obs錄屏核心流程分析

從output入手，梳理一下obs output的結構。這裡需要仔細過一遍，因為接下來需要把視頻寫入Unreal的Rendertarget對象，來渲染成材質。

音頻也需要單獨接入到Unreal引擎中。梳理的過程中，非核心的邏輯和標記我會去掉，只保留主幹。

//碧麟備註版
struct obs_output {
　　        // obs上下文
    struct obs_context_data context;

　　        // 輸出結構信息
　　        struct obs_output_info info;

    /* indicates ownership of the info.id buffer */
    bool owns_info_id;

    int64_t video_offset;
    int64_t audio_offsets[MAX_OUTPUT_AUDIO_ENCODERS];
    int64_t highest_audio_ts;
    int64_t highest_video_ts;
    pthread_t end_data_capture_thread;   

    int total_frames;

    //視頻信息指針
    video_t *video;
    //音頻信息指針
    audio_t *audio;
    //視頻編碼器
    obs_encoder_t *video_encoder;
    //音頻編碼器，因為支持多路音頻合成，所以這裡用的是數組
    obs_encoder_t *audio_encoders[MAX_OUTPUT_AUDIO_ENCODERS];


    struct circlebuf audio_buffer[MAX_AUDIO_MIXES][MAX_AV_PLANES];
    uint64_t audio_start_ts;
    uint64_t video_start_ts;
    size_t audio_size;
    size_t planes;
    size_t sample_rate;
    size_t total_audio_frames;

    uint32_t scaled_width;
    uint32_t scaled_height;

    struct video_scale_info video_conversion;
    struct audio_convert_info audio_conversion;        struct circlebuf caption_data;


    float audio_data[MAX_AUDIO_CHANNELS][AUDIO_OUTPUT_FRAMES];
};

看完上面的output結構，我們實際調試一下，點擊“屏幕錄製”，會進入obs_output_start這個函數，這個函數是個馬甲，簡單帶過

//碧麟精簡標註版
bool obs_output_start(obs_output_t *output)
{
	bool encoded;
	bool has_service;
	
	encoded = (output->info.flags & OBS_OUTPUT_ENCODED) != 0;
    
	if (encoded && output->delay_sec) {
        //延遲運行
		return obs_output_delay_start(output);
	} else {
        //實際運行
		if (obs_output_actual_start(output)) {
            //發送starting signal
			do_output_signal(output, "starting");
			return true;
		}

		return false;
	}
}

//碧麟精簡批註版
bool obs_output_actual_start(obs_output_t *output)
{
	bool success = false;
    
    // 第一步，調用outp->info.start,參數是output->context.data
	if (output->context.data)
		success = output->info.start(output->context.data);

	if (success && output->video) {
		output->starting_frame_count =
			video_output_get_total_frames(output->video);
		output->starting_drawn_count = obs->video.total_frames;
		output->starting_lagged_count = obs->video.lagged_frames;
	}

	if (os_atomic_load_long(&output->delay_restart_refs))
		os_atomic_dec_long(&output->delay_restart_refs);

	output->caption_timestamp = 0;

	circlebuf_free(&output->caption_data);
	circlebuf_init(&output->caption_data);

	return success;
}

第一步是，調用outp->info.start,參數是output->context.data

output->info.start應該是一個函數指針，定義如下

// 碧麟精簡批註版
// output_info結構 ，主要存儲函數指針
struct obs_output_info {
	/* required */
	const char *id;

	uint32_t flags;

	const char *(*get_name)(void *type_data);
    //創建
	void *(*create)(obs_data_t *settings, obs_output_t *output);
    //銷毀
	void (*destroy)(void *data);
    //開始
	bool (*start)(void *data);
    //停止
	void (*stop)(void *data, uint64_t ts);

	void (*raw_video)(void *data, struct video_data *frame);
	void (*raw_audio)(void *data, struct audio_data *frames);

	void (*encoded_packet)(void *data, struct encoder_packet *packet);

    //get預設設置
	void (*get_defaults)(obs_data_t *settings);
    //get屬性
	obs_properties_t *(*get_properties)(void *data);


	uint64_t (*get_total_bytes)(void *data);

	int (*get_dropped_frames)(void *data);

	void *type_data;

