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問題描述

當(dāng)解碼較快的時(shí)候ffmpeg中時(shí)間戳同步，并不是解碼后立馬顯示視頻幀，這樣看著是倍速的效果。如何還原實(shí)際的播放速率？

解決方案

為了解決在解碼后視頻播放還原原來每幀播放的時(shí)刻點(diǎn)。我們需要在解碼較快的情況下對(duì)幀顯示加一定的時(shí)間延時(shí)ffmpeg中時(shí)間戳同步，這個(gè)延時(shí)策略就是計(jì)算出

延時(shí)調(diào)整時(shí)間 =（當(dāng)前幀時(shí)間戳  -  上一幀時(shí)間戳）- （當(dāng)前機(jī)器準(zhǔn)顯示時(shí)間  -  上一幀顯示機(jī)器時(shí)間）

延時(shí)調(diào)整時(shí)間 有可能為負(fù)值則丟棄。如果為正值，可根據(jù)時(shí)長(zhǎng)做一定調(diào)整，畢竟送幀顯示也是耗時(shí)操作。

demo示例：

void dispThread(void *arg)
{
    Input *input = (Input *)arg;
    static const double interval = 1000000.0 / input->fps;
    double q2d = av_q2d(input->tb) * 1000000.0;
    int64_t pre_pts = 0;
    int64_t frame_pre_pts = AV_NOPTS_VALUE;
    AVFrame *pending_frm = NULL;
    while (!input->is_req_exit()) {
        int64_t cur_pts;
        int remaining_time = 10000;
        double duration = interval;
        AVFrame *frm;
        if (pending_frm) {
            frm = pending_frm;
        } else {
            frm = input->PopFrame();
            if (!frm) {
                msleep(10);
                continue;
            }
            // printf("pop frame pts: %ld\n", frm->pts);
        }
        static auto delete_func = [](AVFrame * f) {
            av_frame_free(&f);
        };
        cur_pts = av_gettime_relative();
        if (frame_pre_pts != AV_NOPTS_VALUE)
            duration = (frm->pts - frame_pre_pts) * q2d;
        int countdown = (pre_pts == 0) ? 0 : (int)(duration - (cur_pts - pre_pts));
        remaining_time = std::min<int>(remaining_time, countdown);
        // printf("countdown: %d, remaining_time: %d us\n",
        //         countdown, remaining_time);
        if (input->realtime) {
            countdown = 0;
            remaining_time = 0;
        }
        if (countdown <= 0) {
            frame_pre_pts = frm->pts;
            pre_pts = cur_pts;
            if (frm == pending_frm)
                pending_frm = NULL;
            push_frame(input->join, input->slice_idx,
                       std::shared_ptr<AVFrame>(frm, delete_func));
        } else {
            pending_frm = frm;
        }
        if (remaining_time > 0)
        {
            printf("countdown: %d, remaining_time: %d us\n",
                 countdown, remaining_time);
            std::this_thread::sleep_for(std::chrono::microseconds(remaining_time));
        }
    }
    if (pending_frm)
        av_frame_free(&pending_frm);
}

如果是直播的情況下我們不做延時(shí)。

大致的流程如下

取得解碼視頻幀記錄當(dāng)前機(jī)器時(shí)間計(jì)算當(dāng)前準(zhǔn)顯示時(shí)間與上次顯示時(shí)間差值d1計(jì)算當(dāng)前幀時(shí)間戳與上次顯示時(shí)間戳差值d2計(jì)算延時(shí)時(shí)間 d2 - d1延時(shí)時(shí)間大于0則進(jìn)行sleep延時(shí)并保存當(dāng)前幀在下一次循環(huán)送顯

以上步驟為解決方案。請(qǐng)參考。