記憶體方面
- 經常碰到解碼過程中，野指標崩潰。尤其是當一幀解碼失敗時候。
- ffmepg有引用計數記憶體管理機制，對應_unref方法。
- 使用結束之後，儘量使用_unref方法釋放記憶體，而避免直接_free
int av_packet_ref(AVPacket *dst, const AVPacket *src);
void av_packet_unref(AVPacket *pkt);
複製程式碼解碼器碰到No start code is found.錯誤
[h264 @ 0x7fa318076e00] No start code is found.
[h264 @ 0x7fa318076e00] Error splitting the input into NAL units.
複製程式碼google之後，說沒有使用下列方法
/**
* Fill the codec context based on the values from the supplied codec
* parameters. Any allocated fields in codec that have a corresponding field in
* par are freed and replaced with duplicates of the corresponding field in par.
* Fields in codec that do not have a counterpart in par are not touched.
*
* @return >= 0 on success, a negative AVERROR code on failure.
*/
int avcodec_parameters_to_context(AVCodecContext *codec,
const AVCodecParameters *par);
複製程式碼也就是說AVCodecContext有些值沒有設定，導致解碼失敗。幾經嘗試之後，發現有兩個屬性是必要的。
uint8_t *extradata;
int extradata_size;
複製程式碼除非是自己編碼的裸資料，mp4、flv等視訊檔案解封裝之後，都會有該屬性，對於後續解碼很有必要。
static int h264_decode_frame(AVCodecContext *avctx, void *data,
int *got_frame, AVPacket *avpkt)
{
const uint8_t *buf = avpkt->data;
int buf_size = avpkt->size;
H264Context *h = avctx->priv_data;
AVFrame *pict = data;
int buf_index;
int ret;
h->flags = avctx->flags;
h->setup_finished = 0;
h->nb_slice_ctx_queued = 0;
ff_h264_unref_picture(h, &h->last_pic_for_ec);
/* end of stream, output what is still in the buffers */
if (buf_size == 0)
return send_next_delayed_frame(h, pict, got_frame, 0);
if (h->is_avc && av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, NULL)) {
int side_size;
uint8_t *side = av_packet_get_side_data(avpkt, AV_PKT_DATA_NEW_EXTRADATA, &side_size);
if (is_extra(side, side_size))
ff_h264_decode_extradata(side, side_size,
&h->ps, &h->is_avc, &h->nal_length_size,
avctx->err_recognition, avctx);
}
if(h->is_avc && buf_size >= 9 && buf[0]==1 && buf[2]==0 && (buf[4]&0xFC)==0xFC && (buf[5]&0x1F) && buf[8]==0x67){
if (is_extra(buf, buf_size))
return ff_h264_decode_extradata(buf, buf_size,
&h->ps, &h->is_avc, &h->nal_length_size,
avctx->err_recognition, avctx);
}
buf_index = decode_nal_units(h, buf, buf_size);
if (buf_index < 0)
return AVERROR_INVALIDDATA;
if (!h->cur_pic_ptr && h->nal_unit_type == H264_NAL_END_SEQUENCE) {
av_assert0(buf_index <= buf_size);
return send_next_delayed_frame(h, pict, got_frame, buf_index);
}
if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) && (!h->cur_pic_ptr || !h->has_slice)) {
if (avctx->skip_frame >= AVDISCARD_NONREF ||
buf_size >= 4 && !memcmp("Q264", buf, 4))
return buf_size;
av_log(avctx, AV_LOG_ERROR, "no frame!\n");
return AVERROR_INVALIDDATA;
}
if (!(avctx->flags2 & AV_CODEC_FLAG2_CHUNKS) ||
(h->mb_y >= h->mb_height && h->mb_height)) {
if ((ret = ff_h264_field_end(h, &h->slice_ctx[0], 0)) < 0)
return ret;
/* Wait for second field. */
if (h->next_output_pic) {
ret = finalize_frame(h, pict, h->next_output_pic, got_frame);
if (ret < 0)
return ret;
}
}
av_assert0(pict->buf[0] || !*got_frame);
ff_h264_unref_picture(h, &h->last_pic_for_ec);
return get_consumed_bytes(buf_index, buf_size);
}
複製程式碼上述程式碼位於libcodec/h264dec.c下，是H.264核心的解碼函式，可以看到，某些情況下在decode_nal_units() 獲取NAL單元之前，先呼叫了ff_h264_decode_extradata()來處理extradata資料。主要功能是獲取sps、pps、ps資料，位於libcodec/h264_parse.c下. avcc
int ff_h264_decode_extradata(const uint8_t *data, int size, H264ParamSets *ps,
int *is_avc, int *nal_length_size,
int err_recognition, void *logctx)
