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技术背景

GB/T28181-2022相对2016版，对图像抓拍有了明确的界定，图像抓拍在视频监控行业非常重要, Android平台GB28181设备接入端，无需实时上传音视频实时数据的情况下，就可以抓图上传到指定的图像存储服务器上。

图像抓拍基本要求如下：

1. 源设备向目标设备发送图像抓拍配置命令，需要携带传输路径、会话ID等信息。
2. 目标设备完成图像传输后，发送图像抓拍传输完成通知命令，采用IETF RFC3428中的MESSAGE方法实现。
3. 图像文件命名规则宜采用“设备编码(20位)、图像编码(2位)、时间编码(17位)、序列码(2位)”的形式
4. 图像格式宜使用JPEG，图像分辨率宜采用与主码流相同的分辨率。

需要注意的是，MESSAGE消息头Content-type头域为Content-type:Application/MANSCDP+xml，采用XML封装。设备收到图像抓拍配置命令后，发送配置响应命令，响应命令中包含执行结果信息。

图像抓拍流程如下：

技术实现

大牛直播SDK的SmartGBD已经完成GB28181设备接入侧的图像抓拍。

总体功能设计如下：

•  [视频格式]H.264/H.265(Android H.265硬编码)；
•  [音频格式]G.711 A律、AAC；
•  [音量调节]Android平台采集端支持实时音量调节；
•  [H.264硬编码]支持H.264特定机型硬编码；
•  [H.265硬编码]支持H.265特定机型硬编码；
•  [软硬编码参数配置]支持gop间隔、帧率、bit-rate设置；
•  [软编码参数配置]支持软编码profile、软编码速度、可变码率设置；
•  支持横屏、竖屏推流；
•  Android平台支持后台service推送屏幕(推送屏幕需要5.0+版本)；
• 支持纯视频、音视频PS打包传输；
• 支持RTP OVER UDP和RTP OVER TCP被动模式（TCP媒体流传输客户端）；
• 支持信令通道网络传输协议TCP/UDP设置；
• 支持注册、注销，支持注册刷新及注册有效期设置；
• 支持设备目录查询应答；
• 支持心跳机制，支持心跳间隔、心跳检测次数设置；
• 支持移动设备位置(MobilePosition)订阅和通知；
•  适用国家标准：GB/T 28181—2016；
• 支持语音广播；
• 支持语音对讲；
• 支持图像抓拍；
• 支持历史视音频文件检索；
• 支持历史视音频文件下载；
• 支持历史视音频文件回放；
• 支持云台控制和预置位查询；
•  [实时水印]支持动态文字水印、png水印；
•  [镜像]Android平台支持前置摄像头实时镜像功能；
•  [实时静音]支持实时静音/取消静音；
•  [实时快照]支持实时快照；
•  [降噪]支持环境音、手机干扰等引起的噪音降噪处理、自动增益、VAD检测；
•  [外部编码前视频数据对接]支持YUV数据对接；
•  [外部编码前音频数据对接]支持PCM对接；
•  [外部编码后视频数据对接]支持外部H.264数据对接；
•  [外部编码后音频数据对接]外部AAC数据对接；
•  [扩展录像功能]支持和录像SDK组合使用，录像相关功能。

图像抓拍相关信令处理如下：

/** Author: daniusdk.com*/package com.gb.ntsignalling;public interface GBSIPAgent {void setDeviceConfigListener(GBSIPAgentDeviceConfigListener deviceConfigListener);/** 通知图像抓拍传输完成*/boolean notifyUploadSnapShotFinished(String fromUserName, String fromUserNameAtDomain, String deviceID, String sessionID, java.util.List<String> snapShotList);}

Device配置Listener如下：

package com.gb.ntsignalling;public interface GBSIPAgentDeviceConfigListener {/** 图像抓拍配置*/void ntsOnDeviceSnapShotConfig(String from_user_name, String from_user_name_at_domain,String sn, String device_id, SnapShotConfig config,List<String> extra_info_list);
}

Snapshot配置接口如下：

public interface SnapShotConfig {int snap_num();int interval();String upload_url();String session_id();
}

图像抓拍JNI接口设计如下：

public class SmartPublisherJniV2 {/*** 截图接口, 支持JPEG和PNG两种格式* @param compress_format: 压缩格式, 0:JPEG格式, 1:PNG格式, 其他返回错误* @param quality: 取值范围:[0, 100], 值越大图像质量越好, 仅对JPEG格式有效, 若是PNG格式,请填100* @param file_name: 图像文件名, 例如:/dirxxx/test20231113100739.jpeg, /dirxxx/test20231113100739.png* @param user_data_string: 用户自定义字符串* @return {0} if successful*/public native int CaptureImage(long handle, int compress_format, int quality, String file_name, String user_data_string);}

