rfc2435.txt

很多RFC的中文文档

?	RTP/JPEG data payload to produce a JPEG compressed image in
?	interchange format (except for possible trailing garbage and
?	absence of an EOI marker to terminate the scan).
 */
int MakeHeaders(u_char *p, int type, int w, int h, u_char *lqt,
                u_char *cqt, u_short dri)
{
        u_char *start = p;

        /* convert from blocks to pixels */
        w <<= 3;
        h <<= 3;

        *p++ = 0xff;
        *p++ = 0xd8;            /* SOI */

        p = MakeQuantHeader(p, lqt, 0);
        p = MakeQuantHeader(p, cqt, 1);

        if (dri != 0)
                p = MakeDRIHeader(p, dri);

        *p++ = 0xff;
        *p++ = 0xc0;            /* SOF */
        *p++ = 0;               /* length msb */
        *p++ = 17;              /* length lsb */
        *p++ = 8;               /* 8-bit precision */
        *p++ = h >> 8;          /* height msb */
        *p++ = h;               /* height lsb */
        *p++ = w >> 8;          /* width msb */
        *p++ = w;               /* wudth lsb */
        *p++ = 3;               /* number of components */
        *p++ = 0;               /* comp 0 */
        if (type == 0)
                *p++ = 0x21;    /* hsamp = 2, vsamp = 1 */
        else
                *p++ = 0x22;    /* hsamp = 2, vsamp = 2 */
        *p++ = 0;               /* quant table 0 */
        *p++ = 1;               /* comp 1 */
        *p++ = 0x11;            /* hsamp = 1, vsamp = 1 */
        *p++ = 1;               /* quant table 1 */
        *p++ = 2;               /* comp 2 */
        *p++ = 0x11;            /* hsamp = 1, vsamp = 1 */
        *p++ = 1;               /* quant table 1 */
        p = MakeHuffmanHeader(p, lum_dc_codelens,
sizeof(lum_dc_codelens),
lum_dc_symbols,
sizeof(lum_dc_symbols), 0, 0);
        p = MakeHuffmanHeader(p, lum_ac_codelens,
sizeof(lum_ac_codelens),
lum_ac_symbols,
sizeof(lum_ac_symbols), 0, 1);
        p = MakeHuffmanHeader(p, chm_dc_codelens,
sizeof(chm_dc_codelens),
chm_dc_symbols,
sizeof(chm_dc_symbols), 1, 0);
        p = MakeHuffmanHeader(p, chm_ac_codelens,
sizeof(chm_ac_codelens),
chm_ac_symbols,
sizeof(chm_ac_symbols), 1, 1);

        *p++ = 0xff;
        *p++ = 0xda;            /* SOS */
        *p++ = 0;               /* length msb */
        *p++ = 12;              /* length lsb */
        *p++ = 3;               /* 3 components */
        *p++ = 0;               /* comp 0 */
        *p++ = 0;               /* Success table 0 */
        *p++ = 1;               /* comp 1 */
        *p++ = 0x11;            /* Success table 1 */
        *p++ = 2;               /* comp 2 */
        *p++ = 0x11;            /* Success table 1 */
        *p++ = 0;               /* first DCT coeff */
        *p++ = 63;              /* last DCT coeff */
        *p++ = 0;               /* Successive approx. */

        return (p – start);
};

附录 C
下面这段代码用来阐明RTP/JPEG数据包分片和头的生成过程。

   For clarity and brevity, the structure definitions are only valid for
   32-bit big-endian (most significant octet first) architectures. Bit
   fields are assumed to be packed tightly in big-endian bit order, with
   no additional padding. Modifications would be required to construct a
   portable implementation.

/*
?	RTP data header from RFC1889
 */
typedef struct {
        unsigned int version:2;   /* protocol version */
        unsigned int p:1;         /* padding flag */
        unsigned int x:1;         /* header extension flag */
        unsigned int cc:4;        /* CSRC count */
        unsigned int m:1;         /* marker bit */
        unsigned int pt:7;        /* payload type */
        u_int16 seq;              /* sequence number */
        u_int32 ts;               /* timestamp */
        u_int32 ssrc;             /* synchronization source */
        u_int32 csrc[1];          /* optional CSRC list */
} rtp_hdr_t;

#define RTP_HDR_SZ 12

/* The following definition is from RFC1890 */
#define RTP_PT_JPEG             26

struct jpeghdr {
        unsigned int tspec:8;   /* type-specific field */
        unsigned int off:24;    /* fragment byte offset */
        u_int8 type;            /* id of jpeg decoder params */
        u_int8 q;               /* quantization factor (or table id) */
        u_int8 width;           /* frame width in 8 pixel blocks */
        u_int8 height;          /* frame height in 8 pixel blocks */
};

struct jpeghdr_rst {
        u_int16 dri;
        unsigned int f:1;
        unsigned int l:1;
        unsigned int count:14;
};

struct jpeghdr_qtable {
        u_int8  mbz;
        u_int8  precision;
        u_int16 length;
};