{
int ret;
if (!data || size <= 0)
return -1;
if (data[0] == 1) {
int i, cnt, nalsize;
const uint8_t *p = data;
*is_avc = 1;
if (size < 7) {
av_log(logctx, AV_LOG_ERROR, "avcC %d too short\n", size);
return AVERROR_INVALIDDATA;
}
// Decode sps from avcC
cnt = *(p + 5) & 0x1f; // Number of sps
p += 6;
for (i = 0; i < cnt; i++) {
nalsize = AV_RB16(p) + 2;
if (nalsize > size - (p - data))
return AVERROR_INVALIDDATA;
ret = decode_extradata_ps_mp4(p, nalsize, ps, err_recognition, logctx);
if (ret < 0) {
av_log(logctx, AV_LOG_ERROR,
"Decoding sps %d from avcC failed\n", i);
return ret;
}
p += nalsize;
}
// Decode pps from avcC
cnt = *(p++); // Number of pps
for (i = 0; i < cnt; i++) {
nalsize = AV_RB16(p) + 2;
if (nalsize > size - (p - data))
return AVERROR_INVALIDDATA;
ret = decode_extradata_ps_mp4(p, nalsize, ps, err_recognition, logctx);
if (ret < 0) {
av_log(logctx, AV_LOG_ERROR,
"Decoding pps %d from avcC failed\n", i);
return ret;
}
p += nalsize;
}
// Store right nal length size that will be used to parse all other nals
*nal_length_size = (data[4] & 0x03) + 1;
} else {
*is_avc = 0;
ret = decode_extradata_ps(data, size, ps, 0, logctx);
if (ret < 0)
return ret;
}
return size;
}
複製程式碼static int decode_extradata_ps(const uint8_t *data, int size, H264ParamSets *ps,
int is_avc, void *logctx)
{
H2645Packet pkt = { 0 };
int i, ret = 0;
ret = ff_h2645_packet_split(&pkt, data, size, logctx, is_avc, 2, AV_CODEC_ID_H264, 1);
if (ret < 0) {
ret = 0;
goto fail;
}
for (i = 0; i < pkt.nb_nals; i++) {
H2645NAL *nal = &pkt.nals[i];
switch (nal->type) {
case H264_NAL_SPS:
ret = ff_h264_decode_seq_parameter_set(&nal->gb, logctx, ps, 0);
if (ret < 0)
goto fail;
break;
case H264_NAL_PPS:
ret = ff_h264_decode_picture_parameter_set(&nal->gb, logctx, ps,
nal->size_bits);
if (ret < 0)
goto fail;
break;
default:
av_log(logctx, AV_LOG_VERBOSE, "Ignoring NAL type %d in extradata\n",
nal->type);
break;
}
}
fail:
ff_h2645_packet_uninit(&pkt);
return ret;
}
複製程式碼ff_h2645_packet_split()位於libcodec/h2645_parse.c下
int ff_h2645_packet_split(H2645Packet *pkt, const uint8_t *buf, int length,
void *logctx, int is_nalff, int nal_length_size,
enum AVCodecID codec_id, int small_padding)
{
GetByteContext bc;
int consumed, ret = 0;
int next_avc = is_nalff ? 0 : length;
int64_t padding = small_padding ? 0 : MAX_MBPAIR_SIZE;
bytestream2_init(&bc, buf, length);
av_fast_padded_malloc(&pkt->rbsp.rbsp_buffer, &pkt->rbsp.rbsp_buffer_alloc_size, length + padding);
if (!pkt->rbsp.rbsp_buffer)
return AVERROR(ENOMEM);
pkt->rbsp.rbsp_buffer_size = 0;
pkt->nb_nals = 0;
while (bytestream2_get_bytes_left(&bc) >= 4) {
H2645NAL *nal;
int extract_length = 0;
int skip_trailing_zeros = 1;
if (bytestream2_tell(&bc) == next_avc) {
int i = 0;
extract_length = get_nalsize(nal_length_size,
bc.buffer, bytestream2_get_bytes_left(&bc), &i, logctx);
if (extract_length < 0)
return extract_length;
bytestream2_skip(&bc, nal_length_size);
next_avc = bytestream2_tell(&bc) + extract_length;
} else {
int buf_index;
if (bytestream2_tell(&bc) > next_avc)
av_log(logctx, AV_LOG_WARNING, "Exceeded next NALFF position, re-syncing.\n");
/* search start code */
buf_index = find_next_start_code(bc.buffer, buf + next_avc);
bytestream2_skip(&bc, buf_index);
if (!bytestream2_get_bytes_left(&bc)) {
if (pkt->nb_nals > 0) {
// No more start codes: we discarded some irrelevant
// bytes at the end of the packet.
