C Sync Wrapper

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Contents

About

The freenect driver provides an "asynchronous" interface where you provide callbacks and it calls them when sensor data is available. There is another way of thinking about the data by having a "synchronous" interface where you call a function to get an image "freenect_sync_get_depth" or "freenect_sync_get_video" and it blocks and gives you back the data.

This is implemented by using a thread to handle the callbacks and a buffer to provide an interface for the client. You are guaranteed to get the buffers in order by timestamp (e.g., you won't get something older than your last). You may not get all frames with this interface (e.g., there may be gaps); however, a 'best effort' is made.

Who is involved

Search for Synchronous in People

However (Brandyn) and (amiller) are trying to organize things for the Synchronous interface.

Interface

int freenect_sync_get_video(void **video, uint32_t *timestamp, int index, freenect_video_format fmt);

   Synchronous video function, starts the runloop if it isn't running                                                          
   
   The returned buffer is valid until this function is called again, after which the buffer must not
   be used again.  Make a copy if the data is required.         
                                                                                                                             
   Args:                                                                                                                     
       video: Populated with a pointer to a video buffer with a size of the requested type                                                                                                    
       timestamp: Populated with the associated timestamp
       index: Device index (0 is the first)
       fmt: Valid format                                                   
                                                                                                                             
   Returns:                                                                                                                  
       Nonzero on error.                                                                                                     

int freenect_sync_get_depth(void **depth, uint32_t *timestamp, int index, freenect_depth_format fmt);

   Synchronous depth function, starts the runloop if it isn't running
   
   The returned buffer is valid until this function is called again, after which the buffer must not
   be used again.  Make a copy if the data is required.                                                        
                                                                                                                             
   Args:                                                                                                                     
       depth: Populated with a pointer to a depth buffer with a size of the requested type                                               
       timestamp: Populated with the associated timestamp  
       index: Device index (0 is the first)
       fmt: Valid format                                                     
                                                                                                                             
   Returns:                                                                                                                  
       Nonzero on error.

void freenect_sync_stop(void);

   Stops the runloop (if running), cleans up all buffers, unsets callbacks, and flushes queues.
   
   Returns:
       void.

Example

Here is a very short example using the c_sync wrappers and opencv:

  // Be sure to link with -lfreenect_sync
  #include <stdlib.h>
  #include <stdio.h>
  #include <opencv/cv.h>
  #include <opencv/highgui.h>
  #include <libfreenect_sync.h>
  int main()
  {   
      IplImage *image = cvCreateImageHeader(cvSize(640,480), 8, 3);
      while (cvWaitKey(10) < 0) 
      {
        char *data;
        unsigned int timestamp;
        freenect_sync_get_video((void**)(&data), &timestamp, 0, FREENECT_VIDEO_RGB);
        cvSetData(image, data, 640*3);
        cvCvtColor(image, image, CV_RGB2BGR);
        cvShowImage("RGB", image);
      }
      freenect_sync_stop();       
      cvFree(&image);
      return EXIT_SUCCESS;
  }

Performance

TODO

TODO

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