mpv - write_video 流程解析

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函数概述

write_video 是视频输出管道的核心函数，负责从解码器获取视频帧，处理视频重新配置、帧定时、同步、字幕更新，并将帧发送到视频输出（VO）进行渲染。

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4. 获取视频帧

• 调用 video_output_image 获取视频帧，可能返回多种状态：

• 错误(<0): 跳转到错误处理
• 等待(VD_WAIT): 检查是否发生下溢(underrun)，等待更多数据
• EOF(VD_EOF): 处理结束状态，设置排水(DRAINING)状态，等待VO完成渲染
• 新帧(VD_NEW_FRAME): 继续处理帧

bool logical_eof = false;
int r = video_output_image(mpctx, &logical_eof);
MP_TRACE(mpctx, "video_output_image: r=%d/eof=%d/st=%s\n", r, logical_eof,mp_status_str(mpctx->video_status));if (r < 0)goto error;if (r == VD_WAIT) {if (mpctx->video_status == STATUS_PLAYING && !vo_still_displaying(vo) &&!vo_c->underrun_signaled){vo_c->underrun = true;vo_c->underrun_signaled = true;}return;
}if (r == VD_EOF) {if (check_for_hwdec_fallback(mpctx))return;if (check_for_forced_eof(mpctx)) {uninit_video_chain(mpctx);handle_force_window(mpctx, true);return;}...if (mpctx->video_status == STATUS_DRAINING &&(vo_is_ready_for_frame(vo, -1) || !has_frame)){mpctx->time_frame -= get_relative_time(mpctx);mp_set_timeout(mpctx, mpctx->time_frame);if (mpctx->time_frame <= 0 || !has_frame) {MP_VERBOSE(mpctx, "video EOF reached\n");mpctx->video_status = STATUS_EOF;}}return;
}

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6. 视频参数更新

• 应用视频裁剪设置
• 检查视频参数是否变化，需要时重新配置VO
• 更新动态参数(如色彩空间)

// Inject vo crop to notify and reconfig if needed
apply_video_crop(mpctx, vo);// Filter output is different from VO input?
struct mp_image_params *p = &mpctx->next_frames[0]->params;
if (!vo->params || !mp_image_params_static_equal(p, vo->params)) {if (vo_still_displaying(vo)) {vo_request_wakeup_on_done(vo);return;}int vo_r = vo_reconfig2(vo, mpctx->next_frames[0]);if (vo_r < 0) {mpctx->error_playing = MPV_ERROR_VO_INIT_FAILED;goto error;}mp_notify(mpctx, MPV_EVENT_VIDEO_RECONFIG, NULL);
} else {mp_mutex_lock(&vo->params_mutex);mp_image_params_update_dynamic(vo->params, p, vo->has_peak_detect_values);mp_mutex_unlock(&vo->params_mutex);
}

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7. 同步处理

• 调整帧时间，更新音视频同步信息
• 强制字幕与视频帧同步
• 等待字幕准备就绪

mpctx->time_frame -= get_relative_time(mpctx);
update_avsync_before_frame(mpctx);// Enforce timing subtitles to video frames.
osd_set_force_video_pts(mpctx->osd, MP_NOPTS_VALUE);if (!update_subtitles(mpctx, mpctx->next_frames[0]->pts)) {MP_VERBOSE(mpctx, "Video frame delayed due to waiting on subtitles.\n");return;
}

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8. 帧发送准备

• 检查VO是否准备好接收帧
• 编码模式下等待音频输出初始化

double time_frame = MPMAX(mpctx->time_frame, -1);
int64_t pts = mp_time_ns() + (int64_t)(time_frame * 1e9);if (!vo_is_ready_for_frame(vo, mpctx->display_sync_active ? -1 : pts))return;if (mpctx->encode_lavc_ctx && mpctx->current_track[0][STREAM_AUDIO] && !mpctx->ao)return;

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9. 帧队列管理

• 维护过去帧信息队列
• 计算帧持续时间 calculate_frame_duration
• 构建VO帧数据结构

if (mpctx->num_past_frames >= MAX_NUM_VO_PTS)mpctx->num_past_frames--;
MP_TARRAY_INSERT_AT(mpctx, mpctx->past_frames, mpctx->num_past_frames, 0,(struct frame_info){0});
mpctx->past_frames[0] = (struct frame_info){.pts = mpctx->next_frames[0]->pts,.num_vsyncs = -1,
};
calculate_frame_duration(mpctx);int req = vo_get_num_req_frames(mpctx->video_out);
struct vo_frame dummy = {.pts = pts,.duration = -1,.still = mpctx->step_frames > 0,.can_drop = opts->frame_dropping & 1,.num_frames = MPMIN(mpctx->num_next_frames, req),.num_vsyncs = 1,
};
for (int n = 0; n < dummy.num_frames; n++)dummy.frames[n] = mpctx->next_frames[n];
struct vo_frame *frame = vo_frame_ref(&dummy);

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10. 帧发送和状态更新

• 发送帧到VO进行渲染
• 检查是否需要丢帧
• 更新已显示帧计数
• 处理状态转换(准备就绪→播放中)

schedule_frame(mpctx, frame);
vo_queue_frame(vo, frame);
check_framedrop(mpctx, vo_c);if (mpctx->num_next_frames >= 1)handle_new_frame(mpctx);mpctx->shown_vframes++;
if (mpctx->video_status < STATUS_PLAYING) {mpctx->video_status = STATUS_READY;if (!opts->video_latency_hacks) {vo_wait_frame(vo);MP_VERBOSE(mpctx, "first video frame after restart shown\n");}
}mp_notify(mpctx, MPV_EVENT_TICK, NULL);

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11. 特殊情况处理

• 处理逻辑EOF情况
• 处理步进(step)模式
• 检查最大帧数限制

if (logical_eof && !mpctx->num_next_frames && mpctx->ao_chain)mpctx->video_status = STATUS_EOF;if (mpctx->video_status != STATUS_EOF) {if (mpctx->step_frames > 0) {mpctx->step_frames--;if (!mpctx->step_frames) {set_pause_state(mpctx, true);step_frame_mute(mpctx, false);}}if (mpctx->max_frames == 0 && !mpctx->stop_play)mpctx->stop_play = AT_END_OF_FILE;if (mpctx->max_frames > 0)mpctx->max_frames--;
}vo_c->underrun_signaled = false;if (mpctx->video_status == STATUS_EOF || mpctx->stop_play)mp_wakeup_core(mpctx);

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11. 流程图

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