mediapipe/docs/getting_started/hello_world_cpp.md

layout	title	parent	grand_parent	nav_order
default	Hello World! in C++	MediaPipe in C++	Getting Started	1

Hello World! in C++

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1. TOC {:toc}

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1. Ensure you have a working version of MediaPipe. See installation instructions.

2. To run the hello world example:

   $ git clone https://github.com/google/mediapipe.git
   $ cd mediapipe
   
   $ export GLOG_logtostderr=1
   # Need bazel flag 'MEDIAPIPE_DISABLE_GPU=1' as desktop GPU is not supported currently.
   $ bazel run --define MEDIAPIPE_DISABLE_GPU=1 \
       mediapipe/examples/desktop/hello_world:hello_world
   
   # It should print 10 rows of Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   # Hello World!
   

3. The hello world example uses a simple MediaPipe graph in the PrintHelloWorld() function, defined in a CalculatorGraphConfig proto.

   absl::Status PrintHelloWorld() {
     // Configures a simple graph, which concatenates 2 PassThroughCalculators.
     CalculatorGraphConfig config = ParseTextProtoOrDie<CalculatorGraphConfig>(R"(
       input_stream: "in"
       output_stream: "out"
       node {
         calculator: "PassThroughCalculator"
         input_stream: "in"
         output_stream: "out1"
       }
       node {
         calculator: "PassThroughCalculator"
         input_stream: "out1"
         output_stream: "out"
       }
     )");
   

   You can visualize this graph using MediaPipe Visualizer by pasting the CalculatorGraphConfig content below into the visualizer. See here for help on the visualizer.

       input_stream: "in"
       output_stream: "out"
       node {
         calculator: "PassThroughCalculator"
         input_stream: "in"
         output_stream: "out1"
       }
       node {
         calculator: "PassThroughCalculator"
         input_stream: "out1"
         output_stream: "out"
       }
   

   This graph consists of 1 graph input stream (in) and 1 graph output stream (out), and 2 PassThroughCalculators connected serially.

   

4. Before running the graph, an OutputStreamPoller object is connected to the output stream in order to later retrieve the graph output, and a graph run is started with StartRun.

   CalculatorGraph graph;
   MP_RETURN_IF_ERROR(graph.Initialize(config));
   MP_ASSIGN_OR_RETURN(OutputStreamPoller poller,
                       graph.AddOutputStreamPoller("out"));
   MP_RETURN_IF_ERROR(graph.StartRun({}));
   

5. The example then creates 10 packets (each packet contains a string "Hello World!" with Timestamp values ranging from 0, 1, ... 9) using the MakePacket function, adds each packet into the graph through the in input stream, and finally closes the input stream to finish the graph run.

   for (int i = 0; i < 10; ++i) {
     MP_RETURN_IF_ERROR(graph.AddPacketToInputStream("in",
                        MakePacket<std::string>("Hello World!").At(Timestamp(i))));
   }
   MP_RETURN_IF_ERROR(graph.CloseInputStream("in"));
   

6. Through the OutputStreamPoller object the example then retrieves all 10 packets from the output stream, gets the string content out of each packet and prints it to the output log.

   mediapipe::Packet packet;
   while (poller.Next(&packet)) {
     LOG(INFO) << packet.Get<string>();
   }