前言

仅供参考。
基于0.7.6版本的来做的，较高版本好像也没对这一块做啥修改。

方案一（简单易实现）

找到compute_target_delay的调用位置。

          delay = compute_target_delay(ffp, last_duration, is);

          if(ffp->pf_playback_rate < 0.5)  
               delay = delay / ffp->pf_playback_rate;

方案二（复杂并且还有难解决的问题）

开启AVFILTER

#define CONFIG_AVFILTER 1
#define FFP_AVFILTER_PLAYBACK_RATE 1   

解除setpts的限制

由FFP_AVFILTER_PLAYBACK_RATE宏所包含

#ifdef FFP_AVFILTER_PLAYBACK_RATE
 if (fabsf(ffp->pf_playback_rate) > 0.00001 &&
        fabsf(ffp->pf_playback_rate - 1.0f) > 0.00001) {
        char setpts_buf[256];
        float rate = 1.0f / ffp->pf_playback_rate;
        //rate = av_clipf_c(rate, 0.5f, 2.0f);  //注释掉该行限制解除
        av_log(ffp, AV_LOG_INFO, "vf_rate=%f(1/%f)\n", ffp->pf_playback_rate, rate);
        snprintf(setpts_buf, sizeof(setpts_buf), "%f*PTS", rate);
        INSERT_FILT("setpts", setpts_buf);
    }
#endif

重置video filters

低倍数和恢复1倍数时重置video filters

void ffp_set_playback_rate(FFPlayer *ffp, float rate)
{
    if (!ffp)
        return;

    ffp->pf_playback_rate = rate;
    ffp->pf_playback_rate_changed = 1;
    if(ffp->pf_playback_rate < 0.5 || ffp->pf_playback_rate == 1.0){
        ffp->vf_changed = 1;
    }
}

画面许久才刷新一帧（异常）

compute_target_delay函数计算的delay异常（经avfilter后，frame->pts有概率出现异常的状态）
限制delay的值：不大于当前fps所对应的两帧间隔时间。

        if (!isnan(diff) && fabs(diff) < AV_NOSYNC_THRESHOLD) {
            if (diff <= -sync_threshold)
                delay = FFMAX(0, delay + diff);
            else if (diff >= sync_threshold && delay > AV_SYNC_FRAMEDUP_THRESHOLD)
                delay = delay + diff;
            else if (diff >= sync_threshold)
                delay = 2 * delay;
            // wolf-herd modify
            if(ffp->is && ffp->is->video_st){
                double fps = av_q2d(ffp->is->video_st->avg_frame_rate);
                delay = FFMIN(1/(fps * ffp->pf_playback_rate),delay);
            }
        }

seek操作异常

暂时没有想到方法修复该问题。