How to save a .gif file with a new color table by using GDI+

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SUMMARY

For a Microsoft Visual C# .NET version of this article, see 319061.
For a Microsoft Visual Basic .NET version of this article, see 319591.
The CompuServe Graphics Interchange Format (GIF) is designed with a maximum of 256 colors that are arranged in a color table. To make common modifications to a .gif image file, you must change a custom color table. However, when GDI+ edits an Image object and is then asked to save the image with the GIF encoder, the resulting .gif file contains a halftone color table.

To save an Image with a custom color table by using the GIF encoder, you must work with a 256-color copy of the Image that GDI+ has not modified.

MORE INFORMATION

The .gif image file can express a maximum of 256 colors. Because color is a scarce resource in the .gif file, optimizing those colors is a commonly requested task. To affect an optimized color table, you must be able to set any arbitrary custom color table in a .gif file.

After GDI+ modifies an Image and then writes an image to a file by using the GIF encoder, GDI+ writes the file by using a halftone palette to which the Image object's bits have been color reduced. GDI+ does a color conversion from 32 bits-per-pixel (32 BPP) when it writes the image to the file because all modifications to the image are made with the GDI+ 32-BPP graphics engine.

Although GDI+ supports the creation of Images and Bitmaps of various pixel formats and can therefore load a .gif image, the use of the 32-BPP graphics engine necessitates the conversion to 32 BPP when they are modified by GDI+. However, an Image or Bitmap that is not modified by GDI+ retains its original pixel format and can be written to a file by using the Save method with the appropriate encoder. This property forms the basis for a technique that can save an Image to a .gif file with a custom color table.

You can write an unmodified Bitmap with the GIF encoder and keep the Bitmap color table intact; therefore, you can use this method to save a .gif file with a new color table.

The method is to copy the image data from an original Image object to a temporary Bitmap object. This temporary Bitmap is created as an 8-BPP indexed Bitmap, which is the pixel format that is used to save a .gif file. The Bitmap color table is set by using the SetPalette method, and then the image definition is copied to the temporary Bitmap. After you create the temporary Bitmap with a duplicate definition, you can use the Save method to save it with the GIF encoder, which preserves the 8-BPP color table.

To write a .gif image to a file with a custom color table, follow these steps:
  1. Create a duplicate Bitmap object that is the same size as the source Image.
  2. Set the custom color table of the Bitmap object to the desired color table.
  3. Use the LockBits method to gain write access to the image bits of the copy.
  4. Create an image definition in the copy by writing color indexes to the memory that is obtained from LockBits that duplicate the pixels in the original Image.
  5. Use UnLockBits to release the image bits.
  6. Use the Bitmap copy with the custom color table to save the Image to a file by using Save and the GIF encoder.
  7. Release the Bitmap copy of the Image.

Using the Sample Code

The sample code in this article demonstrates how to use Bitmap.Save to write a .gif file with a custom color table of arbitrary size. The code is not optimized for performance because its purpose is for demonstration only. The best opportunities for optimization are in the pixel processing loops. GetPixel is a convenient abstraction of the pixel format, but it is remarkably slow. The sample code would be much faster if you used LockBits to access the pixel format directly. To increase the speed, do not use the GetPixel method and the Color class abstraction. To improve performance, rewrite the grayscale conversion by using integer math, rather than floating point.

The sample function takes the following five parameters:
  • Any GDI+ Image object.
  • The file name for the target file.
  • The class identifier (CLSID) of the GIF encoder.
  • The number of colors for the .gif file.
  • A flag that indicates whether a transparent color is needed.
For more information about the GDI+ GetEncoderClsid function and to download the sample code, see the MSDN Help documentation at the following link:

Platform SDK: GDI+ - Image::Save(filename, clsidEncoder, encoderParams)
http://msdn.microsoft.com/en-us/library/ms533843.aspx
The function first creates a Bitmap object that has the pixel format of PixelFormat8BPPIndexed because that is the object that is saved to create the .gif file with nColors, and then a color palette with the custom colors is created. The .gif file obtains the size and specific entries for its color table from the Bitmap object's ColorPalette. The sample code creates a gray scale for demonstration purposes because that algorithm is easy to extend over various color table sizes.

To create the .gif file, you must initialize the 8-BPP Bitmap object with the image definition that is to be written to the file. In the sample code, a central set of loops is used to color convert the incoming image to essentially the black and white TV color space.

For demonstration purposes, the source image pixels are accessed by means of the GetPixel method of a Bitmap object that is a copy of the source image. A Bitmap copy is made because the Image class does not implement the GetPixel method.

You can use other techniques to access the pixels, such as direct access to the pixels by using the LockBits method or interop with Windows GDI DIB Sections. When you use the BitBlt function to copy a bitmap from a Gdi+ HDC to a GDI DIB Section memory domain controller, the GBitBlt functions uses the color matching abilities of GDI.

After you create the Bitmap copy, use the Save method with the provided GIF CLSID to write the bitmap to the target file.

GIF Files with Fewer Than 256 Colors

The GIF codec in GDI+ version 1.0 encodes only GDI+ Images that are 8 BPP. All other Image formats are converted before encoding. This code uses the 8-BPP Bitmap format to write .gif files that have fewer than 256 colors because the GIF codec recognizes 8-BPP Bitmap objects that contain fewer than 256 colors by the ColorPalette.Count property.

For more information about GIF codec pixel formats, see the "References" section of this article.

The code in the processing loop that copies the image's pixel definitions to the 8-BPP Bitmap takes into account the size of the palette when the code computes a pixel's index value. The GIF codec limits the size of the palette and restricts the image definition to index values that are compatible with the palette size (that is, the potential GIF color table), and can therefore create .gif files with fewer than 256 colors.

