Takes an image as an input, returns the same image, outlined by 1px, as output.

imageOutliner.cpp

// Copyright (c) 2013 Vittorio Romeo
// License: Academic Free License ("AFL") v. 3.0
// AFL License page: http://opensource.org/licenses/AFL-3.0

#define SSVS_N_USE_JSON

#include <vector>
#include <string>
#include <SFML/System.hpp>
#include <SFML/Graphics.hpp>
#include <SSVUtils/SSVUtils.hpp>
#include <SSVStart/SSVStart.hpp>

constexpr unsigned int tolerance{15};

int main(int argc, char* argv[])
{
	std::vector<std::string> clArgs; for(int i{0}; i < argc; ++i) clArgs.emplace_back(argv[i]);

	if(clArgs.size() < 2)
	{
		ssvu::lo("Error") << "Please specify an image file name." << std::endl;
		return 1;
	}

	std::string imgPath{clArgs[1]};
	sf::Image img;
	
	if(!img.loadFromFile(imgPath))
	{
		ssvu::lo("Error") << "<" + clArgs[1] + "> is not a valid image file." << std::endl;
		return 1;
	}

	unsigned int width{img.getSize().x}, height{img.getSize().y};
	sf::Color outlineColor{sf::Color::Black};

	sf::Image result;
	result.create(width, height, img.getPixelsPtr());

	for(auto iX(1u); iX < width - 1; ++iX)
		for(auto iY(1u); iY < height - 1; ++iY)
			{
				auto px(img.getPixel(iX, iY));
				auto alpha(px.a);

				auto diffR(std::abs(px.r - outlineColor.r));
				auto diffG(std::abs(px.g - outlineColor.g));
				auto diffB(std::abs(px.b - outlineColor.b));

				auto diffOverall((diffR + diffG + diffB) / 3);

				if(alpha > tolerance)
				{
					for(int nX{-1}; nX <= 1; ++nX)
						for(int nY{-1}; nY <= 1; ++nY)
							if(result.getPixel(iX + nX, iY + nY).a == 0)
								result.setPixel(iX + nX, iY + nY, sf::Color{0, 0, 0, diffOverall});
				}	
			}

	result.saveToFile(clArgs[1] + "_outlined.png");

	return 0;
}