ffmpeg断流黑屏问题分析

转载请标明出处，尊重原作者，https://blog.youkuaiyun.com/qq_36491519/article/details/86535967

做安防+AI的很多人都遇到过黑屏问题，这各问题在个平台又大不一样，这一次我分析了一下在win系统下rtsp协议情况下的断流黑屏问题（同样的在Android下就没有问题）。

是这样的，在断网等情况下拉流端会拉不到流造成黑屏，这样用户体验非常不好，后来我在播放段加了断流检测回调，简单的就是每10微妙检测一下网络，如果网络超时达到自己的设定值就回调通知上层，让上层弹框通知等。然而没想到此回调再win系统会有ones的不起作用，但是再Android等就完全没问题。

首先分析一下win问题的log。

log会频繁打印

"corrupted macroblock 8 10 (total_coeff=-1)\n"

这是什么情况那？来看看他的位置。

//转载请标明出处，尊敬原作者，https://blog.youkuaiyun.com/qq_36491519/article/details/86535967
/**
 * Decode a residual block.
 * @param n block index
 * @param scantable scantable
 * @param max_coeff number of coefficients in the block
 * @return <0 if an error occurred
 */
static int decode_residual(const H264Context *h, H264SliceContext *sl,
                           GetBitContext *gb, int16_t *block, int n,
                           const uint8_t *scantable, const uint32_t *qmul,
                           int max_coeff)
{
    static const int coeff_token_table_index[17]= {0, 0, 1, 1, 2, 2, 2, 2, 3, 3, 3, 3, 3, 3, 3, 3, 3};
    int level[16];
    int zeros_left, coeff_token, total_coeff, i, trailing_ones, run_before;

    //FIXME put trailing_onex into the context

    if(max_coeff <= 8){
        if (max_coeff == 4)
            coeff_token = get_vlc2(gb, chroma_dc_coeff_token_vlc.table, CHROMA_DC_COEFF_TOKEN_VLC_BITS, 1);
        else
            coeff_token = get_vlc2(gb, chroma422_dc_coeff_token_vlc.table, CHROMA422_DC_COEFF_TOKEN_VLC_BITS, 1);
        total_coeff= coeff_token>>2;
    }else{
        if(n >= LUMA_DC_BLOCK_INDEX){
            total_coeff= pred_non_zero_count(h, sl, (n - LUMA_DC_BLOCK_INDEX)*16);
            coeff_token= get_vlc2(gb, coeff_token_vlc[ coeff_token_table_index[total_coeff] ].table, COEFF_TOKEN_VLC_BITS, 2);
            total_coeff= coeff_token>>2;
        }else{
            total_coeff= pred_non_zero_count(h, sl, n);
            coeff_token= get_vlc2(gb, coeff_token_vlc[ coeff_token_table_index[total_coeff] ].table, COEFF_TOKEN_VLC_BITS, 2);
            total_coeff= coeff_token>>2;
        }
    }
    sl->non_zero_count_cache[scan8[n]] = total_coeff;

    //FIXME set last_non_zero?

    if(total_coeff==0)
        return 0;
    if(total_coeff > (unsigned)max_coeff) {
        av_log(h->avctx, AV_LOG_ERROR, "corrupted macroblock %d %d (total_coeff=%d)\n", sl->mb_x, sl->mb_y, total_coeff);
        return -1;
    }

    trailing_ones= coeff_token&3;
    ff_tlog(h->avctx, "trailing:%d, total:%d\n", trailing_ones, total_coeff);
    av_assert2(total_coeff<=16);

    i = show_bits(gb, 3);
    skip_bits(gb, trailing_ones);
    level[0] = 1-((i&4)>>1);
    level[1] = 1-((i&2)   );
    level[2] = 1-((i&1)<<1);

    if(trailing_ones<total_coeff) {
        int mask, prefix;
        int suffix_length = total_coeff > 10 & trailing_ones < 3;
        int bitsi= show_bits(gb, LEVEL_TAB_BITS);
        int level_code= cavlc_level_tab[suffix_length][bitsi][0];

        skip_bits(gb, cavlc_level_tab[suffix_length][bitsi][1]);
        if(level_code >= 100){
            prefix= level_code - 100;
            if(prefix == LEVEL_TAB_BITS)
                prefix += get_level_prefix(gb);

            //first coefficient has suffix_length equal to 0 or 1
            if(prefix<14){ //FIXME try to build a large unified VLC table for all this
                if(suffix_length)
                    level_code= (prefix<<1) + get_bits1(gb); //part
                else
                    level_code= prefix; //part
            }else if(prefix==14){
                if(suffix_length)
                    level_code= (prefix<<1) + get_bits1(gb); //part
                else
                    level_code= prefix + get_bits(gb, 4); //part
            }else{
                level_code= 30;
                if(prefix>=16){
                    if(prefix > 25+3){
                        av_log(h->avctx, AV_LOG_ERROR, "Invalid level prefix\n");
                        return -1;
                    }
                    level_code += (1<<(prefix-3))-4096;
                }
                level_code += get_bits(gb, prefix-3); //part
            }

            if(trailing_ones < 3) level_code += 2;

            suffix_length = 2;
            mask= -(level_code&1);
            level[trailing_ones]= (((2+level_code)>>1) ^ mask) - mask;
        }else{
            level_code += ((level_code>>31)|1) & -(trailing_ones < 3);

            suffix_length = 1 + (level_code + 3U > 6U);
            level[trailing_ones]= level_code;
        }