return 0;
} else {
av_log(logctx, AV_LOG_ERROR, "No start code is found.\n");
return AVERROR_INVALIDDATA;
}
}
extract_length = FFMIN(bytestream2_get_bytes_left(&bc), next_avc - bytestream2_tell(&bc));
if (bytestream2_tell(&bc) >= next_avc) {
/* skip to the start of the next NAL */
bytestream2_skip(&bc, next_avc - bytestream2_tell(&bc));
continue;
}
}
if (pkt->nals_allocated < pkt->nb_nals + 1) {
int new_size = pkt->nals_allocated + 1;
void *tmp = av_realloc_array(pkt->nals, new_size, sizeof(*pkt->nals));
if (!tmp)
return AVERROR(ENOMEM);
pkt->nals = tmp;
memset(pkt->nals + pkt->nals_allocated, 0,
(new_size - pkt->nals_allocated) * sizeof(*pkt->nals));
nal = &pkt->nals[pkt->nb_nals];
nal->skipped_bytes_pos_size = 1024; // initial buffer size
nal->skipped_bytes_pos = av_malloc_array(nal->skipped_bytes_pos_size, sizeof(*nal->skipped_bytes_pos));
if (!nal->skipped_bytes_pos)
return AVERROR(ENOMEM);
pkt->nals_allocated = new_size;
}
nal = &pkt->nals[pkt->nb_nals];
consumed = ff_h2645_extract_rbsp(bc.buffer, extract_length, &pkt->rbsp, nal, small_padding);
if (consumed < 0)
return consumed;
if (is_nalff && (extract_length != consumed) && extract_length)
av_log(logctx, AV_LOG_DEBUG,
"NALFF: Consumed only %d bytes instead of %d\n",
consumed, extract_length);
pkt->nb_nals++;
bytestream2_skip(&bc, consumed);
/* see commit 3566042a0 */
if (bytestream2_get_bytes_left(&bc) >= 4 &&
bytestream2_peek_be32(&bc) == 0x000001E0)
skip_trailing_zeros = 0;
nal->size_bits = get_bit_length(nal, skip_trailing_zeros);
ret = init_get_bits(&nal->gb, nal->data, nal->size_bits);
if (ret < 0)
return ret;
if (codec_id == AV_CODEC_ID_HEVC)
ret = hevc_parse_nal_header(nal, logctx);
else
ret = h264_parse_nal_header(nal, logctx);
if (ret <= 0 || nal->size <= 0) {
if (ret < 0) {
av_log(logctx, AV_LOG_ERROR, "Invalid NAL unit %d, skipping.\n",
nal->type);
}
pkt->nb_nals--;
}
}
return 0;
}
複製程式碼BOX TYPE avcc
0x61 0x76 0x63 0x43
複製程式碼對於mp4檔案，以下是avcc具體資料，也就是extradata內容。
0x1 version
0x4d sps[1]
0x40 sps[2]
0x1e sps[3]
0b111111 6位reserved
0b11 2位h264中NALU長度，1+(size&3) = 4
0b111. 3位reserved
0b00001 sps個數1
sps 資料長度23
0x0 0x17
sps長度
0x67 0x4d 0x40 0x1e 0xe8 0x80 0x50 0x8 0x34 0x20 0x0 0x0 0x3 0x0 0x20 0x0 0x0 0x6 0x51 0xe2 0xc5 0xa2 0x40
0x1 pps個數
0x0 0x5 pps長度
pps資料
0x68 0xeb 0xec 0x4c 0x80複製程式碼