/// <summary>
/// Load a bitmap and create ASCII art from it using linear optimization
/// </summary>
/// <param name="blockSize"></param>
private void ASCIILinearOptimization(int blockSize)
{
    char[] tiles = new char[] { '■', '@', '#', '?', '!', 'x', '-', ',', '.', ' ' };
    double[] tilesWeight = new double[] { 1, 0.9, 0.8, 0.7, 0.6, 0.5, 0.3, 0.2, 0.1, 0.0 };
    double[,] image = GetBlockBrightness("./Input/Lena.jpg", blockSize);

    CpModel model = new CpModel();

    // variables
    Dictionary<string, IntVar> variableSet = new Dictionary<string, IntVar>();

    // cost function
    LinearExprBuilder cost = LinearExpr.NewBuilder();
    // for each block
    for (int i = 0; i < image.GetLength(0); i++)
    {
        for (int j = 0; j < image.GetLength(1); j++)
        {
            // create variables for each combination block-character
            List<IntVar> forOneSquare = new List<IntVar>();
            for (int c = 0; c < tiles.Length; c++)
            {
                string varName = "x_" + i + "_" + j + "_" + c;
                IntVar xijc = model.NewIntVar(0, 1, varName);
                variableSet.Add(varName, xijc);
                forOneSquare.Add(xijc);

                // weight has to be integer !!
                long weight = (long)(100 * Math.Abs(image[i, j] - tilesWeight[c]));
                // add to cost function
                cost.AddTerm(xijc, weight);
            }

            // for each square we can pick only one tile
            model.Add(LinearExpr.Sum(forOneSquare) == 1); // = one constraint per block
        }
    }

    // minimize cost function with given constraints
    model.Minimize(cost);
    CpSolver solver = new CpSolver();
    CpSolverStatus status = solver.Solve(model);

    // if successfull solving print result
    if (status == CpSolverStatus.Optimal || status == CpSolverStatus.Feasible)
    {
        // check each variable
        for (int i = 0; i < image.GetLength(0); i++)
        {
            for (int j = 0; j < image.GetLength(1); j++)
            {
                for (int c = 0; c < tiles.Length; c++)
                {
                    string varName = "x_" + i + "_" + j + "_" + c;
                    // if variable is true, print corresponding character
                    if (solver.Value(variableSet[varName]) == 1)
                        Console.Write(tiles[c]);
                }
            }
            Console.WriteLine();
        }
    }
    else
    {
        Console.WriteLine("No solution found.");
    }
}