        //remaining coefficients have suffix_length > 0
        for(i=trailing_ones+1;i<total_coeff;i++) {
            static const unsigned int suffix_limit[7] = {0,3,6,12,24,48,INT_MAX };
            int bitsi= show_bits(gb, LEVEL_TAB_BITS);
            level_code= cavlc_level_tab[suffix_length][bitsi][0];

            skip_bits(gb, cavlc_level_tab[suffix_length][bitsi][1]);
            if(level_code >= 100){
                prefix= level_code - 100;
                if(prefix == LEVEL_TAB_BITS){
                    prefix += get_level_prefix(gb);
                }
                if(prefix<15){
                    level_code = (prefix<<suffix_length) + get_bits(gb, suffix_length);
                }else{
                    level_code = 15<<suffix_length;
                    if (prefix>=16) {
                        if(prefix > 25+3){
                            av_log(h->avctx, AV_LOG_ERROR, "Invalid level prefix\n");
                            return AVERROR_INVALIDDATA;
                        }
                        level_code += (1<<(prefix-3))-4096;
                    }
                    level_code += get_bits(gb, prefix-3);
                }
                mask= -(level_code&1);
                level_code= (((2+level_code)>>1) ^ mask) - mask;
            }
            level[i]= level_code;
            suffix_length+= suffix_limit[suffix_length] + level_code > 2U*suffix_limit[suffix_length];
        }
    }

    if(total_coeff == max_coeff)
        zeros_left=0;
    else{
        if (max_coeff <= 8) {
            if (max_coeff == 4)
                zeros_left = get_vlc2(gb, chroma_dc_total_zeros_vlc[total_coeff].table,
                                      CHROMA_DC_TOTAL_ZEROS_VLC_BITS, 1);
            else
                zeros_left = get_vlc2(gb, chroma422_dc_total_zeros_vlc[total_coeff].table,
                                      CHROMA422_DC_TOTAL_ZEROS_VLC_BITS, 1);
        } else {
            zeros_left= get_vlc2(gb, total_zeros_vlc[ total_coeff ].table, TOTAL_ZEROS_VLC_BITS, 1);
        }
    }

#define STORE_BLOCK(type) \
    scantable += zeros_left + total_coeff - 1; \
    if(n >= LUMA_DC_BLOCK_INDEX){ \
        ((type*)block)[*scantable] = level[0]; \
        for(i=1;i<total_coeff && zeros_left > 0;i++) { \
            if(zeros_left < 7) \
                run_before= get_vlc2(gb, run_vlc[zeros_left].table, RUN_VLC_BITS, 1); \
            else \
                run_before= get_vlc2(gb, run7_vlc.table, RUN7_VLC_BITS, 2); \
            zeros_left -= run_before; \
            scantable -= 1 + run_before; \
            ((type*)block)[*scantable]= level[i]; \
        } \
        for(;i<total_coeff;i++) { \
            scantable--; \
            ((type*)block)[*scantable]= level[i]; \
        } \
    }else{ \
        ((type*)block)[*scantable] = ((int)(level[0] * qmul[*scantable] + 32))>>6; \
        for(i=1;i<total_coeff && zeros_left > 0;i++) { \
            if(zeros_left < 7) \
                run_before= get_vlc2(gb, run_vlc[zeros_left].table, RUN_VLC_BITS, 1); \
            else \
                run_before= get_vlc2(gb, run7_vlc.table, RUN7_VLC_BITS, 2); \
            zeros_left -= run_before; \
            scantable -= 1 + run_before; \
            ((type*)block)[*scantable]= ((int)(level[i] * qmul[*scantable] + 32))>>6; \
        } \
        for(;i<total_coeff;i++) { \
            scantable--; \
            ((type*)block)[*scantable]= ((int)(level[i] * qmul[*scantable] + 32))>>6; \
        } \
    }

    if (h->pixel_shift) {
        STORE_BLOCK(int32_t)
    } else {
        STORE_BLOCK(int16_t)
    }

    if(zeros_left<0){
        av_log(h->avctx, AV_LOG_ERROR, "negative number of zero coeffs at %d %d\n", sl->mb_x, sl->mb_y);
        return -1;
    }

    return 0;
}

问题就发生再H264解码的时候，在解码残余部分的时候发生。

   //转载请标明出处，尊重原作者，https://blog.youkuaiyun.com/qq_36491519/article/details/86535967
 //FIXME set last_non_zero?

    if(total_coeff==0)
        return 0;
    if(total_coeff > (unsigned)max_coeff) {
        av_log(h->avctx, AV_LOG_ERROR, "corrupted macroblock %d %d (total_coeff=%d)\n", sl->mb_x, sl->mb_y, total_coeff);
        return -1;
    }

在这里由于total_coeff值没有传入，造成total_coeff为-1，进而造成total_coeff<max_coeff。

而decode_residual函数是在下面的h264解码函数调用的。函数如下：

int ff_h264_decode_mb_cavlc(const H264Context *h, H264SliceContext *sl)

那上面的函数又在static int decode_slice(struct AVCodecContext *avctx, void *arg)

static int decode_slice(struct AVCodecContext *avctx, void *arg)

static int decode_slice(struct AVCodecContext *avctx, void *arg)中有一个无限循环，讲到这里大家就知道该怎么处理了吧，在这个无限循环里处理，比如超时判断，增加回调等等。