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Նկար (nkar) image comparison and differences highlighting C++ API

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# Nkar image comparison and differences highlighting C++ API

Nkar (Armenian: նկար) is an image processing and comparison utility. The comparison
of two images of the same size based on comparing each pixel of one image with the
corresponding pixel of another one. After all differences are identified, algorithm
generates contours (polygons) that outlines the differences. The API provides both
error reporting and differences analysis. For instance results contains information
on the number of difference contours generated (see usage example below).

According to [stb single-file public domain libraries](https://github.com/nothings/stb)
we use for image loading/decoding/saving the following image formats are supported:
JPG, PNG, TGA, BMP, PSD, GIF, HDR, PIC.

[![Latest release](https://img.shields.io/github/v/release/vahancho/nkar?include_prereleases)](https://github.com/vahancho/nkar/releases)
[![Test (CMake)](https://github.com/vahancho/nkar/actions/workflows/cmake.yml/badge.svg)](https://github.com/vahancho/nkar/actions/workflows/cmake.yml)
[![Build status](https://ci.appveyor.com/api/projects/status/gh9v3ynrm1dt1w7t/branch/master?svg=true)](https://ci.appveyor.com/project/vahancho/nkar/branch/master)
[![codecov](https://codecov.io/gh/vahancho/nkar/branch/master/graph/badge.svg)](https://codecov.io/gh/vahancho/nkar)

### Installation

No installation required. Just incorporate header and source files from *src/* directory
in your project and compile them. Use `nkar::Comparator` class to compare two images.
All library classes are in *nkar* namespace.

#### Integration with `CMake` projects

However, if you use `CMake` and want to integrate the library into your project
you might want to install it first by invoking a `CMake` command from the build directory:

```
cmake --install . --prefix=<install_path> --config=Release
```

Once the library is installed you can use it from in your project by adjusting its
`CMake` script. For example:

```
[..]
find_package(nkar REQUIRED)

add_executable(example main.cpp)
target_link_libraries(example nkar)
[..]
```

### Prerequisites

There are no special requirements and dependencies except *C++11* compliant compiler.
For more details see the CI badges (*GitHub Actions*, *AppVeyor CI* etc.).

### Usage Example:

```cpp
// A simple application that compares two image files and saves differences in a third one.

#include <string>

#include "comparator.h"
#include "image.h"

using namespace nkar;

int main(int argc, char **argv)
{
  if (argc != 4) {
    fprintf(stderr, "Incorrect number of arguments\n");
    return -1;
  }

// Compare two image files.
  auto highlightColor = Color{0, 255, 0}; // The diff outline color.
  auto result = Comparator::compare(argv[1], argv[2], highlightColor);

if (result.error() != Result::Error::NoError) {
    // An error occurred during comparison.
    fprintf(stderr, "%s\n", result.errorMessage().c_str());
    return -1;
  }

if (result.status() == Result::Status::Different) {
    if (result.resultImage().save(argv[3])) {
      fprintf(stdout, "Images are different: %d contours found. Image with highlighting is saved to '%s'\n",
              result.contourCount(), argv[3]);
      return -1;
    } else {
      fprintf(stderr, "Failed to save result image\n");
      return -1;
    }
  }

fprintf(stdout, "Images are identical\n");
  return 0;
}
```

### Test

There are unit tests provided for `nkar::Comparator` class. You can find them in the *test/* directory.
To run them you have to build and run the test application. For doing that you must invoke the following
commands from the terminal, assuming that compiler and environment are already configured:

##### Linux (gcc)

```
mkdir build && cd build
cmake .. -DCMAKE_BUILD_TYPE=Release -DENABLE_TESTING=True
cmake --build .
ctest
```

##### Windows (MSVC Toolchain)

```
mkdir build && cd build
cmake .. -DENABLE_TESTING=True -A x64
cmake --build . --config=Release
ctest -C Release
```

For x86 builds use `-A Win32` option instead.

### Examples

Below are some examples of image comparison with results. Third image in each row
represents an image generated by *Comparator* and contains contours (red outlines)
that highlight the differences:

| expected | actual | diff with contours |
| --- | --- | --- |
| <img src="./test/images/map1.png" width="200"> | <img src="./test/images/map2.png" width="200"> | <img src="./test/images/map_result.png" width="200"> |
| ![](https://raw.githubusercontent.com/vahancho/nkar/master/test/images/empty.png) | ![](https://raw.githubusercontent.com/vahancho/nkar/master/test/images/13.png) | ![1diff](https://raw.githubusercontent.com/vahancho/nkar/master/test/images/13_result.png) |
| ![](https://raw.githubusercontent.com/vahancho/nkar/master/test/images/lenna.png) | ![](https://raw.githubusercontent.com/vahancho/nkar/master/test/images/lenna_changed.png) | ![1diff](https://raw.githubusercontent.com/vahancho/nkar/master/test/images/lenna_result.png) |
| <img src="./test/images/empty_large.png" width="200"> | <img src="./test/images/large.png" width="200"> | <img src="./test/images/large_result.png" width="200"> |

### See Also

For reading and writing image files we used [stb single-file public domain libraries](https://github.com/nothings/stb)

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