opencv/3rdparty/openvx/README.md

# C++ wrappers for OpenVX-1.x C API

## Core ideas:

* lightweight - minimal overhead vs standard C API
* automatic references counting
* exceptions instead of return codes
* object-oriented design
* (NYI) helpers for user-defined kernels & nodes
* C++ 11 friendly

## Quick start sample

The following short sample gives basic knowledge on the wrappers usage:

```cpp
#include "ivx.hpp"
#include "ivx_lib_debug.hpp" // ivx::debug::*

int main()
{
	vx_uint32 width = 640, height = 480;
	try
	{
		ivx::Context context = ivx::Context::create();
		ivx::Graph graph = ivx::Graph::create(context);
		ivx::Image
		    gray = ivx::Image::create(context, width, height, VX_DF_IMAGE_U8),
		    gb   = ivx::Image::createVirtual(graph),
		    res  = ivx::Image::create(context, width, height, VX_DF_IMAGE_U8);

		context.loadKernels("openvx-debug");  // ivx::debug::*

		ivx::debug::fReadImage(context, inputPath, gray);

		ivx::Node::create(graph, VX_KERNEL_GAUSSIAN_3x3, gray, gb);
		ivx::Node::create(
		    graph,
		    VX_KERNEL_THRESHOLD,
		    gb,
		    ivx::Threshold::createBinary(context, VX_TYPE_UINT8, 50),
		    res
		);

		graph.verify();
		graph.process();

		ivx::debug::fWriteImage(context, res, "ovx-res-cpp.pgm");
    }
    catch (const ivx::RuntimeError& e)
    {
        printf("ErrorRuntime: code = %d(%x), message = %s\n", e.status(), e.status(), e.what());
        return e.status();
    }
    catch (const ivx::WrapperError& e)
    {
        printf("ErrorWrapper: message = %s\n", e.what());
        return -1;
    }
    catch(const std::exception& e)
    {
        printf("runtime_error: message = %s\n", e.what());
        return -1;
    }

    return 0;
}
	
```
## C++ API overview

The wrappers have **header-only** implementation that simplifies their integration to projects.
All the API is inside `ivx` namespace (E.g. `class ivx::Graph`).

While the C++ API is pretty much the same for underlying OpenVX version **1.0** and **1.1**, there are alternative code branches for some features implementation  that are selected at **compile time** via `#ifdef` preprocessor directives.
E.g. external ref-counting is implemented for 1.0 version and native OpenVX one is used (via  `vxRetainReference()` and `vxReleaseXYZ()`) for version 1.1.

Also there are some **C++ 11** features are used (e.g. rvalue ref-s) when their availability is detected at ***compile time***.

C++ exceptions are used for errors indication instead of return codes. There are two types of exceptions are defined:  `RuntimeError` is thrown when OpenVX C call returned unsuccessful result and `WrapperError` is thrown when a problem is occurred in the wrappers code. Both exception classes are derived from `std::exception` (actually from its inheritance).

The so called **OpenVX objects** (e.g. `vx_image`) are represented as C++ classes in wrappers.
All these classes use automatic ref-counting that allows development of exception-safe code.
All these classes have `create()` or `createXYZ()` `static` methods for instances creation. (E.g. `Image::create()`, `Image::createVirtual()` and  `Image::createFromHandle()`)
Most of the wrapped OpenVX functions are represented as methods of the corresponding C++ classes, but in most cases they still accept C "object" types (e.g. `vx_image` or `vx_context`) that allows mixing of C and C++ OpenVX API use.
E.g.:
```cpp
class Image
{
    static Image create(vx_context context, vx_uint32 width, vx_uint32 height, vx_df_image format);
    static Image createVirtual(vx_graph graph, vx_uint32 width = 0, vx_uint32 height = 0, vx_df_image format = VX_DF_IMAGE_VIRT);
    // ...
}
```
All the classes instances can automatically be converted to the corresponding C "object" types.

For more details please refer to C++ wrappers reference manual or directly to their source code.