A basic app that converts .png images to ASCII art

png2ascii.cpp

#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <stdexcept>
#include <vector>

#include "lodepng/lodepng.h" // you have to have lodepng in this folder to compile

bool open_input(std::string filename, std::vector<unsigned char> &img, unsigned int &width,
				unsigned int &height);
std::string transform_to_ascii(std::vector<unsigned char> &img, unsigned int width,
								unsigned int height);
bool write_output(std::string filename, std::string img);

int main(int argc, char *argv[])
{
	if (argc != 3)
	{
		std::cerr << "Usage: png2ascii input output" << std::endl;
		return EXIT_FAILURE;
	}
	
	#define INPUT_FILE  argv[1]
	#define OUTPUT_FILE argv[2]
	
	std::vector<unsigned char> img;
	unsigned int width, height;
	
	if (!open_input(INPUT_FILE, img, width, height))
		return EXIT_FAILURE;
	
	std::string ascii_img = transform_to_ascii(img, width, height);
	
	if (!write_output(OUTPUT_FILE, ascii_img))
		return EXIT_FAILURE;
	
	return EXIT_SUCCESS;
}

bool open_input(std::string filename, std::vector<unsigned char> &img, unsigned int &width,
				unsigned int &height)
{
	unsigned int err = lodepng::decode(img, width, height, filename);
	if (err)
	{
		std::cerr << "ERROR: " << "lodepng decoder error " << err << ": "
			<< lodepng_error_text(err) << std::endl;
		
		return false;
	}
	
	return true;
}

std::string transform_to_ascii(std::vector<unsigned char> &img, unsigned int width,
								unsigned int height)
{
	const char symbols[] = "@%#*+=-:. ";
	const int sym_count = strlen(symbols);
	
	std::string result = "";
	
	for (size_t i = 0; i < img.size(); i += 4)
	{
		float luminance = (0.2126 * img[i] + 0.7152 * img[i + 1] +
							0.0722 * img[i + 2]) / 255;
		
		result += symbols[(img[i + 3] ? (size_t) (luminance * (sym_count - 1)) : sym_count - 1)];
		
		if ((result.size() % (width + 1) == width) &&
			(result.size() != (width + 1) * height - 1))
		{
			result += '\n';
		}
	}
	
	return result;
}

bool write_output(std::string filename, std::string img)
{
	std::ofstream file(filename);
	if (!file.is_open())
	{
		std::cerr << "ERROR: Could not write to output file" << std::endl;
		return false;
	}
	
	file << img;
	file.close();
	
	return true;
}