#define RTP_JPEG_RESTART           0x40

/* Procedure SendFrame:
 *
?	Arguments:
?	start_seq: The sequence number for the first packet of the current
?	frame.
?	ts: RTP timestamp for the current frame
?	ssrc: RTP SSRC value
?	jpeg_data: Huffman encoded JPEG scan data
?	len: Length of the JPEG scan data
?	type: The value the RTP/JPEG type field should be set to
?	typespec: The value the RTP/JPEG type-specific field should be set
?	to
?	width: The width in pixels of the JPEG image
?	height: The height in pixels of the JPEG image
?	dri: The number of MCUs between restart markers (or 0 if there
?	are no restart markers in the data
?	q: The Q factor of the data, to be specified using the Independent
?	JPEG group's algorithm if 1 <= q <= 99, specified explicitly
?	with lqt and cqt if q >= 128, or undefined otherwise.
?	lqt: The quantization table for the luminance channel if q >= 128
?	cqt: The quantization table for the chrominance channels if
?	q >= 128
 *
?	Return value:
?	the sequence number to be sent for the first packet of the next
?	frame.
 *
?	The following are assumed to be defined:
 *
 * PACKET_SIZE                         - The size of the outgoing packet
?	send_packet(u_int8 *data, int len)  - Sends the packet to the network
 */

u_int16 SendFrame(u_int16 start_seq, u_int32 ts, u_int32 ssrc,
                   u_int8 *jpeg_data, int len, u_int8 type,
                   u_int8 typespec, int width, int height, int dri,
                   u_int8 q, u_int8 *lqt, u_int8 *cqt) {
        rtp_hdr_t rtphdr;
        struct jpeghdr jpghdr;
        struct jpeghdr_rst rsthdr;

        struct jpeghdr_qtable qtblhdr;
        u_int8 packet_buf[PACKET_SIZE];
        u_int8 *ptr;
        int bytes_left = len;
        int seq = start_seq;
        int pkt_len, data_len;

        /* Initialize RTP header
         */
        rtphdr.version = 2;
        rtphdr.p = 0;
        rtphdr.x = 0;
        rtphdr.cc = 0;
        rtphdr.m = 0;
        rtphdr.pt = RTP_PT_JPEG;
        rtphdr.seq = start_seq;
        rtphdr.ts = ts;
        rtphdr.ssrc = ssrc;

        /* Initialize JPEG header
         */
        jpghdr.tspec = typespec;
        jpghdr.off = 0;
        jpghdr.type = type | ((dri != 0) ? RTP_JPEG_RESTART : 0);
        jpghdr.q = q;
        jpghdr.width = width / 8;
        jpghdr.height = height / 8;

        /* Initialize DRI header
         */
        if (dri != 0) {
                rsthdr.dri = dri;
                rsthdr.f = 1;        /* This code does not align Ris */
                rsthdr.l = 1;
                rsthdr.count = 0x3fff;
        }

        /* Initialize quantization table header
         */
        if (q >= 128) {
                qtblhdr.mbz = 0;
                qtblhdr.precision = 0; /* This code uses 8 bit tables only */
                qtblhdr.length = 128;  /* 2 64-byte tables */
        }

        while (bytes_left > 0) {
                ptr = packet_buf + RTP_HDR_SZ;
                memcpy(ptr, &jpghdr, sizeof(jpghdr));

                ptr += sizeof(jpghdr);

                if (dri != 0) {
                        memcpy(ptr, &rsthdr, sizeof(rsthdr));
                        ptr += sizeof(rsthdr);
                }

                if (q >= 128 && jpghdr.off == 0) {
                        memcpy(ptr, &qtblhdr, sizeof(qtblhdr));
                        ptr += sizeof(qtblhdr);
                        memcpy(ptr, lqt, 64);
                        ptr += 64;
                        memcpy(ptr, cqt, 64);
                        ptr += 64;
                }

                data_len = PACKET_SIZE – (ptr – packet_buf);
                if (data_len >= bytes_left) {
                        data_len = bytes_left;
                        rtphdr.m = 1;
                }

                memcpy(packet_buf, &rtphdr, RTP_HDR_SZ);
                memcpy(ptr, jpeg_data + jpghdr.off, data_len);

                send_packet(packet_buf, (ptr – packet_buf) + data_len);

                jpghdr.off += data_len;
                bytes_left -= data_len;
                rtphdr.seq++;
        }
        return rtphdr.seq;
}

附录 D
这一部分给出了本文档相对于RFC 2035的改动。这些改动着眼于尽可能保持新版本对于
旧版本的兼容性。事实上，很多已经废弃的约定仍然能够在新版中正常地解码。尽管如此，
我们仍然强烈反对在新版中使用一些旧地约定。
    o   类型0和类型1现在可以编码隔行扫描的视频图象，用类型特定域中的两个比特给
出指示即可。参见4.1节。
o   JPEG工作组曾经就如何更灵活地描述JPEG量化表发生过争论。本备忘录允许通过使
用一个可选地量化表头来显式地描述表系数。这些内容在节3.1.8和4.2中讨论。
       

o   在RFC 2035中，在类型域中描述复位标记的信息，这样就很难再加入新的类型。并
且，类型特定域用于记录复位计数，这样其它的一些类型特定信息就无法编码。在
本备忘录中，复位标记的指示移到了类型域中的一个特定比特位，并且加入了可选
头来编码一些必要的额外信息，而把类型特定域留出来用于它本来的用途。对于部
分帧解码的处理提高了码流的健壮性，能够对抗一定程度的丢包。详细信息参见
3.1.7和4.4。

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RFC2435—RTP Payload Format for JPEG-compressed Video        针对JPEG压缩视频的RTP荷载格式


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RFC文档中文翻译计划