# Graph API {#gapi}

# Introduction {#gapi_root_intro}

OpenCV Graph API (or G-API) is a new OpenCV module targeted to make
regular image processing fast and portable. These two goals are
achieved by introducing a new graph-based model of execution.

G-API is a special module in OpenCV -- in contrast with the majority
of other main modules, this one acts as a framework rather than some
specific CV algorithm. G-API provides means to define CV operations,
construct graphs (in form of expressions) using it, and finally
implement and run the operations for a particular backend.

# Contents

G-API documentation is organized into the following chapters:

- @subpage gapi_purposes

  The motivation behind G-API and its goals.

- @subpage gapi_hld

  General overview of G-API architecture and its major internal
  components.

- @subpage gapi_kernel_api

  Learn how to introduce new operations in G-API and implement it for
  various backends.

- @subpage gapi_impl

  Low-level implementation details of G-API, for those who want to
  contribute.

- API Reference: functions and classes

    - @subpage gapi_core

      Core G-API operations - arithmetic, boolean, and other matrix
      operations;

    - @subpage gapi_imgproc

      Image processing functions: color space conversions, various
      filters, etc.

# API Example {#gapi_example}

A very basic example of G-API pipeline is shown below:

@include modules/gapi/samples/api_example.cpp

<!-- TODO align this code with text using marks and itemized list -->

G-API is a separate OpenCV module so its header files have to be
included explicitly. The first four lines of `main()` create and
initialize OpenCV's standard video capture object, which fetches
video frames from either an attached camera or a specified file.

G-API pipelie is constructed next. In fact, it is a series of G-API
operation calls on cv::GMat data. The important aspect of G-API is
that this code block is just a declaration of actions, but not the
actions themselves. No processing happens at this point, G-API only
tracks which operations form pipeline and how it is connected. G-API
_Data objects_ (here it is cv::GMat) are used to connect operations
each other. `in` is an _empty_ cv::GMat signalling that it is a
beginning of computation.

After G-API code is written, it is captured into a call graph with
instantiation of cv::GComputation object. This object takes
input/output data references (in this example, `in` and `out`
cv::GMat objects, respectively) as parameters and reconstructs the
call graph based on all the data flow between `in` and `out`.

cv::GComputation is a thin object in sense that it just captures which
operations form up a computation. However, it can be used to execute
computations -- in the following processing loop, every captured frame (a
cv::Mat `input_frame`) is passed to cv::GComputation::apply().

![Example pipeline running on sample video 'vtest.avi'](pics/demo.jpg)

cv::GComputation::apply() is a polimorphic method which accepts a
variadic number of arguments. Since this computation is defined on one
input, one output, a special overload of cv::GComputation::apply() is
used to pass input data and get output data.

Internally, cv::GComputation::apply() compiles the captured graph for
the given input parameters and executes the compiled graph on data
immediately.

There is a number important concepts can be outlines with this examle:
* Graph declaration and graph execution are distinct steps;
* Graph is built implicitly from a sequence of G-API expressions;
* G-API supports function-like calls -- e.g. cv::gapi::resize(), and
  operators, e.g operator|() which is used to compute bitwise OR;
* G-API syntax aims to look pure: every operation call within a graph
  yields a new result, thus forming a directed acyclic graph (DAG);
* Graph declaration is not bound to any data -- real data objects
  (cv::Mat) come into picture after the graph is already declared.

<!-- FIXME: The above operator|() link links to MatExpr not GAPI -->

See Tutorial[TBD] and Porting examples[TBD] to learn more on various
G-API features and concepts.

<!-- TODO Add chapter on declaration, compilation, execution -->