2020-12-10 04:13:05 +01:00
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---
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layout: default
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title: MediaPipe Python Framework
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parent: MediaPipe in Python
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grand_parent: Getting Started
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nav_order: 1
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---
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# MediaPipe Python Framework
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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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The MediaPipe Python framework grants direct access to the core components of
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the MediaPipe C++ framework such as Timestamp, Packet, and CalculatorGraph,
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whereas the
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[ready-to-use Python solutions](./python.md#ready-to-use-python-solutions) hide
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the technical details of the framework and simply return the readable model
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inference results back to the callers.
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MediaPipe framework sits on top of
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[the pybind11 library](https://pybind11.readthedocs.io/en/stable/index.html).
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The C++ core framework is exposed in Python via a C++/Python language binding.
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The content below assumes that the reader already has a basic understanding of
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the MediaPipe C++ framework. Otherwise, you can find useful information in
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[Framework Concepts](../framework_concepts/framework_concepts.md).
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### Packet
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The packet is the basic data flow unit in MediaPipe. A packet consists of a
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numeric timestamp and a shared pointer to an immutable payload. In Python, a
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MediaPipe packet can be created by calling one of the packet creator methods in
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the
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[`mp.packet_creator`](https://github.com/google/mediapipe/tree/master/mediapipe/python/pybind/packet_creator.cc)
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module. Correspondingly, the packet payload can be retrieved by using one of the
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packet getter methods in the
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[`mp.packet_getter`](https://github.com/google/mediapipe/tree/master/mediapipe/python/pybind/packet_getter.cc)
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module. Note that the packet payload becomes **immutable** after packet
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creation. Thus, the modification of the retrieved packet content doesn't affect
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the actual payload in the packet. MediaPipe framework Python API supports the
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most commonly used data types of MediaPipe (e.g., ImageFrame, Matrix, Protocol
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Buffers, and the primitive data types) in the core binding. The comprehensive
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table below shows the type mappings between the Python and the C++ data type
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along with the packet creator and the content getter method for each data type
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supported by the MediaPipe Python framework API.
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Python Data Type | C++ Data Type | Packet Creator | Content Getter
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------------------------------------ | ----------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------ | --------------
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bool | bool | create_bool(True) | get_bool(packet)
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int or np.intc | int_t | create_int(1) | get_int(packet)
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int or np.int8 | int8_t | create_int8(2**7-1) | get_int(packet)
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int or np.int16 | int16_t | create_int16(2**15-1) | get_int(packet)
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int or np.int32 | int32_t | create_int32(2**31-1) | get_int(packet)
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int or np.int64 | int64_t | create_int64(2**63-1) | get_int(packet)
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int or np.uint8 | uint8_t | create_uint8(2**8-1) | get_uint(packet)
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int or np.uint16 | uint16_t | create_uint16(2**16-1) | get_uint(packet)
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int or np.uint32 | uint32_t | create_uint32(2**32-1) | get_uint(packet)
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int or np.uint64 | uint64_t | create_uint64(2**64-1) | get_uint(packet)
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float or np.float32 | float | create_float(1.1) | get_float(packet)
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float or np.double | double | create_double(1.1) | get_float(packet)
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str (UTF-8) | std::string | create_string('abc') | get_str(packet)
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bytes | std::string | create_string(b'\xd0\xd0\xd0') | get_bytes(packet)
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mp.Packet | mp::Packet | create_packet(p) | get_packet(packet)
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List\[bool\] | std::vector\<bool\> | create_bool_vector(\[True, False\]) | get_bool_list(packet)
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List\[int\] or List\[np.intc\] | int\[\] | create_int_array(\[1, 2, 3\]) | get_int_list(packet, size=10)
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List\[int\] or List\[np.intc\] | std::vector\<int\> | create_int_vector(\[1, 2, 3\]) | get_int_list(packet)
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List\[float\] or List\[np.float\] | float\[\] | create_float_arrary(\[0.1, 0.2\]) | get_float_list(packet, size=10)
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List\[float\] or List\[np.float\] | std::vector\<float\> | create_float_vector(\[0.1, 0.2\]) | get_float_list(packet, size=10)
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List\[str\] | std::vector\<std::string\> | create_string_vector(\['a'\]) | get_str_list(packet)
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List\[mp.Packet\] | std::vector\<mp::Packet\> | create_packet_vector(<br> \[packet1, packet2\]) | get_packet_list(p)
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Mapping\[str, Packet\] | std::map<std::string, Packet> | create_string_to_packet_map(<br> {'a': packet1, 'b': packet2}) | get_str_to_packet_dict(packet)
