean useHdr;
+  public final float alpha;
   public final float[] matrix;
 
-  private OverlaySettings(boolean useHdr, float[] matrix) {
+  private OverlaySettings(boolean useHdr, float alpha, float[] matrix) {
     this.useHdr = useHdr;
+    this.alpha = alpha;
     this.matrix = matrix;
   }
 
   /** A builder for {@link OverlaySettings} instances. */
   public static final class Builder {
     private boolean useHdr;
+    private float alpha = 1;
     private float[] matrix;
 
     /** Creates a new {@link Builder}. */
@@ -63,9 +69,23 @@ public final class OverlaySettings {
       return this;
     }
 
+    /**
+     * Sets the alpha value of the overlay, altering its transparency.
+     *
+     * <p>Alpha values range from 0 (all transparent) to 1 (completely opaque).
+     *
+     * <p>Set to always return {@code 1} by default.
+     */
+    @CanIgnoreReturnValue
+    public Builder setAlpha(@FloatRange(from = 0, to = 1) float alpha) {
+      checkArgument(0 <= alpha && alpha <= 1, "Alpha needs to be in the interval [0, 1].");
+      this.alpha = alpha;
+      return this;
+    }
+
     /** Creates an instance of {@link OverlaySettings}, using defaults if values are unset. */
     public OverlaySettings build() {
-      return new OverlaySettings(useHdr, matrix);
+      return new OverlaySettings(useHdr, alpha, matrix);
     }
   }
 }

libraries/effect/src/main/java/androidx/media3/effect/OverlayTextureProcessor.java

@@ -99,13 +99,14 @@ import java.io.IOException;
             videoHeight / (float) overlayTextureSize.second,
             /* z= */ 1);
         glProgram.setFloatsUniform("uAspectRatioMatrix", aspectRatioMatrix);
-
         Matrix.invertM(
             overlayMatrix,
             MATRIX_OFFSET,
             overlay.getOverlaySettings(presentationTimeUs).matrix,
             MATRIX_OFFSET);
         glProgram.setFloatsUniform("uOverlayMatrix", overlayMatrix);
+        glProgram.setFloatUniform(
+            "uOverlayAlpha1", overlay.getOverlaySettings(presentationTimeUs).alpha);
 
       } else {
         glProgram.setSamplerTexIdUniform(