Device Snap Shot Listener 核心代码如下：

/** Author: daniusdk.com*/public class GBDeviceSnapShotListenerImpl implements GBSIPAgentDeviceControlListener {@Overridepublic void ntsOnDeviceSnapShotConfig(String from_user_name, final String from_user_name_at_domain,String sn, String device_id, final SnapShotConfig config,List<String> extra_info_list) {if (null == config)return;handler_.postDelayed(new Runnable() {@Overridepublic void run() {Log.i(TAG, "ntsOnDeviceSnapShotConfig device_id:" + device_id_ + " session_id:" + config.session_id()+ ", snap_num:" + config.snap_num() + ", interval:" + config.interval() + ", upload_url:" + config.upload_url());if (null == gb28181_agent_)return;if (null == snap_shot_impl_) {snap_shot_impl_ = new SnapShotGBImpl(image_path_, context_, handler_, lib_publisher_jni, snap_shot_publisher_);snap_shot_impl_.start();}snap_shot_impl_.add_config(gb28181_agent_, from_user_name_, from_user_name_at_domain_, sn_,device_id_, snap_shot_config_, extra_info_list_);}private String from_user_name_;private String from_user_name_at_domain_;private String sn_;private String device_id_;private SnapShotConfig snap_shot_config_;private List<String> extra_info_list_;public Runnable set(String from_user_name, String from_user_name_at_domain,String sn, String device_id, SnapShotConfig config,List<String> extra_info_list) {this.from_user_name_ = from_user_name;this.from_user_name_at_domain_ = from_user_name_at_domain;this.sn_ = sn;this.device_id_ = device_id;this.snap_shot_config_ = config;this.extra_info_list_ = extra_info_list;return this;}}.set(from_user_name, from_user_name_at_domain, sn, device_id, config, extra_info_list), 0);}
}public class SnapShotGBImpl extends SnapShotImpl {private List<SnapConfig> config_list_ = new LinkedList<>();public SnapShotGBImpl(String dir, Context context, android.os.Handler handler,SmartPublisherJniV2 lib_sdk, LibPublisherWrapper publisher) {super(dir, context, handler, lib_sdk, publisher);}public boolean add_config(GBSIPAgent agent, String from_user_name, String from_user_name_at_domain, String sn,String device_id, SnapShotConfig config, List<String> extra_info_list) {if (null == agent)return false;if (is_null_or_empty(device_id))return false;if (null == config)return false;if (config.snap_num() < 1)return false;if (config.interval() < 1)return false;if (is_null_or_empty(config.session_id()))return false;SnapConfig c = new SnapConfig(agent, from_user_name, from_user_name_at_domain, sn, device_id, config, extra_info_list);config_list_.add(c);return true;}public void on_captured_image(long result, String file_name, long file_date_time_ms, String user_data) {SnapConfig config = find_config(user_data);if (null == config) {super.on_captured_image(result, file_name, file_date_time_ms, user_data);return;}SnapItem item = config.find_capturing_item(file_name);if (null == item) {super.on_captured_image(result, file_name, file_date_time_ms, user_data);return;}if (result != 0) {item.set_status(SnapItem.ERROR_STATUS);item.set_error_info("capture failed");Log.e(TAG, "capture failed, file:" + file_name + ", session_id:" + user_data);return;}item.set_status(SnapItem.CAPTURE_COMPLETION_STATUS);}public void on_uploaded(boolean is_ok, String file_name, String session_id, String gb_name) {SnapConfig config = find_config(session_id);if (null == config) {Log.w(TAG, "on_uploaded cannot find config, session_id:" + session_id + ", gb_name:" + gb_name);return;}SnapItem item = config.find_uploading_item(gb_name);if (null == item) {Log.w(TAG, "on_uploaded cannot find item, session_id:" + session_id + ", gb_name:" + gb_name);return;}if (is_ok) {item.set_status(SnapItem.UPLOAD_COMPLETION_STATUS);Log.i(TAG, "on_uploaded ok, session_id:" + session_id + ", file:" + file_name);}else {item.set_status(SnapItem.ERROR_STATUS);item.set_error_info("upload failed");Log.e(TAG, "on_uploaded failed, session_id:" + session_id + ", file:" + file_name);}}@Overridepublic void on_stop() {this.config_list_.clear();shutdown(200, TimeUnit.MILLISECONDS);}private void process_upload() {android.os.Handler app_handler = os_handler();for(SnapConfig c : config_list_)c.upload_files(app_handler, this);}private void process_finished() {List<String> notified_files = null;Iterator<SnapConfig> iterator = config_list_.iterator();while(iterator.hasNext()) {SnapConfig c = iterator.next();if (!c.is_can_notify_server())continue;iterator.remove();if (null == notified_files)notified_files = new LinkedList<>();c.notify_server(notified_files);}// 暂时删除这些文件, 根据业务需求随时调整就好if(notified_files != null && !notified_files.isEmpty())execute(new DeleteFilesTask(notified_files));}private static class SnapItem {private int status_ = INITIALIZATION_STATUS;private final String device_id_;private final int sn_; // 序列码, 40~41private final String dir_;private String file_name_;}private static class SnapConfig {private WeakReference<GBSIPAgent> agent_;private final String from_user_name_;private final String from_user_name_at_domain_;private final String sn_;private final String device_id_;private final String session_id_;private final int snap_num_;private final String upload_url_;private final int interval_sec_;private final List<String> extra_info_list_;private ArrayList<SnapItem> items_;public final String session_id() {return this.session_id_;}public void upload_files(android.os.Handler os_handler, SnapShotGBImpl snap) {if (null == items_)return;for (SnapItem i : items_) {if (i.is_capture_completion_status()) {i.set_status(SnapItem.UPLOADING_STATUS);BaseUploadTask upload_task = new MyUploadTask(upload_url_, i.file_name(), i.gb_snap_shot_file_id(),session_id(), i.gb_name(), os_handler, snap);if (!snap.submit(upload_task) ) {i.set_status(SnapItem.ERROR_STATUS);i.set_error_info("submit upload task failed");}}}}public void notify_server(List<String> notified_files) {ArrayList<String> snap_shot_list = new ArrayList(items_.size());for (SnapItem i : items_) {if (SnapItem.UPLOAD_COMPLETION_STATUS == i.status())snap_shot_list.add(i.gb_snap_shot_file_id());if (notified_files != null)notified_files.add(i.file_name());}if (null == agent_)return;GBSIPAgent agent = agent_.get();if (null == agent)return;agent.notifyUploadSnapShotFinished(from_user_name_, from_user_name_at_domain_, device_id_, this.session_id(), snap_shot_list);}}private static class DeleteFilesTask implements Runnable {private List<String> file_name_list_;public DeleteFilesTask(List<String> file_name_list) {this.file_name_list_ = file_name_list;}@Overridepublic void run() {if (null == file_name_list_)return;if (file_name_list_.isEmpty()) {file_name_list_ = null;return;}for (String i : file_name_list_) {try  {File f = new File(i);if (!f.exists()||!f.isFile() )continue;if (f.delete())Log.i(TAG, "delete file:" + i);elseLog.w(TAG, "delete file failed, " + i);}catch(Exception e) {Log.e(TAG, "DeleteFilesTask.run Exception:", e);}}file_name_list_.clear();file_name_list_ = null;}}public static class BaseUploadTask extends CancellableTask {private final String upload_url_;private final String file_name_;private final String gb_snap_shot_file_id_;private final String session_id_;private final String gb_name_;private WeakReference<android.os.Handler> os_handler_;private WeakReference<SnapShotGBImpl> snap_;public BaseUploadTask(String upload_url, String file_name, String gb_snap_shot_file_id,String session_id, String gb_name, android.os.Handler os_handler, SnapShotGBImpl snap) {this.upload_url_ = upload_url;this.file_name_ = file_name;this.gb_snap_shot_file_id_ = gb_snap_shot_file_id;this.session_id_ = session_id;this.gb_name_ = gb_name;if (os_handler !=null)this.os_handler_ = new WeakReference<>(os_handler);if (snap != null)this.snap_ = new WeakReference<>(snap);}protected final String upload_url() {return this.upload_url_;}protected final String file_name() {return this.file_name_;}protected final String gb_snap_shot_file_id() {return this.gb_snap_shot_file_id_;}protected final String session_id() {return this.session_id_;}protected final String gb_name() { return this.gb_name_; }protected final android.os.Handler os_handler() {if (os_handler_ != null)return os_handler_.get();return null;}protected final SnapShotGBImpl snap() {if (snap_ != null)return snap_.get();return null;}private static class ResultRunnable implements Runnable {private final boolean result_;private final String file_name_;private final String session_id_;private final String gb_name_;private WeakReference<SnapShotGBImpl> snap_;public ResultRunnable(boolean result, String file_name, String session_id,String gb_name, SnapShotGBImpl snap) {this.result_ = result;this.file_name_ = file_name;this.session_id_ = session_id;this.gb_name_ = gb_name;if (snap != null)this.snap_ = new WeakReference<>(snap);}@Overridepublic void run(){if (null == this.snap_)return;SnapShotGBImpl snap = this.snap_.get();if (null == snap)return;snap.on_uploaded(result_, file_name_, session_id_, gb_name_);}}protected void post_result(boolean is_ok) {android.os.Handler handler = os_handler();if (null == handler)return;SnapShotGBImpl gb_snap = snap();if (null == gb_snap)return;handler.post(new ResultRunnable(is_ok, file_name_,session_id_, gb_name_, gb_snap));}}
}

总结

以上是GB28181图像抓拍大概的流程和设计参考，权当抛砖引玉，Android终端除支持常规的音视频数据接入外，还可以支持移动设备位置(MobilePosition)订阅和通知、图像抓拍、语音广播和语音对讲、历史视音频下载和回放。感兴趣的开发者，可以单独跟我探讨。