Overlay: Rename alpha to alphaScale.

The actual logic scales the alpha, instead of setting it, so rename
this to what it does.

We would also prefer alpha to be scaled here, to not lose alpha information
(for example for PNGs with transparent regions).

PiperOrigin-RevId: 560121708

demos/transformer/src/main/java/androidx/media3/demo/transformer/TransformerActivity.java

@@ -617,7 +617,7 @@ public final class TransformerActivity extends AppCompatActivity {
     if (selectedEffects[ConfigurationActivity.BITMAP_OVERLAY_INDEX]) {
       OverlaySettings overlaySettings =
           new OverlaySettings.Builder()
-              .setAlpha(
+              .setAlphaScale(
                   bundle.getFloat(
                       ConfigurationActivity.BITMAP_OVERLAY_ALPHA, /* defaultValue= */ 1))
               .build();
@@ -631,7 +631,7 @@ public final class TransformerActivity extends AppCompatActivity {
     if (selectedEffects[ConfigurationActivity.TEXT_OVERLAY_INDEX]) {
       OverlaySettings overlaySettings =
           new OverlaySettings.Builder()
-              .setAlpha(
+              .setAlphaScale(
                   bundle.getFloat(ConfigurationActivity.TEXT_OVERLAY_ALPHA, /* defaultValue= */ 1))
               .build();
       SpannableString overlayText =

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

@@ -253,7 +253,7 @@ public class OverlayShaderProgramPixelTest {
   @RequiresNonNull("testId")
   public void drawFrame_translucentBitmapOverlay_blendsBitmapIntoFrame() throws Exception {
     Bitmap bitmap = readBitmap(OVERLAY_PNG_ASSET_PATH);
-    OverlaySettings overlaySettings = new OverlaySettings.Builder().setAlpha(0.5f).build();
+    OverlaySettings overlaySettings = new OverlaySettings.Builder().setAlphaScale(0.5f).build();
     BitmapOverlay translucentBitmapOverlay =
         BitmapOverlay.createStaticBitmapOverlay(bitmap, overlaySettings);
     overlayShaderProgram =
@@ -277,7 +277,7 @@ public class OverlayShaderProgramPixelTest {
   @RequiresNonNull("testId")
   public void drawFrame_transparentTextOverlay_blendsBitmapIntoFrame() throws Exception {
     SpannableString overlayText = new SpannableString(/* source= */ "Text styling");
-    OverlaySettings overlaySettings = new OverlaySettings.Builder().setAlpha(0f).build();
+    OverlaySettings overlaySettings = new OverlaySettings.Builder().setAlphaScale(0f).build();
     overlayText.setSpan(
         new ForegroundColorSpan(Color.GRAY),
         /* start= */ 0,
@@ -400,7 +400,7 @@ public class OverlayShaderProgramPixelTest {
         new OverlaySettings.Builder().setVideoFrameAnchor(0.5f, 0.5f).build();
     TextOverlay textOverlay = TextOverlay.createStaticTextOverlay(overlayText, overlaySettings1);
     Bitmap bitmap = readBitmap(OVERLAY_PNG_ASSET_PATH);
-    OverlaySettings overlaySettings2 = new OverlaySettings.Builder().setAlpha(0.5f).build();
+    OverlaySettings overlaySettings2 = new OverlaySettings.Builder().setAlphaScale(0.5f).build();
     BitmapOverlay bitmapOverlay = BitmapOverlay.createStaticBitmapOverlay(bitmap, overlaySettings2);
     overlayShaderProgram =
         new OverlayEffect(ImmutableList.of(textOverlay, bitmapOverlay))

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

@@ -26,7 +26,7 @@ import com.google.errorprone.annotations.CanIgnoreReturnValue;
 @UnstableApi
 public final class OverlaySettings {
   public final boolean useHdr;
-  public final float alpha;
+  public final float alphaScale;
   public final Pair<Float, Float> videoFrameAnchor;
   public final Pair<Float, Float> overlayAnchor;
   public final Pair<Float, Float> scale;
@@ -34,13 +34,13 @@ public final class OverlaySettings {
 
