video - 解压缩 MP4 文件中的 time-To-Sample 表 (STTS)

Question

我有一个 mp4 视频字节数组，我需要使用它的中间帧为它生成一个缩略图(例如，如果视频长度为 10 秒，那么我需要从第 5 秒开始获取图片)。

我设法解析文件并提取其框(原子)。我还设法从 mvhd 盒子中获取视频长度。我也设法提取
1. stts 框中的 time-To-Sample 表，
2. stcs 框中的 sample-To-Chunk 表，
3. 来自 stco box 的 chunk-Offset 表，
4. stsz box的样本尺寸表，
5. 从 stss 框中同步样本表

我知道所有实际媒体都在 mdat 框中可用，我需要关联上表以找到文件中的确切帧偏移，但我的问题是如何？表数据似乎被压缩(特别是 time-To-Sample 表)，但我不知道如何解压缩它们。

任何帮助表示赞赏。

以下是代码示例

将字节转换为十六进制的代码

public static char[] bytesToHex(byte[] bytes) {
    char[] hexChars = new char[bytes.length * 2];
    for ( int j = 0; j < bytes.length; j++ ) {
        int v = bytes[j] & 0xFF;

        hexChars[j * 2] = hexArray[v >>> 4];
        hexChars[j * 2 + 1] = hexArray[v & 0x0F];            
    }
    return hexChars;
}

获取框偏移的代码

final static String MOOV                          = "6D6F6F76";
final static String MOOV_MVHD                     = "6D766864";
final static String MOOV_TRAK                     = "7472616B";
final static String MOOV_TRAK_MDIA                = "6D646961";
final static String MOOV_TRAK_MDIA_MINF           = "6D696E66";
final static String MOOV_TRAK_MDIA_MINF_STBL      = "7374626C";
final static String MOOV_TRAK_MDIA_MINF_STBL_STSD = "73747364";
final static String MOOV_TRAK_MDIA_MINF_STBL_STTS = "73747473";
final static String MOOV_TRAK_MDIA_MINF_STBL_STSS = "73747373";
final static String MOOV_TRAK_MDIA_MINF_STBL_STSC = "73747363";
final static String MOOV_TRAK_MDIA_MINF_STBL_STCO = "7374636F";
final static String MOOV_TRAK_MDIA_MINF_STBL_STSZ = "7374737A";

static int getBox(char[] s, int offset, String type) {
    int typeOffset = -1;
    for (int i = offset*2; i<s.length; ) {
        String sizeHex = new String(Arrays.copyOfRange(s, i, i + 8));
        String typeHex = new String(Arrays.copyOfRange(s, i + 8, i + 16));
        int size = Integer.parseInt(sizeHex, 16);

        if (typeHex.equals(type)) {
            typeOffset = (i/2);
            break;
        } else if (typeHex.equals(MOOV) 
            || typeHex.equals(MOOV_TRAK) 
            || typeHex.equals(MOOV_TRAK_MDIA) 
            || typeHex.equals(MOOV_TRAK_MDIA_MINF)
            || typeHex.equals(MOOV_TRAK_MDIA_MINF_STBL)) {
            int x = (i/2) + 8;
            typeOffset = getBox(s, x, type);
            if (typeOffset>-1) {
                break;
            } 
        }
        i+=(size*2);
    }

    return typeOffset;
}

获取持续时间和时间刻度的代码

static int[] getDuration(char[] s) {
    int mvhdOffset = getBox(s, 0, MOOV_MVHD);
    int timeScaleStart = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4)*2;
    int timeScaleEnd   = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;

    int durationStart  = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4)*2;
    int durationEnd    = (mvhdOffset*2) + (4 + 4 + 1 + 3 + 4 + 4 + 4 + 4)*2;

    String timeScaleHex = new String(Arrays.copyOfRange(s, timeScaleStart, timeScaleEnd));
    String durationHex = new String(Arrays.copyOfRange(s, durationStart, durationEnd));

    int timeScale = Integer.parseInt(timeScaleHex, 16);
    int duration = Integer.parseInt(durationHex, 16);

    int[] result = {duration, timeScale};
    return result;
}

获取 time-To-Sample 表的代码

static int[][] getTimeToSampleTable(char[] s, int trakOffset) {
    int offset = getBox(s, trakOffset, MOOV_TRAK_MDIA_MINF_STBL_STTS);
    int sizeStart = offset*2;
    int sizeEnd   = offset*2 + (4)*2;

    int typeStart = offset*2 + (4)*2;
    int typeEnd   = offset*2 + (4 + 4)*2;

    int noOfEntriesStart = offset*2 + (4 + 4 + 1 + 3)*2;
    int noOfEntriesEnd   = offset*2 + (4 + 4 + 1 + 3 + 4)*2;

    String sizeHex = new String(Arrays.copyOfRange(s, sizeStart, sizeEnd));
    String typeHex = new String(Arrays.copyOfRange(s, typeStart, typeEnd));
    String noOfEntriesHex = new String(Arrays.copyOfRange(s, noOfEntriesStart, noOfEntriesEnd));

    int size = Integer.parseInt(sizeHex, 16);
    int noOfEntries = Integer.parseInt(noOfEntriesHex, 16);

    int[][] timeToSampleTable = new int[noOfEntries][2];

    for (int i = 0; i<noOfEntries; i++) {
        int sampleCountStart = noOfEntriesEnd + ((i)*((4 + 4)*2));
        int sampleCountEnd   = noOfEntriesEnd + ((i)*((4 + 4)*2)) + (4)*2;

        int sampleDurationStart = noOfEntriesEnd + ((i)*((4 + 4)*2)) + (4)*2;
        int sampleDurationEnd   = noOfEntriesEnd + ((i)*((4 + 4)*2)) + (4 + 4)*2;

        String sampleCountHex = new String(Arrays.copyOfRange(s, sampleCountStart, sampleCountEnd));
        String sampleDurationHex = new String(Arrays.copyOfRange(s, sampleDurationStart, sampleDurationEnd));

        timeToSampleTable[i][0] = Integer.parseInt(sampleCountHex, 16);
        timeToSampleTable[i][1] = Integer.parseInt(sampleDurationHex, 16);
    }

    return timeToSampleTable;
} 

Answer 1

ISO/IEC 14496-12 标准描述了您需要的所有步骤。
附录 7 提供了解码步骤。第 8 节描述了这些表格。

希望这可以帮助。