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np.ndarray<br>(cv.mat and PIL.Image) | mp::ImageFrame | create_image_frame(<br> format=ImageFormat.SRGB,<br> data=mat) | get_image_frame(packet)
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np.ndarray | mp::Matrix | create_matrix(data) | get_matrix(packet)
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Google Proto Message | Google Proto Message | create_proto(proto) | get_proto(packet)
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List\[Proto\] | std::vector\<Proto\> | n/a | get_proto_list(packet)
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2020-12-10 04:13:05 +01:00
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It's not uncommon that users create custom C++ classes and and send those into
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the graphs and calculators. To allow the custom classes to be used in Python
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with MediaPipe, you may extend the Packet API for a new data type in the
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following steps:
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1. Write the pybind11
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[class binding code](https://pybind11.readthedocs.io/en/stable/advanced/classes.html)
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or
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[a custom type caster](https://pybind11.readthedocs.io/en/stable/advanced/cast/custom.html?highlight=custom%20type%20caster)
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for the custom type in a cc file.
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```c++
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#include "path/to/my_type/header/file.h"
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#include "pybind11/pybind11.h"
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namespace py = pybind11;
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PYBIND11_MODULE(my_type_binding, m) {
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// Write binding code or a custom type caster for MyType.
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py::class_<MyType>(m, "MyType")
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.def(py::init<>())
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.def(...);
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}
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```
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2. Create a new packet creator and getter method of the custom type in a
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separate cc file.
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```c++
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#include "path/to/my_type/header/file.h"
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#include "mediapipe/framework/packet.h"
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#include "pybind11/pybind11.h"
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namespace mediapipe {
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namespace py = pybind11;
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PYBIND11_MODULE(my_packet_methods, m) {
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m.def(
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"create_my_type",
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[](const MyType& my_type) { return MakePacket<MyType>(my_type); });
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m.def(
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"get_my_type",
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[](const Packet& packet) {
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if(!packet.ValidateAsType<MyType>().ok()) {
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PyErr_SetString(PyExc_ValueError, "Packet data type mismatch.");
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return py::error_already_set();
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}
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return packet.Get<MyType>();
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});
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} // namespace mediapipe
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```
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3. Add two bazel build rules for the custom type binding and the new packet
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methods in the BUILD file.
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```
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load("@pybind11_bazel//:build_defs.bzl", "pybind_extension")
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pybind_extension(
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name = "my_type_binding",
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srcs = ["my_type_binding.cc"],
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deps = [":my_type"],
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)
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pybind_extension(
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name = "my_packet_methods",
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srcs = ["my_packet_methods.cc"],
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deps = [
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":my_type",
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"//mediapipe/framework:packet"
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],
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)
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```
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4. Build the pybind extension targets (with the suffix .so) by Bazel and move the generated dynamic libraries into one of the $LD_LIBRARY_PATH dirs.
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5. Use the binding modules in Python.
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```python
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import my_type_binding
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import my_packet_methods
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packet = my_packet_methods.create_my_type(my_type_binding.MyType())
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my_type = my_packet_methods.get_my_type(packet)
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```
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### Timestamp
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Each packet contains a timestamp that is in units of microseconds. In Python,
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the Packet API provides a convenience method `packet.at()` to define the numeric
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timestamp of a packet. More generally, `packet.timestamp` is the packet class
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property for accessing the underlying timestamp. To convert an Unix epoch to a
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MediaPipe timestamp,
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[the Timestamp API](https://github.com/google/mediapipe/tree/master/mediapipe/python/pybind/timestamp.cc)
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offers a method `mp.Timestamp.from_seconds()` for this purpose.
