VisionWorks: Losing reference to images obtained from 'vxGetReferenceFromDelay' within thread context

I am attempting to modify the feature_tracker demo to work with our camera.

Our implementation is different from the demo in that the LK Tracker is running in a background thread (created by subclassing the ‘Thread.cpp’ found in the tegra_multimedia_api/argus/samples/utils/Thread.cpp) while the main thread is capturing camera images using the ‘select’ function.

Within the constructor (not the thread initializer) of my image processor thread class I setup the lk_tracker and, similar to the main function in the lk_tracker demo, I obtain a current image and previous image from the ‘vx_delay’ element. I keep a reference to these iamges as class members so I do not need to keep calling the ‘vxGetReferenceFromDelay’ in the time sensitive Thread Execute function.

When I first obtain the reference to the images within the constructor I can map patches and umap them just fine but when I try and access those same images within the ‘Thread Execute’ function I receive a -12 from the vx_status, which is: VX_ERROR_INVALID_REFERENCE

I have called the function: ‘vxRetainReference((vx_reference) frame)’ with the idea that perhaps the ‘vx_delay’ had decided that it will destroy my current image references after the the execution left the scope of the constructor but it didn’t work.

Just to be clear I call the constructor from the main thread context and then call the ‘Thread Initialize’ and ‘Thread Execute’ functions within the child thread context.

As a reference here is my constructor:

VisionWorksController::VisionWorksController( uint32_t image_width, uint32_t image_height,
                                              nvxio::ThreadSafeQueue<FRAME *> *ready_image_in_queue,
                                              nvxio::ThreadSafeQueue<FRAME *> *empty_image_in_queue,
                                              nvxio::ThreadSafeQueue<uint8_t *> *ready_image_out_queue,
                                              nvxio::ThreadSafeQueue<uint8_t *> *empty_image_out_queue,
                                              bool debug)
  vx_status status;
  this->debug = debug;
  this->image_width = image_width;
  this->image_height = image_height;
  this->ready_image_in_queue = ready_image_in_queue;
  this->empty_image_in_queue = empty_image_in_queue;

  this->ready_image_out_queue = ready_image_out_queue;
  this->empty_image_out_queue = empty_image_out_queue;
  this->first = true;

  ovxio::ContextGuard context;
  //Setup the logs
  vxRegisterLogCallback(context, &ovxio::stdoutLogCallback, vx_false_e);

printf("%s: Setting up LK Tracker for image with size: %d x %d...", __func__, this->image_width, this->image_height );
  //Feature Tracker Specific
  nvx::FeatureTracker::Params feature_tracker_params;

  // parameters for optical flow node
  feature_tracker_params.pyr_levels             = TRACKER_PYR_LEVELS;
  feature_tracker_params.lk_num_iters           = TRACKER_LK_NUM_ITERS;
  feature_tracker_params.lk_win_size            = TRACKER_LK_WIN_SIZE;

  // common parameters for corner detector node
  feature_tracker_params.array_capacity         = TRACKER_ARRAY_CAPACITY;
  feature_tracker_params.detector_cell_size     = TRACKER_DETECTOR_CELL_SIZE;
  feature_tracker_params.use_harris_detector    = TRACKER_USE_DETECTOR_HARRIS;

  // parameters for harris_track node
  feature_tracker_params.harris_k               = TRACKER_HARRIS_K;
  feature_tracker_params.harris_thresh          = TRACKER_HARRIS_THRESH;

  // parameters for fast_track node
  feature_tracker_params.fast_type              = TRACKER_FAST_TYPE;
  feature_tracker_params.fast_thresh            = TRACKER_FAST_THRESH;

  //Frame Delay Object is used to hold previous and current frames from video sources in the frame_delay variable
  vx_image image_ex = vxCreateImage(context, this->image_width, this->image_height, VX_DF_IMAGE_U8);
  this->frame_delay = vxCreateDelay(context, (vx_reference)image_ex, 2);
  if (this->frame_delay == NULL){
    printf ("Failed to get Frame Delay\n");

  this->prev_image = (vx_image) vxGetReferenceFromDelay(this->frame_delay, -1);
  vxRetainReference((vx_reference) this->prev_image);
  this->curr_image = (vx_image) vxGetReferenceFromDelay(this->frame_delay,  0);
  status = vxRetainReference((vx_reference) this->curr_image);

  this->tracker = nvx::FeatureTracker::create(context, feature_tracker_params);
  this->sync_timer = nvxio::createSyncTimer();

  //The following is just to test out the mapping and unmapping of patches from the current and previous frame

  //Current Image Full Map
  this->image_in_rect.start_x = 0u;                           //Start X
  this->image_in_rect.start_y = 0u;                           //Start Y
  this->image_in_rect.end_x   = this->image_width;            //End X
  this->image_in_rect.end_y   = this->image_height ;          //End Y
  vxMapImagePatch(this->curr_image, &this->image_in_rect, 0, &this->image_in_id, &this->image_in_addr, &this->image_in_ptr, VX_WRITE_ONLY, VX_MEMORY_TYPE_HOST, VX_NOGAP_X);

  printf("Image In Patch Dimensions:\n");
  printf("  Width x Height: %dx%d\n", this->image_in_addr.dim_x,    this->image_in_addr.dim_y);
  printf("  StepX x StepY:  %d,%d\n", this->image_in_addr.step_x,   this->image_in_addr.step_y);
  printf("  StriX x StriY:  %d,%d\n", this->image_in_addr.stride_x, this->image_in_addr.stride_y);

