113 lines
4.3 KiB
Markdown
113 lines
4.3 KiB
Markdown
# Graph API {#gapi}
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# Introduction {#gapi_root_intro}
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OpenCV Graph API (or G-API) is a new OpenCV module targeted to make
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regular image processing fast and portable. These two goals are
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achieved by introducing a new graph-based model of execution.
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G-API is a special module in OpenCV -- in contrast with the majority
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of other main modules, this one acts as a framework rather than some
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specific CV algorithm. G-API provides means to define CV operations,
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construct graphs (in form of expressions) using it, and finally
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implement and run the operations for a particular backend.
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@note G-API is a new module and now is in active development. It's API
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is volatile at the moment and there may be minor but
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compatibility-breaking changes in the future.
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# Contents
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G-API documentation is organized into the following chapters:
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- @subpage gapi_purposes
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The motivation behind G-API and its goals.
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- @subpage gapi_hld
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General overview of G-API architecture and its major internal
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components.
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- @subpage gapi_kernel_api
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Learn how to introduce new operations in G-API and implement it for
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various backends.
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- @subpage gapi_impl
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Low-level implementation details of G-API, for those who want to
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contribute.
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- API Reference: functions and classes
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- @subpage gapi_core
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Core G-API operations - arithmetic, boolean, and other matrix
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operations;
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- @subpage gapi_imgproc
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Image processing functions: color space conversions, various
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filters, etc.
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# API Example {#gapi_example}
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A very basic example of G-API pipeline is shown below:
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@include modules/gapi/samples/api_example.cpp
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<!-- TODO align this code with text using marks and itemized list -->
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G-API is a separate OpenCV module so its header files have to be
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included explicitly. The first four lines of `main()` create and
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initialize OpenCV's standard video capture object, which fetches
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video frames from either an attached camera or a specified file.
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G-API pipeline is constructed next. In fact, it is a series of G-API
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operation calls on cv::GMat data. The important aspect of G-API is
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that this code block is just a declaration of actions, but not the
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actions themselves. No processing happens at this point, G-API only
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tracks which operations form pipeline and how it is connected. G-API
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_Data objects_ (here it is cv::GMat) are used to connect operations
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each other. `in` is an _empty_ cv::GMat signalling that it is a
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beginning of computation.
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After G-API code is written, it is captured into a call graph with
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instantiation of cv::GComputation object. This object takes
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input/output data references (in this example, `in` and `out`
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cv::GMat objects, respectively) as parameters and reconstructs the
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call graph based on all the data flow between `in` and `out`.
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cv::GComputation is a thin object in sense that it just captures which
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operations form up a computation. However, it can be used to execute
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computations -- in the following processing loop, every captured frame (a
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cv::Mat `input_frame`) is passed to cv::GComputation::apply().
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cv::GComputation::apply() is a polimorphic method which accepts a
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variadic number of arguments. Since this computation is defined on one
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input, one output, a special overload of cv::GComputation::apply() is
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used to pass input data and get output data.
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Internally, cv::GComputation::apply() compiles the captured graph for
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the given input parameters and executes the compiled graph on data
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immediately.
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There is a number important concepts can be outlines with this example:
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* Graph declaration and graph execution are distinct steps;
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* Graph is built implicitly from a sequence of G-API expressions;
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* G-API supports function-like calls -- e.g. cv::gapi::resize(), and
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operators, e.g operator|() which is used to compute bitwise OR;
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* G-API syntax aims to look pure: every operation call within a graph
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yields a new result, thus forming a directed acyclic graph (DAG);
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* Graph declaration is not bound to any data -- real data objects
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(cv::Mat) come into picture after the graph is already declared.
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<!-- FIXME: The above operator|() link links to MatExpr not GAPI -->
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See [tutorials and porting examples](@ref tutorial_table_of_content_gapi)
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to learn more on various G-API features and concepts.
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<!-- TODO Add chapter on declaration, compilation, execution -->
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