Secondary Processing of Preview Streams (C/C++)

You can use the APIs in the ImageReceiver class to create a PreviewOutput instance and obtain real-time data of the preview stream for secondary processing. For example, you can add a filter algorithm to the preview stream.

How to Develop

Read Camera for the API reference.

1. Import the NDK, which provides camera-related attributes and methods.

    // Include the NDK header files.
    #include "hilog/log.h"
    #include "ohcamera/camera.h"
    #include "ohcamera/camera_input.h"
    #include "ohcamera/capture_session.h"
    #include "ohcamera/photo_output.h"
    #include "ohcamera/preview_output.h"
    #include "ohcamera/video_output.h"
    #include "ohcamera/camera_manager.h"
   

2. Link the dynamic library in the CMake script.

    target_link_libraries(entry PUBLIC libohcamera.so libhilog_ndk.z.so)
   

3. Obtain the surface ID.

   Call createImageReceiver() of the image module to create an ImageReceiver instance, and call getReceivingSurfaceId() of the instance to obtain the surface ID.

4. Call OH_CameraManager_GetSupportedCameraOutputCapability() to obtain the preview capability supported by the current device based on the surface ID. Then call OH_CameraManager_CreatePreviewOutput() to create a PreviewOutput instance, with the parameters set to the cameraManager pointer, the first item in the previewProfiles array, the surface ID obtained in step 3, and the returned previewOutput pointer, respectively.

    NDKCamera::NDKCamera(char *str)
    {
      Camera_Manager *cameraManager = nullptr;
      Camera_Device* cameras = nullptr;
      Camera_OutputCapability* cameraOutputCapability = nullptr;
      Camera_PreviewOutput* previewOutput = nullptr;
      const Camera_Profile* previewProfile = nullptr;
      uint32_t size = 0;
      uint32_t cameraDeviceIndex = 0;
      char* previewSurfaceId = str;
      Camera_ErrorCode ret = OH_Camera_GetCameraManager(&cameraManager);
      if (cameraManager == nullptr || ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_Camera_GetCameraManager failed.");
      }
      ret = OH_CameraManager_GetSupportedCameras(cameraManager, &cameras, &size);
      if (cameras == nullptr || size < 0 || ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_CameraManager_GetSupportedCameras failed.");
      }
      ret = OH_CameraManager_GetSupportedCameraOutputCapability(cameraManager, &cameras[cameraDeviceIndex],
                                                                      &cameraOutputCapability);
      if (cameraOutputCapability == nullptr || ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_CameraManager_GetSupportedCameraOutputCapability failed.");
      }
      if (cameraOutputCapability->previewProfilesSize < 0) {
        OH_LOG_ERROR(LOG_APP, "previewProfilesSize == null");
      }
      previewProfile = cameraOutputCapability->previewProfiles[0];
      
      ret = OH_CameraManager_CreatePreviewOutput(cameraManager, previewProfile, previewSurfaceId, &previewOutput);
      if (previewProfile == nullptr || previewOutput == nullptr || ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_CameraManager_CreatePreviewOutput failed.");
      }
    }
   

5. Configure the session. Call commitConfig() to commit the session configuration, and then call start() to start outputting the preview stream. If the call fails, an error code is returned. For details, see Camera_ErrorCode.

    ret = OH_PreviewOutput_Start(previewOutput);
    if (ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_PreviewOutput_Start failed.");
    }
   

6. Call stop() to stop outputting the preview stream. If the call fails, an error code is returned. For details, see Camera_ErrorCode.

    ret = OH_PreviewOutput_Stop(previewOutput);
    if (ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_PreviewOutput_Stop failed.");
    }
   

Status Listening

During camera application development, you can listen for the preview output stream status, including preview stream start, preview stream end, and preview stream output errors.

• Register the 'frameStart' event to listen for preview start events. This event can be registered when a PreviewOutput instance is created and is triggered when the bottom layer starts exposure for the first time. The preview stream starts as long as a result is returned.

    ret = OH_PreviewOutput_RegisterCallback(previewOutput, GetPreviewOutputListener());
    if (ret != CAMERA_OK) {
        OH_LOG_ERROR(LOG_APP, "OH_PreviewOutput_RegisterCallback failed.");
    }
  

    void PreviewOutputOnFrameStart(Camera_PreviewOutput* previewOutput)
    {
        OH_LOG_INFO(LOG_APP, "PreviewOutputOnFrameStart");
    }
    PreviewOutput_Callbacks* GetPreviewOutputListener(void)
    {
        static PreviewOutput_Callbacks previewOutputListener = {
            .onFrameStart = PreviewOutputOnFrameStart,
            .onFrameEnd = PreviewOutputOnFrameEnd,
            .onError = PreviewOutputOnError
        };
        return &previewOutputListener;
    }
  

• Register the 'frameEnd' event to listen for preview end events. This event can be registered when a PreviewOutput instance is created and is triggered when the last frame of preview ends. The preview stream ends as long as a result is returned.

    void PreviewOutputOnFrameEnd(Camera_PreviewOutput* previewOutput, int32_t frameCount)
    {
        OH_LOG_INFO(LOG_APP, "PreviewOutput frameCount = %{public}d", frameCount);
    }
  

• Register the 'error' event to listen for preview output errors. The callback function returns an error code when an API is incorrectly used. For details about the error code types, see Camera_ErrorCode.

    void PreviewOutputOnError(Camera_PreviewOutput* previewOutput, Camera_ErrorCode errorCode)
    {
        OH_LOG_INFO(LOG_APP, "PreviewOutput errorCode = %{public}d", errorCode);
    }