   private OverlaySettings(
       boolean useHdr,
-      float alpha,
+      float alphaScale,
       Pair<Float, Float> videoFrameAnchor,
       Pair<Float, Float> overlayAnchor,
       Pair<Float, Float> scale,
       float rotationDegrees) {
     this.useHdr = useHdr;
-    this.alpha = alpha;
+    this.alphaScale = alphaScale;
     this.videoFrameAnchor = videoFrameAnchor;
     this.overlayAnchor = overlayAnchor;
     this.scale = scale;
@@ -50,7 +50,7 @@ public final class OverlaySettings {
   /** A builder for {@link OverlaySettings} instances. */
   public static final class Builder {
     private boolean useHdr;
-    private float alpha;
+    private float alphaScale;
     private Pair<Float, Float> videoFrameAnchor;
     private Pair<Float, Float> overlayAnchor;
     private Pair<Float, Float> scale;
@@ -58,7 +58,7 @@ public final class OverlaySettings {
 
     /** Creates a new {@link Builder}. */
     public Builder() {
-      alpha = 1f;
+      alphaScale = 1f;
       videoFrameAnchor = Pair.create(0f, 0f);
       overlayAnchor = Pair.create(0f, 0f);
       scale = Pair.create(1f, 1f);
@@ -80,15 +80,15 @@ public final class OverlaySettings {
     /**
      * Sets the alpha scale value of the overlay, altering its translucency.
      *
-     * <p>An {@code alpha} value of {@code 1} means no change is applied. A value below {@code 1}
-     * increases translucency, and a value above {@code 1} reduces translucency.
+     * <p>An {@code alphaScale} value of {@code 1} means no change is applied. A value below {@code
+     * 1} increases translucency, and a value above {@code 1} reduces translucency.
      *
      * <p>Set to always return {@code 1} by default.
      */
     @CanIgnoreReturnValue
-    public Builder setAlpha(@FloatRange(from = 0) float alpha) {
-      checkArgument(0 <= alpha, "Alpha needs to be more than or equal to zero.");
-      this.alpha = alpha;
+    public Builder setAlphaScale(@FloatRange(from = 0) float alphaScale) {
+      checkArgument(0 <= alphaScale, "alphaScale needs to be greater than or equal to zero.");
+      this.alphaScale = alphaScale;
       return this;
     }
 
@@ -165,7 +165,7 @@ public final class OverlaySettings {
     /** Creates an instance of {@link OverlaySettings}, using defaults if values are unset. */
     public OverlaySettings build() {
       return new OverlaySettings(
-          useHdr, alpha, videoFrameAnchor, overlayAnchor, scale, rotationDegrees);
+          useHdr, alphaScale, videoFrameAnchor, overlayAnchor, scale, rotationDegrees);
     }
   }
 }

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

@@ -276,7 +276,8 @@ import com.google.common.collect.ImmutableList;
               Util.formatInvariant("uTransformationMatrix%d", texUnitIndex), transformationMatrix);
 
           glProgram.setFloatUniform(
-              Util.formatInvariant("uOverlayAlpha%d", texUnitIndex), overlaySettings.alpha);
+              Util.formatInvariant("uOverlayAlphaScale%d", texUnitIndex),
+              overlaySettings.alphaScale);
         }
       }
       glProgram.setSamplerTexIdUniform("uVideoTexSampler0", inputTexId, /* texUnitIndex= */ 0);
@@ -387,7 +388,7 @@ import com.google.common.collect.ImmutableList;
     for (int texUnitIndex = 1; texUnitIndex <= numOverlays; texUnitIndex++) {
       shader
           .append(Util.formatInvariant("uniform sampler2D uOverlayTexSampler%d;\n", texUnitIndex))
-          .append(Util.formatInvariant("uniform float uOverlayAlpha%d;\n", texUnitIndex))
+          .append(Util.formatInvariant("uniform float uOverlayAlphaScale%d;\n", texUnitIndex))
           .append(Util.formatInvariant("varying vec2 vOverlayTexSamplingCoord%d;\n", texUnitIndex));
     }
 
@@ -405,7 +406,7 @@ import com.google.common.collect.ImmutableList;
                   texUnitIndex))
           .append(
               Util.formatInvariant(
-                  "    uOverlayTexSampler%d, vOverlayTexSamplingCoord%d, uOverlayAlpha%d);\n",
+                  "    uOverlayTexSampler%d, vOverlayTexSamplingCoord%d, uOverlayAlphaScale%d);\n",
                   texUnitIndex, texUnitIndex, texUnitIndex))
           .append(Util.formatInvariant("  vec4 opticalOverlayColor%d = vec4(\n", texUnitIndex))
           .append(