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### ImageFrame
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ImageFrame is the container for storing an image or a video frame. Formats
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supported by ImageFrame are listed in
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[the ImageFormat enum](https://github.com/google/mediapipe/tree/master/mediapipe/python/pybind/image_frame.cc#l=170).
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Pixels are encoded row-major with interleaved color components, and ImageFrame
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supports uint8, uint16, and float as its data types. MediaPipe provides
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[an ImageFrame Python API](https://github.com/google/mediapipe/tree/master/mediapipe/python/pybind/image_frame.cc)
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to access the ImageFrame C++ class. In Python, the easiest way to retrieve the
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pixel data is to call `image_frame.numpy_view()` to get a numpy ndarray. Note
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that the returned numpy ndarray, a reference to the internal pixel data, is
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unwritable. If the callers need to modify the numpy ndarray, it's required to
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explicitly call a copy operation to obtain a copy. When MediaPipe takes a numpy
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ndarray to make an ImageFrame, it assumes that the data is stored contiguously.
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Correspondingly, the pixel data of an ImageFrame will be realigned to be
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contiguous when it's returned to the Python side.
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### Graph
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In MediaPipe, all processing takes places within the context of a
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CalculatorGraph.
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[The CalculatorGraph Python API](https://github.com/google/mediapipe/tree/master/mediapipe/python/pybind/calculator_graph.cc)
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is a direct binding to the C++ CalculatorGraph class. The major difference is
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the CalculatorGraph Python API raises a Python error instead of returning a
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non-OK Status when an error occurs. Therefore, as a Python user, you can handle
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the exceptions as you normally do. The life cycle of a CalculatorGraph contains
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three stages: initialization and setup, graph run, and graph shutdown.
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1. Initialize a CalculatorGraph with a CalculatorGraphConfig protobuf or binary
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protobuf file, and provide callback method(s) to observe the output
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stream(s).
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Option 1. Initialize a CalculatorGraph with a CalculatorGraphConfig protobuf
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or its text representation, and observe the output stream(s):
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```python
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import mediapipe as mp
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config_text = """
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input_stream: 'in_stream'
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output_stream: 'out_stream'
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node {
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calculator: 'PassThroughCalculator'
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input_stream: 'in_stream'
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output_stream: 'out_stream'
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}
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"""
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graph = mp.CalculatorGraph(graph_config=config_text)
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output_packets = []
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graph.observe_output_stream(
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'out_stream',
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lambda stream_name, packet:
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output_packets.append(mp.packet_getter.get_str(packet)))
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```
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Option 2. Initialize a CalculatorGraph with with a binary protobuf file, and
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observe the output stream(s).
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```python
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import mediapipe as mp
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# resources dependency
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graph = mp.CalculatorGraph(
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binary_graph=os.path.join(
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resources.GetRunfilesDir(), 'path/to/your/graph.binarypb'))
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graph.observe_output_stream(
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'out_stream',
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lambda stream_name, packet: print(f'Get {packet} from {stream_name}'))
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```
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2. Start the graph run and feed packets into the graph.
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```python
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graph.start_run()
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graph.add_packet_to_input_stream(
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'in_stream', mp.packet_creator.create_string('abc').at(0))
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rgb_img = cv2.cvtColor(cv2.imread('/path/to/your/image.png'), cv2.COLOR_BGR2RGB)
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graph.add_packet_to_input_stream(
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'in_stream',
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mp.packet_creator.create_image_frame(image_format=mp.ImageFormat.SRGB,
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data=rgb_img).at(1))
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```
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3. Close the graph after finish. You may restart the graph for another graph
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run after the call to `close()`.
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```python
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graph.close()
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```
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The Python script can be run by your local Python runtime.
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