② C++ elements

② C++ elements#

Intel® DL Streamer C++ interface allows to develop new functional blocks (source, transform and sink elements) with minimal effort. The interface does not use any of GStreamer types, so elements/blocks written in this interface can be used in both GStreamer-based and non-GStreamer applications.

The C++ elements implementation typically inherit one of base classes which partially implement some functions in corresponding abstract interfaces:

  • BaseSource partially implements Source

  • BaseTransform partially implements Transform

  • BaseTransformInplace partially implements Transform

  • BaseSink partially implements Sink

so that C++ element implements only remaining virtual functions (mostly read, process and write functions) as shown in diagram below

digraph { node[shape=record,style=filled,fillcolor=lightskyblue1] edge[dir=back, arrowtail=empty] Desc[label = "{ElementDesc|+name\l+description\l+author\l+params\l+input_info\l+output_info\l+flags\l+create\l}"] Element[label = "{«interface»\nElement|+ init()\l+ get_context(...)\l}"] Source[label = "{«interface»\Source|+ set_output_info(...)\l+ get_output_info(...)\l+ read()\l}"] Transform[label = "{«interface»\nTransform|+ set_input_info(...)\l+ set_output_info(...)\l+ get_input_info(...)\l+ get_output_info(...)\l+ process(TensorPtr, TensorPtr)\l+ process(FramePtr, FramePtr)\l}"] TransformInplace[label = "{«interface»\nTransformInplace|+ set_info(const FrameInfo&)\l+ process(TensorPtr)\l+ process(FramePtr)\l}"] Sink[label = "{«interface»\Sink|+ set_input_info(...)\l+ get_input_info(...)\l+ write(FramePtr)\l}"] BaseESource[label = "{BaseElement\<Source\>|+ init_once()\l+ init()\l+ get_context(...)\l}"] BaseT1[label = "{BaseElement\<Transform\>|+ init_once()\l+ init()\l+ get_context(...)\l}"] BaseT2[label = "{BaseElement\<TransformInplace\>|+ init_once()\l+ init()\l+ get_context(...)\l}"] BaseESink[label = "{BaseElement\<Sink\>|+ init_once()\l+ init()\l+ get_context(...)\l}"] BaseSource[label = "BaseSource"] BaseTransform[label = "BaseTransform"] BaseInplace[label = "BaseTransformInplace"] BaseSink[label = "BaseSink"] Element->{Source Transform TransformInplace Sink} Source->BaseESource Transform->BaseT1 TransformInplace->BaseT2 Sink->BaseESink BaseESource->BaseSource BaseT1->BaseTransform BaseT2->BaseInplace BaseESink->BaseSink }

C++ interfaces and base classes#

Many examples how to create C++ elements can be found on github repository in folder src and sub-folders.

Element description#

The ElementDesc structure is used to describe an element input/output capabilities and supported parameters, and provide instance creation function. The most important are the following fields:

  • name – Name of element. Same name is used for both GStreamer and direct programming applications.

  • params – Array of parameters supported by element. In case of GStreamer, those will be translated to GStreamer properties.

  • input_info – Types of input Frames that element can consume. In case of GStreamer interop, it will be represented as sink capabilities.

  • output_info – Types of output Frames that element can produce. In case of GStreamer interop, it will be represented as source (src) capabilities.

  • create – Pointer to a function that creates an instance of an element.

Here’s example of ElementDesc structure for simple post-processing element:

// Element parameters
ParamDescVector params_desc = {
  {"method", "Method used to post-process tensor data", "max", {"max", "softmax", "compound", "index"}},
  {"labels_file", "Path to .txt file containing object classes (one per line)", std::string()},
  {"layer_name", "Name of output layer to process (in case of multiple output tensors)", std::string()},
  {"threshold", "Threshold for confidence values", dflt::threshold, 0.0, 1.0},
};

// Element description
ElementDesc tensor_postproc_label = {
  .name = "tensor_postproc_label",
  .description = "Post-processing of classification inference to extract object classes",
  .author = "Intel Corporation",
  .params = &params_desc,
  .input_info = {MediaType::Tensors},
  .output_info = {MediaType::Tensors},
  .create = create_element<PostProcLabel>,
  .flags = 0};

How to create instance of C++ element#

Instance of C++ element can be created using functions create_source, create_transform, create_sink. These functions take pointer to ElementDesc, initialization parameters (as std::map) and optional context pointer as parameters:

auto ffmpeg_source = create_source(ffmpeg_multi_source, {{"inputs", inputs}}, ffmpeg_ctx);

See direct programming samples ffmpeg_openvino and ffmpeg_dpcpp for examples.

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