132 lines
4.3 KiB
Markdown
132 lines
4.3 KiB
Markdown
---
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layout: forward
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target: https://developers.google.com/mediapipe/framework/getting_started/hello_world_cpp
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title: Hello World! in C++
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parent: MediaPipe in C++
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grand_parent: Getting Started
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nav_order: 1
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---
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# Hello World! in C++
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{: .no_toc }
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1. TOC
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{:toc}
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---
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1. Ensure you have a working version of MediaPipe. See
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[installation instructions](./install.md).
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2. To run the [`hello world`] example:
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```bash
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$ git clone https://github.com/google/mediapipe.git
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$ cd mediapipe
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$ export GLOG_logtostderr=1
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# Need bazel flag 'MEDIAPIPE_DISABLE_GPU=1' as desktop GPU is not supported currently.
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$ bazel run --define MEDIAPIPE_DISABLE_GPU=1 \
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mediapipe/examples/desktop/hello_world:hello_world
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# It should print 10 rows of Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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# Hello World!
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```
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3. The [`hello world`] example uses a simple MediaPipe graph in the
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`PrintHelloWorld()` function, defined in a [`CalculatorGraphConfig`] proto.
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```C++
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absl::Status PrintHelloWorld() {
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// Configures a simple graph, which concatenates 2 PassThroughCalculators.
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CalculatorGraphConfig config = ParseTextProtoOrDie<CalculatorGraphConfig>(R"(
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input_stream: "in"
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output_stream: "out"
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node {
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calculator: "PassThroughCalculator"
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input_stream: "in"
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output_stream: "out1"
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}
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node {
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calculator: "PassThroughCalculator"
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input_stream: "out1"
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output_stream: "out"
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}
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)");
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```
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You can visualize this graph using
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[MediaPipe Visualizer](https://viz.mediapipe.dev) by pasting the
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CalculatorGraphConfig content below into the visualizer. See
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[here](../tools/visualizer.md) for help on the visualizer.
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```bash
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input_stream: "in"
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output_stream: "out"
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node {
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calculator: "PassThroughCalculator"
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input_stream: "in"
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output_stream: "out1"
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}
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node {
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calculator: "PassThroughCalculator"
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input_stream: "out1"
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output_stream: "out"
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}
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```
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This graph consists of 1 graph input stream (`in`) and 1 graph output stream
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(`out`), and 2 [`PassThroughCalculator`]s connected serially.
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4. Before running the graph, an `OutputStreamPoller` object is connected to the
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output stream in order to later retrieve the graph output, and a graph run
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is started with [`StartRun`].
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```c++
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CalculatorGraph graph;
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MP_RETURN_IF_ERROR(graph.Initialize(config));
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MP_ASSIGN_OR_RETURN(OutputStreamPoller poller,
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graph.AddOutputStreamPoller("out"));
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MP_RETURN_IF_ERROR(graph.StartRun({}));
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```
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5. The example then creates 10 packets (each packet contains a string "Hello
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World!" with Timestamp values ranging from 0, 1, ... 9) using the
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[`MakePacket`] function, adds each packet into the graph through the `in`
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input stream, and finally closes the input stream to finish the graph run.
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```c++
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for (int i = 0; i < 10; ++i) {
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MP_RETURN_IF_ERROR(graph.AddPacketToInputStream("in",
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MakePacket<std::string>("Hello World!").At(Timestamp(i))));
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}
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MP_RETURN_IF_ERROR(graph.CloseInputStream("in"));
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```
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6. Through the `OutputStreamPoller` object the example then retrieves all 10
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packets from the output stream, gets the string content out of each packet
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and prints it to the output log.
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```c++
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mediapipe::Packet packet;
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while (poller.Next(&packet)) {
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LOG(INFO) << packet.Get<string>();
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}
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```
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[`hello world`]: https://github.com/google/mediapipe/tree/master/mediapipe/examples/desktop/hello_world/hello_world.cc
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[`CalculatorGraphConfig`]: https://github.com/google/mediapipe/tree/master/mediapipe/framework/calculator.proto
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[`PassThroughCalculator`]: https://github.com/google/mediapipe/tree/master/mediapipe/calculators/core/pass_through_calculator.cc
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[`MakePacket`]: https://github.com/google/mediapipe/tree/master/mediapipe/framework/packet.h
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[`StartRun`]: https://github.com/google/mediapipe/tree/master/mediapipe/framework/calculator_graph.h
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