为什么我的所有视频缩略图都是黑色的

Question

我的视频播放器效果很好，但每个视频格式的缩略图都是黑色的。我的代码是

public static Bitmap getVideoBitmap(String path, int widthWanted, int heightWanted) {
    Bitmap thumbnail = null;
    MediaMetadataRetriever retriever = new MediaMetadataRetriever();
    try {
        retriever.setDataSource(path);
    } catch (IllegalArgumentException e) {
        retriever.release();
        retriever = null;
        return null;
    }

    String duration = retriever.extractMetadata(MediaMetadataRetriever.METADATA_KEY_DURATION);
    int timeUs = Integer.valueOf(duration);
    for(int i = 1; i <= timeUs; i++) {
        thumbnail = retriever.getFrameAtTime(i * 1000 * 1000, MediaMetadataRetriever.OPTION_NEXT_SYNC);
    }
    thumbnail = getWantedBitmap(thumbnail, widthWanted, heightWanted);
    return thumbnail;
}