Create 16bit pgm/ppm images

Program.cs

using System.Text;

File.WriteAllText("_p2_16b.pgm", CreateP2_16b());
File.WriteAllText("_p3_16b_gray.ppm", CreateP3_Gray16b());
File.WriteAllText("_p3_16b_red.ppm", CreateP3_Red16b());
File.WriteAllText("_p3_16b_green_blue.ppm", CreateP3_GreenBlue16b());

File.WriteAllBytes("_p5_16b.pgm", CreateP5_16b());
File.WriteAllBytes("_p6_16b_red.pgm", CreateP6_Red16b());
File.WriteAllBytes("_p6_16b_green_blue.pgm", CreateP6_GreenBlue16b());

static string CreateP2_16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var sb = new StringBuilder($"""
            P2
            # Created by myself
            {width} {height}
            {max}

            """);

    const int newLineCount = 16;

    var values = new List<ushort>();
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x += newLineCount)
        {
            for (int n = x; n < x + newLineCount; n++)
            {
                var value = (ushort)Math.Min(0xffff, y * 256 + n);
                values.Add(value);
            }
            sb.AppendLine(string.Join(' ', values));
            values.Clear();
        }
    }

    return sb.ToString();
}

static string CreateP3_Gray16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var sb = new StringBuilder($"""
            P3
            # Created by myself : Actual red image
            {width} {height}
            {max}

            """);

    const int newLineCount = 8;

    var values = new List<ushort>();
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x += newLineCount)
        {
            for (int n = x; n < x + newLineCount; n++)
            {
                var value = (ushort)Math.Min(0xffff, y * 256 + n);
                values.Add(value);  // 3ch RGB
                values.Add(value);
                values.Add(value);
            }
            sb.AppendLine(string.Join(' ', values));
            values.Clear();
        }
    }

    return sb.ToString();
}

static string CreateP3_Red16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var sb = new StringBuilder($"""
            P3
            # Created by myself : Actual gray image
            {width} {height}
            {max}

            """);

    const int newLineCount = 8;

    var values = new List<ushort>();
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x += newLineCount)
        {
            for (int n = x; n < x + newLineCount; n++)
            {
                var value = (ushort)Math.Min(0xffff, y * 256 + n);
                values.Add(value);  // 3ch RGB
                values.Add(0);
                values.Add(0);
            }
            sb.AppendLine(string.Join(' ', values));
            values.Clear();
        }
    }

    return sb.ToString();
}

static string CreateP3_GreenBlue16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var sb = new StringBuilder($"""
            P3
            # Created by myself : Actual green and blue image
            {width} {height}
            {max}

            """);

    const int newLineCount = 8;

    var values = new List<ushort>();
    for (int y = 0; y < height; y++)
    {
        for (int x = 0; x < width; x += newLineCount)
        {
            for (int n = x; n < x + newLineCount; n++)
            {
                var value = (ushort)Math.Min(0xffff, y * 256 + n);
                values.Add(0);      // 3ch RGB
                values.Add((ushort)(0xffff - value));
                values.Add(value);
            }
            sb.AppendLine(string.Join(' ', values));
            values.Clear();
        }
    }

    return sb.ToString();
}

static unsafe byte[] CreateP5_16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var headerBinary = Encoding.ASCII.GetBytes($"""
        P5
        # Created by myself
        {width} {height}
        {max}

        """);
    var headerSize = headerBinary.Length;

    var pixelSize = width * height * sizeof(ushort);
    var bs = new byte[headerSize + pixelSize];

    for (int i = 0; i < headerSize; i++)
        bs[i] = headerBinary[i];

    fixed (byte* pixelHead = &bs[headerSize])
    {
        byte* ptr = pixelHead;
        for (int y = 0; y < height; y++)
        {
            for (int x = 0; x < width; x++)
            {
                var value = (ushort)Math.Min(0xffff, y * 256 + x);

                // BigEndian
                *(ptr++) = (byte)(value >> 8);
                *(ptr++) = (byte)(value & 0x00ff);
            }
        }
    }

    return bs;
}

static unsafe byte[] CreateP6_Red16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var headerBinary = Encoding.ASCII.GetBytes($"""
        P6
        # Created by myself
        {width} {height}
        {max}

        """);
    var headerSize = headerBinary.Length;

    var pixelSize = width * height * sizeof(ushort) * 3;
    var bs = new byte[headerSize + pixelSize];

    for (int i = 0; i < headerSize; i++)
        bs[i] = headerBinary[i];

    fixed (byte* pixelHead = &bs[headerSize])
    {
        byte* ptr = pixelHead;
        for (int y = 0; y < height; y++)
        {
            for (int x = 0; x < width; x++)
            {
                var value = (ushort)Math.Min(0xffff, y * 256 + x);

                // BigEndian
                *(ptr++) = (byte)(value >> 8);
                *(ptr++) = (byte)(value & 0x00ff);
                *(ptr++) = 0;
                *(ptr++) = 0;
                *(ptr++) = 0;
                *(ptr++) = 0;
            }
        }
    }

    return bs;
}

static unsafe byte[] CreateP6_GreenBlue16b()
{
    (int width, int height, int max) = (256, 256, 65535);

    var headerBinary = Encoding.ASCII.GetBytes($"""
        P6
        # Created by myself
        {width} {height}
        {max}

        """);
    var headerSize = headerBinary.Length;

    var pixelSize = width * height * sizeof(ushort) * 3;
    var bs = new byte[headerSize + pixelSize];

    for (int i = 0; i < headerSize; i++)
        bs[i] = headerBinary[i];

    fixed (byte* pixelHead = &bs[headerSize])
    {
        byte* ptr = pixelHead;
        for (int y = 0; y < height; y++)
        {
            for (int x = 0; x < width; x++)
            {
                var value1 = (ushort)Math.Min(0xffff, y * 256 + x);
                var value2 = (ushort)0xffff - value1;

                // BigEndian
                *(ptr++) = 0;
                *(ptr++) = 0;
                *(ptr++) = (byte)(value2 >> 8);
                *(ptr++) = (byte)(value2 & 0x00ff);
                *(ptr++) = (byte)(value1 >> 8);
                *(ptr++) = (byte)(value1 & 0x00ff);
            }
        }
    }

    return bs;
}