色の濃淡のみでASCII ARTを描画してみる

今回は、画像から色の濃淡のみでASCII ARTを描画します。

必要なものの宣言

#include <iostream>
#include <opencv2/opencv.hpp>
#include <string>

using namespace cv;
using namespace std;

明るさに応じた文字を返す関数を作る

char mapGrayscaleToAscii(int grayscale_value) {
  const string ascii_chars = " .:-=+*#%@";

  // Map 0-255 grayscale value to character index
  int index = (grayscale_value * (ascii_chars.size() - 1)) / 255;

  return ascii_chars[index];
}

受け取った画像をもとにASCII ARTに変換

string imageToAscii(Mat &image, int output_width) {
  // 画像が白黒でなければ変換する
  Mat grayscale;
  if (image.channels() == 3) {
    cvtColor(image, grayscale, COLOR_BGR2GRAY);
  } else {
    grayscale = image.clone();
  }

  // 大きさを計算する
  float aspect_ratio = (float)grayscale.rows / (float)grayscale.cols;
  int output_height = (int)(output_width * aspect_ratio *
                            0.5); // 0.5 because chars are taller than wide

  // 画像を出力したい大きさに合わせてスケールし直す
  Mat resized;
  resize(grayscale, resized, Size(output_width, output_height), 0, 0,
         INTER_AREA);

  // 各ピクセルをアスキーアートに変換する
  string ascii_art;
  for (int y = 0; y < resized.rows; y++) {
    for (int x = 0; x < resized.cols; x++) {
      int pixel_value = (int)resized.at<uchar>(y, x);
      ascii_art += mapGrayscaleToAscii(pixel_value);
    }
    ascii_art += '\n';
  }

  return ascii_art;
}

引数を受け取って処理する

int main(int argc, char **argv) {
  // Check command line arguments
  if (argc < 2) {
    cout << "Usage: " << argv[0] << " <image_path> [output_width]" << endl;
    cout << "  image_path: Path to the input image" << endl;
    cout << "  output_width: Width of ASCII output in characters (default: 100)"
         << endl;
    return -1;
  }

  string image_path = argv[1];
  int output_width = 100; // Default width

  if (argc >= 3) {
    output_width = stoi(argv[2]);
  }

  // Load the image
  Mat image = imread(image_path, IMREAD_COLOR);

  if (image.empty()) {
    cerr << "Error: Could not open or find the image: " << image_path << endl;
    return -1;
  }

  // Convert to ASCII
  string ascii_art = imageToAscii(image, output_width);

  // Output the result
  cout << ascii_art << endl;

  return 0;
}

最後に

コード全体

#include <iostream>
#include <opencv2/opencv.hpp>
#include <string>

using namespace cv;
using namespace std;

/**
 * Maps a grayscale value (0-255) to an ASCII character
 *
 * This function determines which ASCII character represents a given brightness
 * level. Since we assume white characters on black background:
 * - Dark pixels (low values) should map to sparse/light characters
 * - Bright pixels (high values) should map to dense/heavy characters
 *
 * @param grayscale_value: Pixel brightness (0 = black, 255 = white)
 * @return: ASCII character representing that brightness
 */
char mapGrayscaleToAscii(int grayscale_value) {
  // Simple character set from light to dark (for white text on black
  // background) Ordered by visual density: space is lightest, @ is darkest
  const string ascii_chars = " .:-=+*#%@";

  // Map 0-255 grayscale value to character index
  int index = (grayscale_value * (ascii_chars.size() - 1)) / 255;

  return ascii_chars[index];
}

/**
 * Converts an image to ASCII art
 *
 * @param image: Input image (will be converted to grayscale)
 * @param output_width: Desired width of ASCII output (height scaled
 * proportionally)
 * @return: String containing the ASCII art
 */
string imageToAscii(Mat &image, int output_width) {
  // Convert to grayscale if not already
  Mat grayscale;
  if (image.channels() == 3) {
    cvtColor(image, grayscale, COLOR_BGR2GRAY);
  } else {
    grayscale = image.clone();
  }

  // Calculate scaling factor to resize image
  float aspect_ratio = (float)grayscale.rows / (float)grayscale.cols;
  int output_height = (int)(output_width * aspect_ratio *
                            0.5); // 0.5 because chars are taller than wide

  // Resize image to match desired ASCII dimensions
  Mat resized;
  resize(grayscale, resized, Size(output_width, output_height), 0, 0,
         INTER_AREA);

  // Convert each pixel to ASCII
  string ascii_art;
  for (int y = 0; y < resized.rows; y++) {
    for (int x = 0; x < resized.cols; x++) {
      int pixel_value = (int)resized.at<uchar>(y, x);
      ascii_art += mapGrayscaleToAscii(pixel_value);
    }
    ascii_art += '\n';
  }

  return ascii_art;
}

int main(int argc, char **argv) {
  // Check command line arguments
  if (argc < 2) {
    cout << "Usage: " << argv[0] << " <image_path> [output_width]" << endl;
    cout << "  image_path: Path to the input image" << endl;
    cout << "  output_width: Width of ASCII output in characters (default: 100)"
         << endl;
    return -1;
  }

  string image_path = argv[1];
  int output_width = 100; // Default width

  if (argc >= 3) {
    output_width = stoi(argv[2]);
  }

  // Load the image
  Mat image = imread(image_path, IMREAD_COLOR);

  if (image.empty()) {
    cerr << "Error: Could not open or find the image: " << image_path << endl;
    return -1;
  }

  // Convert to ASCII
  string ascii_art = imageToAscii(image, output_width);

  // Output the result
  cout << ascii_art << endl;

  return 0;
}