  //Previous Image Full Map
  this->image_out_rect.start_x = 0u;                            //Start X
  this->image_out_rect.start_y = 0u;                            //Start Y
  this->image_out_rect.end_x   = this->image_width;             //End X
  this->image_out_rect.end_y   = this->image_height;            //End Y
  vxMapImagePatch(this->prev_image, &this->image_out_rect, 0, &this->image_out_id, &this->image_out_addr, &this->image_out_ptr, VX_READ_ONLY, VX_MEMORY_TYPE_HOST, VX_NOGAP_X);

  printf("Image Out Patch Dimensions:\n");
  printf("  Width x Height: %dx%d\n", this->image_out_addr.dim_x,    this->image_out_addr.dim_y);
  printf("  StepX x StepY:  %d,%d\n", this->image_out_addr.step_x,   this->image_out_addr.step_y);
  printf("  StriX x StriY:  %d,%d\n", this->image_out_addr.stride_x, this->image_out_addr.stride_y);

  vxUnmapImagePatch(this->curr_image, this->image_in_id);
  vxUnmapImagePatch(this->prev_image, this->image_out_id);

Within the above constructor I call map and unmap patch to make sure I got a reference to the images and I have verified that the patch address information looks correct. Here is a reference to the ‘Thread Execute’ function that gets called over and over from Thread base class until the user requests a shutdown/join.

inline bool VisionWorksController::threadExecute()
  vx_status status;
  bool frame_ready = false;
  FRAME *fpga_frame = NULL;
  uint8_t *output_buffer = NULL;
  double total_ms = 0.00;
  //uint8_t temp_buffer[1024 * 768];

  frame_ready = ready_image_in_queue->pop(fpga_frame, 1000);
  if (!frame_ready){
    printf("VW Controller: 1S Timed Out\n");
    return true;

  //XXX: Do I need to keep mapping and unmapping these patches or can I do it one time??
  status = vxMapImagePatch(this->curr_image, &this->image_in_rect, 0, &this->image_in_id, &this->image_in_addr, &this->image_in_ptr, VX_WRITE_ONLY, VX_MEMORY_TYPE_HOST, 0);
  if (status != VX_SUCCESS)
    printf ("Failed to map image patch!: %d\n", status);
    printf("Successfully mapped iamge patch!\n");
  //XXX: Is this the best way to do this? Can I use the GPU to copy the data?
  memcpy(this->image_in_ptr, fpga_frame->buffer, this->image_height * this->image_width  * sizeof(uint8_t));
  vxUnmapImagePatch(this->curr_image, this->image_in_id);

  //Put the data back in the empty queue

  if (this->first)
    this->tracker->init(this->curr_image, NULL);
    this->sync_timer->arm(1. / VID_FRAMERATE);
    proc_ms = 0;
    this->first = false;
    nvx::Timer proc_timer;
    this->tracker->track(this->curr_image, NULL);
    proc_ms = proc_timer.toc();
    total_ms = this->total_timer.toc();
    printf("LK Tracking Total Time Between Frames: %f ms, Track Processing Time: %f\n", total_ms, proc_ms);

//Publish Data!
  if (this->empty_image_out_queue->pop(output_buffer, 0)){
    //XXX: Do I need to keep mapping and unmapping these patches or can I do it one time??
    vxMapImagePatch(this->prev_image, &this->image_out_rect, 0, &this->image_out_id, &this->image_out_addr, &this->image_out_ptr, VX_READ_ONLY, VX_MEMORY_TYPE_HOST, 0);
    //XXX: Is this the best way to do this? Can I use the GPU to copy the data?
    memcpy(output_buffer, this->image_out_ptr, this->image_height * this->image_width * sizeof(uint8_t));
    vxUnmapImagePatch(this->prev_image, this->image_out_id);
    printf("VX Controller: No Output buffer available\n");

  return true;

Is there something I’m doing wrong?

Besides this issue I have a couple of other questions:

  • Do I need to map and unmap a patch from an image every time I want to use it (like I do in the above example?) or can I map a patch one time at the beginning and unmap at the end?
  • I found reference to 'nvx_cv::convertCVMatTypeToVXImageFormat' in the documentation but I couldn't find it in any samples or demos, is there any example that uses this? Where is the header file for this? I grepped around the /usr/share/visionworks folder and couldn't find a reference to -i 'CVMat'

Thanks in advance for any help.



1. Argus use non-buffered strategy and create a new buffer each frame and then destroy when callback.
You may need to map and unmap image every time for the different address.

2. Here is some discussion about vx_image ⇠⇢ cv::mat for your reference:



Thanks for the response. I moved the LK Tracker initialization into the thread initialization and used vxRetainReference on the context and that solved the loss of reference issue. I haven’t tested if retaining the reference within the constructor would also work but as of now things are working.

  1. Once I got the vxRetainReference I experimented with keeping a patch available from start to finish vs grabbing a patch only as needed. When I attempted to keep the patch the entire time I received a runtime error with an error message basically saying what you are saying.

  2. Thanks, I should have looked for it on the forums, sorry to waste your time.

Thanks again for your response.