How to convert a TTF character to an 8bpp BMP file

Introduction

In this article, I'll show you how to convert a TTF character to a bitmap file with the option to define the color depth of the bitmap picture. You can use this code to create icons for your application by extracting the appropriate characters from a Windows font file. For example, you can see that the Wingdings MS Word font is full of characters that can be used like cool icons.

Step 1

The first step is to create a BMP file. In the attached code, you can see the way to open a font file, create a new bitmap image, and to draw the given character. This part of the code is very simple, all that we need is supported by the .NET framework. May be, the only thing to remember is that the Graphics class can not be used with an indexed color, so create a Bitmap object with default definitions.

/// <summary>
/// Converts TTF character to bitmap
/// </summary>
/// <param name="fontFileName">
/// Full name of font file
/// </param>
/// <param name="charCode">Charcter code</param>
/// <param name="encoding">ASCII, UNICODE ...</param>
/// <param name="color">Color to draw given character</param>
/// <returns></returns>

public static Bitmap Convert(
    string fontFileName, 
    byte[] charCode, 
    Encoding encoding,
    Color color
    )
{

    // Create font 
    PrivateFontCollection fonts = new  
        PrivateFontCollection();
    fonts.AddFontFile( fontFileName );

    // Get charTodraw by given unicode
    char[] charToDraw = encoding.GetChars( charCode ); 

    // Get font family 
    FontFamily family = 
        (FontFamily)fonts.Families.GetValue( 0 );

    // Create bitmap 
    Bitmap bitmap = new Bitmap( 32, 32 );

    // Create graphics from image 
    Graphics g = Graphics.FromImage( bitmap );

    // Draw string 
    g.DrawString( 
        new string( charToDraw ), 
        new Font( family, 12, FontStyle.Regular ),
        new SolidBrush( color ),
        6, 6
        );

    // Set background to be transparent
    Color transColor = bitmap.GetPixel( 0, 0 );
    bitmap.MakeTransparent( transColor );
    return bitmap;  

}

Step 2

Now, we need to convert a rich color bitmap image to a bitmap with an 8bpp format. For some reason, the method Bitmap.Save doesn't really work for me. I mean, it's possible to save a bitmap, but it's impossible to change the color depth by passing EncoderParameter. So changing the color depth is a little bit tricky. Since it's impossible to work with Graphics because of the indexed color, we need to iterate all pixels in the source (rich color) bitmap image, get the pixel's color, and find a similar color from the Color palette (256 Colors) and save its index in the destination bitmap image (8bpp). See the code below:

/// <summary>
/// Converts bitmap format to 8bpp
/// </summary>
/// <param name="image">Source bitmap image</param>
/// <returns></returns>

public static Bitmap ConvertTo8bppFormat(Bitmap image)
{

    // Create new image with 8BPP format
    Bitmap destImage = new Bitmap(
        image.Width,
        image.Height,
        PixelFormat.Format8bppIndexed
        );

    // Lock bitmap in memory
    BitmapData bitmapData = destImage.LockBits( 
        new Rectangle( 0, 0, image.Width, image.Height ), 
        ImageLockMode.ReadWrite, 
        destImage.PixelFormat 
        ); 

    for( int i = 0; i < image.Width; i++ )
    {
        for( int j = 0; j < image.Height; j++ )
        {
            // Get source color
            Color color = image.GetPixel( i, j );

            // Get index of similar color 
            byte index = GetSimilarColor( destImage.Palette, color );

            WriteBitmapData( i, j, index, bitmapData, 8 );  
        }
    }

    destImage.UnlockBits( bitmapData );

    return destImage;
}

/// <summary>
/// Returns Similar color 
/// </summary>
/// <param name="palette"></param>
/// <param name="color"></param>
/// <returns></returns>

private static byte GetSimilarColor(ColorPalette palette, Color color)
{

    byte minDiff = byte.MaxValue;
    byte index = 0;

    for( int i = 0; i < palette.Entries.Length-1; i++ )
    {

        byte currentDiff = GetMaxDiff( color, palette.Entries[i] );

        if( currentDiff < minDiff )
        {
            minDiff = currentDiff;
            index = (byte)i;
        }
    }

    return index;
}

/// <summary>
/// Return similar color
/// </summary>
/// <param name="a"></param>
/// <param name="b"></param>
/// <returns></returns>

private static byte GetMaxDiff(Color a, Color b)
{

    byte bDiff = System.Convert.ToByte( 
        Math.Abs( (short)( a.B - b.B ) ) );

    byte gDiff = System.Convert.ToByte(
        Math.Abs( ( short )( a.G - b.G ) ) );

    byte rDiff = System.Convert.ToByte(
        Math.Abs( ( short )( a.R - b.R ) ) );

    return Math.Max( rDiff, Math.Max( bDiff, gDiff ) );  
}

/// <summary>
/// Writing to bitmap
/// </summary>
/// <param name="i"></param>
/// <param name="j"></param>
/// <param name="index"></param>
/// <param name="data"></param>
/// <param name="pixelSize"></param>

private static void WriteBitmapData(
    int i,
    int j,
    byte index,
    BitmapData data,
    int pixelSize)
{

    double entry = pixelSize/8;

    // Get unmanaged address of needed byte
    IntPtr realByteAddr = new IntPtr( System.Convert.ToInt32( 
                          data.Scan0.ToInt32()  + 
                          ( j*data.Stride ) + i*entry ) );

    // Create array with data to copy
    byte[] dataToCopy = new byte[] { index };

    // Perfrom copy
    Marshal.Copy( dataToCopy, 0, realByteAddr, 
                  dataToCopy.Length );
}

Some more explanations ...

Some more explanations about the WriteBitmapData method: the BitmapData member Scan0 points to the first byte (physical memory that contains the pixel's information) and Stride contains the row size. In our case (8 bit per pixel), finding the appropriate information for a given pixel is very easy (row index * row length + column index). So first, we create a pointer to the required byte, and then by using the Marshal class, we can copy the color's index to the bitmap.

Enjoy - :)