Fix overlapping subtitles with negative line numbers

libraries/ui/src/main/java/androidx/media3/ui/CanvasSubtitleOutput.java

@@ -26,7 +26,9 @@
 import androidx.media3.common.text.Cue;
 import java.util.ArrayList;
 import java.util.Collections;
+import java.util.HashMap;
 import java.util.List;
+import java.util.Map;
 
 /**
  * A {@link SubtitleView.Output} that uses Android's native layout framework via {@link
@@ -104,12 +106,39 @@ public void dispatchDraw(Canvas canvas) {
       return;
     }
 
+    // Track cumulative offsets per line value to prevent overlapping only among cues that share
+    // the same line number. Cues with different line numbers are already separated by the
+    // line-number positioning math in SubtitlePainter, so they don't need viewport adjustment.
+    // We use Float as the key: DIMEN_UNSET for unpositioned cues, or the actual line value.
+    Map<Float, Integer> cumulativeOffsetByLine = new HashMap<>();
+
     int cueCount = cues.size();
     for (int i = 0; i < cueCount; i++) {
       Cue cue = cues.get(i);
       if (cue.verticalType != Cue.TYPE_UNSET) {
         cue = repositionVerticalCue(cue);
       }
+
+      // Determine the effective line key for grouping.
+      float lineKey = cue.line;
+      boolean isBottomStackedCue =
+          cue.line == Cue.DIMEN_UNSET
+              || (cue.lineType == Cue.LINE_TYPE_NUMBER && cue.line < 0);
+      boolean isTopStackedCue =
+          cue.line != Cue.DIMEN_UNSET && cue.lineType == Cue.LINE_TYPE_NUMBER && cue.line >= 0;
+
+      // Get the cumulative offset for cues at this same line value.
+      int cumulativeOffset = cumulativeOffsetByLine.getOrDefault(lineKey, 0);
+
+      // Adjust boundaries to account for previously drawn cues at the same line.
+      int adjustedTop = top;
+      int adjustedBottom = bottom;
+      if (isBottomStackedCue && cumulativeOffset > 0) {
+        adjustedBottom = bottom - cumulativeOffset;
+      } else if (isTopStackedCue && cumulativeOffset > 0) {
+        adjustedTop = top + cumulativeOffset;
+      }
+
       float cueTextSizePx =
           SubtitleViewUtils.resolveTextSize(
               cue.textSizeType, cue.textSize, rawViewHeight, viewHeightMinusPadding);
@@ -122,9 +151,14 @@ public void dispatchDraw(Canvas canvas) {
           bottomPaddingFraction,
           canvas,
           left,
-          top,
+          adjustedTop,
           right,
-          bottom);
+          adjustedBottom);
+
+      // Accumulate offset so subsequent cues at the same line don't overlap.
+      if (isBottomStackedCue || isTopStackedCue) {
+        cumulativeOffsetByLine.put(lineKey, cumulativeOffset + painter.getLastDrawnCueHeight());
+      }
     }
   }
 

libraries/ui/src/main/java/androidx/media3/ui/SubtitlePainter.java

@@ -52,6 +52,9 @@
   /** Ratio of inner padding to font size. */
   private static final float INNER_PADDING_RATIO = 0.125f;
 
+  /** The height of the last drawn cue, or 0 if no cue was drawn. */
+  private int lastDrawnCueHeight;
+
   // Styled dimensions.
   private final float outlineWidth;
   private final float shadowRadius;
@@ -218,13 +221,27 @@ public void draw(
     if (isTextCue) {
       checkNotNull(cueText);
       setupTextLayout();
+      lastDrawnCueHeight = textLayout != null ? textLayout.getHeight() : 0;
     } else {
       checkNotNull(cueBitmap);
       setupBitmapLayout();
+      lastDrawnCueHeight = bitmapRect != null ? bitmapRect.height() : 0;
     }
     drawLayout(canvas, isTextCue);
   }
 
+  /**
+   * Returns the height of the last drawn cue.
+   *
+   * <p>This can be used to stack multiple cues without overlap by adjusting the drawing bounds for
+   * subsequent cues by this amount.
+   *
+   * @return The height of the last drawn cue in pixels, or 0 if no cue was drawn.
+   */
+  public int getLastDrawnCueHeight() {
+    return lastDrawnCueHeight;
+  }
+
   @RequiresNonNull("cueText")
   private void setupTextLayout() {
     SpannableStringBuilder cueText =