Use anti-aliasing and bitmap filtering for bitmap subtitles

issue:#6950
PiperOrigin-RevId: 307411067

RELEASENOTES.md

@@ -95,6 +95,8 @@
         (a
         [previous draft](https://www.w3.org/TR/2014/WD-webvtt1-20141111/#dfn-webvtt-text-position-cue-setting)
         used `start`, `middle` and `end`).
+    *   Use anti-aliasing and bitmap filtering when displaying bitmap subtitles
+        ([#6950](https://github.com/google/ExoPlayer/pull/6950)).
 *   DRM:
     *   Add support for attaching DRM sessions to clear content in the demo app.
     *   Remove `DrmSessionManager` references from all renderers.

library/ui/src/main/java/com/google/android/exoplayer2/ui/SubtitlePainter.java

@@ -65,7 +65,8 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
   private final float spacingAdd;
 
   private final TextPaint textPaint;
-  private final Paint paint;
+  private final Paint windowPaint;
+  private final Paint bitmapPaint;
 
   // Previous input variables.
   @Nullable private CharSequence cueText;
@@ -124,9 +125,13 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
     textPaint.setAntiAlias(true);
     textPaint.setSubpixelText(true);
 
-    paint = new Paint();
-    paint.setAntiAlias(true);
-    paint.setStyle(Style.FILL);
+    windowPaint = new Paint();
+    windowPaint.setAntiAlias(true);
+    windowPaint.setStyle(Style.FILL);
+
+    bitmapPaint = new Paint();
+    bitmapPaint.setAntiAlias(true);
+    bitmapPaint.setFilterBitmap(true);
   }
 
   /**
@@ -442,9 +447,13 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
     canvas.translate(textLeft, textTop);
 
     if (Color.alpha(windowColor) > 0) {
-      paint.setColor(windowColor);
+      windowPaint.setColor(windowColor);
       canvas.drawRect(
-          -textPaddingX, 0, textLayout.getWidth() + textPaddingX, textLayout.getHeight(), paint);
+          -textPaddingX,
+          0,
+          textLayout.getWidth() + textPaddingX,
+          textLayout.getHeight(),
+          windowPaint);
     }
 
     if (edgeType == CaptionStyleCompat.EDGE_TYPE_OUTLINE) {
@@ -478,7 +487,7 @@ import org.checkerframework.checker.nullness.qual.RequiresNonNull;
 
   @RequiresNonNull({"cueBitmap", "bitmapRect"})
   private void drawBitmapLayout(Canvas canvas) {
-    canvas.drawBitmap(cueBitmap, /* src= */ null, bitmapRect, /* paint= */ null);
+    canvas.drawBitmap(cueBitmap, /* src= */ null, bitmapRect, bitmapPaint);
   }
 
   /**