	/* only used with encoded outputs, separated with semicolon */
	const char *encoded_video_codecs;
	const char *encoded_audio_codecs;

	/* raw audio callback for multi track outputs */
	void (*raw_audio2)(void *data, size_t idx, struct audio_data *frames);
};

context定義如下

//context數據結構
struct obs_context_data {
	char *name;
	const char *uuid;
    // 這個是最重要的
	void *data;
    
	obs_data_t *settings;
	signal_handler_t *signals;
	proc_handler_t *procs;
	enum obs_obj_type type;

	struct obs_weak_object *control;

	DARRAY(obs_hotkey_id) hotkeys;
	DARRAY(obs_hotkey_pair_id) hotkey_pairs;
	obs_data_t *hotkey_data;
    
    //多路傳輸使用，鏈表
	struct obs_context_data *next;
	struct obs_context_data **prev_next;

	bool private;
};

這裡，data是context最重要的內容

因為我是用的ffmpeg多路傳輸做錄屏，所以data是一個ffmpeg_muxer

因此output->info.start(output->context.data)在這裡展開是這樣的結構

static bool ffmpeg_mux_start(void *data)
{
	struct ffmpeg_muxer *stream = data;

	//讀取設置
    obs_data_t *settings = obs_output_get_settings(stream->output);
    //實際開始執行多路傳輸
	bool success = ffmpeg_mux_start_internal(stream, settings);
	obs_data_release(settings);

	return success;
}

//碧麟精簡批註版
static inline bool ffmpeg_mux_start_internal(struct ffmpeg_muxer *stream,
					     obs_data_t *settings)
{
    //讀取保存路徑
    //調試結果：C:/Users/86180/Videos/2023-03-16 13-36-11.mp4
	const char *path = obs_data_get_string(settings, "path");
    //設定保存路徑
	update_encoder_settings(stream, path);

    //網路版
	if (stream->is_network) {
		obs_service_t *service;
		service = obs_output_get_service(stream->output);
		if (!service)
			return false;
		path = obs_service_get_url(service);
		stream->split_file = false;
	} else {
       //本地版
		stream->max_time =
			obs_data_get_int(settings, "max_time_sec") * 1000000LL;
		stream->max_size = obs_data_get_int(settings, "max_size_mb") *
				   (1024 * 1024);
		stream->split_file = obs_data_get_bool(settings, "split_file");
		stream->allow_overwrite =
			obs_data_get_bool(settings, "allow_overwrite");
		stream->cur_size = 0;
		stream->sent_headers = false;
	}

	ts_offset_clear(stream);
    //錄屏信息嘗試寫本地文件，確保文件可寫入
	if (!stream->is_network) {
		/* ensure output path is writable to avoid generic error
		 * message.
		 *
		 * TODO: remove once ffmpeg-mux is refactored to pass
		 * errors back */
		FILE *test_file = os_fopen(path, "wb");
		if (!test_file) {
			set_file_not_readable_error(stream, settings, path);
			return false;
		}

		fclose(test_file);
		os_unlink(path);
	}
    //這裡會生成一個command
    //"D:/dev/obs/obs-studio/build/rundir/Debug/bin/64bit/obs-ffmpeg-mux.exe" "C:/Users/86180/Videos/2023-03-16 14-58-19.mp4" 1 1 h264 850 1280 720 1 1 1 1 1 0 30 1 0 aac "simple_aac" 192 48000 1024 2 "" "" 
	//可以看出是使用外部程式進行錄屏
    start_pipe(stream, path);

	if (!stream->pipe) {
		obs_output_set_last_error(
			stream->output, obs_module_text("HelperProcessFailed"));
		warn("Failed to create process pipe");
		return false;
	}

	/* write headers and start capture */
	os_atomic_set_bool(&stream->active, true);
	os_atomic_set_bool(&stream->capturing, true);
	stream->total_bytes = 0;
	obs_output_begin_data_capture(stream->output, 0);

	info("Writing file '%s'...", stream->path.array);
	return true;
}

ffmpeg_mux實際工作核心邏輯都在這裡

可以看出錄屏是用這個外部程式obs-ffmpeg-mux.exe來進行的。