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Frame management

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librealsense2 provides flexible model for frame management and synchronization. The document will overview frame memory management, passing frames between threads and synchronization.

API Overview

The core C++ abstraction when dealing is the rs2::frame class and the rs2::device::start method. All other management and synchronization primitives can be derived from those two APIs. 

/**
 * Start passing frames into user provided callback
 * \param[in] callback   Stream callback, can be any callable object accepting rs2::frame
 */
template<class T>
void start(T callback) const;

Once you call start, the library will start dispatching new frames from selected device into the callback you provided.
The callback will be invoked from the same thread handling the low-level IO ensuring minimal latency. Any object implementing void operator()(rs2::frame) can be used as a callback. In particular, you can pass an anonymous function (lambda with capture) as the frame callback:

dev.start([](rs::frame f){
    std::cout << "This line be printed every frame!" << std::endl; 
});

As a side-note, rs2::device::stop will block until all pending callbacks return. This way within callback scope you can be sure the device object is available.

Frame Memory Management

rs2::frame is a smart reference to the underlying frame - as long as you hold ownership of the rs2::frame the underlying memory is exclusively yours and will not be modified or freed.

  • If no processing was necessary on the frame, rs2::frame::get_data will provide a direct pointer to the buffer provided by the underlying driver stack. No extra memory copies are performed in this case.
  • If some processing was required (for example, whenever you configure RS2_FORMAT_RGB8 it is likely librealsense will do the conversion from YUY format internally) librealsense will store the processing output in an internal buffer, and rs2::frame::get_data will point to it.
  • You can extend the lifetime of the rs2::frame object by moving it out of the callback into some global, thread-safe, data structure. (See below) Moving rs2::frame does not involve a mem-copy of its content.
  • Except some initial stabilization period, librealsense ensures no heap allocations are being made when using frame callbacks. (This also applies to rs2::frame_queue but not to rs2::syncer primitive)
  • If you are not releasing rs2::frame objects in less then the 1000 / fps milliseconds, you will likely encounter frame drops. These events will be visible in the log, if you decrease the severity to DEBUG level.

Frames and Threads

Callbacks are invoked from an internal thread to minimize latency. If you have a lot of processing to do, or simply want to handle the frame in your main event loop, librealsense provides rs2::frame_queue primitive to move frames from one thread to another in a thread-safe fashion:

rs2::frame_queue q;

dev.start([](rs2::frame f){
    q.enqueue(std::move(f)); // enqueue any new frames into q
});

while(true)
{
    rs2::frame f = q.wait_for_frame(); // wait until new frame is available and dequeue it
    // handle frames in the main event loop
}

Since rs2::frame_queue implements operator() you can also pass the queue directly to start:

rs2::frame_queue q;
dev.start(q);

You could also have a separate queue for each stream type:

rs2::frame_queue depth_q;
dev.start(RS2_STREAM_DEPTH, depth_q);
rs2::frame_queue ir_q;
dev.start(RS2_STREAM_INFRARED, ir_q);

This is particularly handy if you want to set-up different processing pipeline for each stream type.

Frame-Drops vs. Latency

There are two common types of applications of the streaming API:

  • Those who need the most relevant data as soon as possible (low latency)
  • Those who want all the data, but don't mind waiting for it (low frame-drops)

librealsense provides some degree of control over this trade-off using RS2_OPTION_FRAMES_QUEUE_SIZE option. If you increase this number, your application will consume more memory and some frames might potentially wait in line more time, but frame drops will be less likely to happen. On the flip side, if you decrease this number, you will get frames faster, but if new frame will arrive while you are busy it will get dropped.

Frame Syncer

Often the input to an image processing application is not simply a frame, but rather a coherent set of frames, preferably taken at the same time. librealsense provides rs2::syncer primitive to help with this problem:

auto sync = dev.create_syncer(); // syncronization algorithm can be device specific
dev.start(sync);
while(true)
{
    auto frameset = sync.wait_for_frames(); // wait for a coherent set of frames
    for (auto&& frame : frameset)
    {
        // handle frame
    }
}
  • In general, there is no guarantee on the quality of the temporal synchronization.
  • If hardware timestamps are available, librealsense will take advantage of them.
  • If the device supports hardware sync, librealsense will try to take advantage of it if it's enabled, but will not implicitly enable it.
  • You can also use a single rs2::syncer to synchronize between devices, assuming it makes sense.

Updated about 1 year ago