GIF Transparency

In the sample code, the ColorPalette creation routine sets the first entry to be the GIF transparent color to demonstrate the use of the transparency feature. The code does this by setting the Alpha component of the Color entry to ZERO. The sample code in this article is for demonstration purposes only, therefore, the transparency color is an arbitrary choice and may have unexpected results that depend entirely on the source Image.

The GIF encoder identifies the first color in the ColorPalette that has an Alpha value of ZERO as the transparent color. This means that the transparent color does not have to be the first entry in the ColorPalette. It can be any one of the possible 256 colors in the palette, on the condition that all preceeding entries contain Alpha components with non-zero values. Any later entries with Alpha component values of ZERO are ignored. All entries that have non-zero Alpha components are considered opaque.

NOTE: when you use this code to overwrite an existing file, you might see a problem with the size of the resulting file. This occurs because of a bug in GDI+ version 1.0 that does not truncate the file.

For more information about Image file sizes, see the "References" section of this article. Although the article referenced there discusses the use of the Bitmap class in the System.Drawing namespace of the .NET Framework, the topic applies to GDI+ because System.Drawing is implemented by GDI+.

The GIF/LZW Licensing Issue

Microsoft has obtained a license from Unisys to use the .gif file format and other LZW technologies that are covered by the Unisys-owned U.S. and foreign patents in a number of Microsoft products. However, this license does not extend to third-party developers who use Microsoft development products or toolkits to develop applications. As a third-party developer, you need to determine whether you must obtain a license from Unisys to use the GIF format or the LZW technologies.

For additional information about LZW licenses and GIF, click the article number below to view the article in the Microsoft Knowledge Base:
193543 INFO: Unisys GIF and LZW Technology License Information

Sample Code

Status SaveGIFWithNewColorTable(
  Image *pImage,
  const WCHAR* filename,
  const CLSID* clsidEncoder,
  DWORD nColors,
  BOOL fTransparent
)
{
    Status stat = GenericError;

    // GIF codec supports 256 colors maximum.
    if (nColors > 256)
        nColors = 256;
    if (nColors < 2)
        nColors = 2;

    // Make a new 8-BPP pixel indexed bitmap that is the same size as the source image.
    DWORD   dwWidth = pImage->GetWidth();
    DWORD   dwHeight = pImage->GetHeight();

    // Always use PixelFormat8BPPIndexed, because that is the color table
    // based interface to the GIF codec.
    Bitmap  bitmap(dwWidth, dwHeight, PixelFormat8BppIndexed); 

    stat = bitmap.GetLastStatus();

    if (stat != Ok)
        return stat;        // No point in continuing.

    // Allocate the new color table.
    DWORD   dwSizeColorPalette;
    dwSizeColorPalette = sizeof(ColorPalette);      // Size of core structure.
    dwSizeColorPalette += sizeof(ARGB)*(nColors-1);   // The other entries.

    // Allocate some raw space to back the ColorPalette structure pointer.
    ColorPalette *ppal = (ColorPalette *)new BYTE[dwSizeColorPalette];
    if (ppal == NULL) return OutOfMemory;

    ZeroMemory(ppal, dwSizeColorPalette);

    // Initialize a new color table with entries that are determined by
    // some optimal palette finding algorithm; for demonstration 
    // purposes, just do a grayscale. 
    if (fTransparent)
        ppal->Flags = PaletteFlagsHasAlpha;
    else
        ppal->Flags = 0; 
    ppal->Flags |= PaletteFlagsGrayScale;
    ppal->Count = nColors;
    for (UINT i = 0; i < nColors; i++)
    {
        int Alpha = 0xFF;       // Colors are opaque by default.
        int Intensity = i*0xFF/(nColors-1); // even distribution 

        // The GIF encoder makes the first entry in the palette with a
        // zero alpha the "transparent" color in the GIF.
        // For demonstration purposes, the first one is picked arbitrarily.

        if ( i == 0 && fTransparent) // Make this color index...
            Alpha = 0;          // Transparent
        
        // Create a gray scale for demonstration purposes.
        // Otherwise, use your favorite color reduction algorithm
        // and an optimum palette for that algorithm generated here.
        // For example, a color histogram, or a median cut palette.
        ppal->Entries[i] = Color::MakeARGB( Alpha, 
                                            Intensity, 
                                            Intensity, 
                                            Intensity );
    }

    // Set the palette into the new Bitmap object.
    bitmap.SetPalette(ppal);


    // Use GetPixel below to pull out the color data of Image.
    // Because GetPixel isn't defined on an Image, make a copy in a Bitmap 
    // instead. Make a new Bitmap that is the same size of the image that
    // you want to export. Otherwise, you might try to interpret the native 
    // pixel format of the image by using a LockBits call.
    // Use PixelFormat32BppARGB so that you can wrap a Graphics around it.
    Bitmap BmpCopy(dwWidth, dwHeight, PixelFormat32BppARGB); 
    stat = BmpCopy.GetLastStatus();
    if (stat == Ok)
    {
        Graphics g(&BmpCopy);

        // Transfer the Image to the Bitmap.
        stat = g.DrawImage(pImage, 0, 0, dwWidth, dwHeight);

        // g goes out of scope here and cleans up.
    }

    if (stat != Ok)
    {
        delete [] (LPBYTE) ppal;
        ppal = NULL;
        return stat;
    }

    // Lock the whole of the bitmap for writing.
    BitmapData  bitmapData;
    Rect        rect(0, 0, dwWidth, dwHeight);

    stat = bitmap.LockBits(
      &rect,
      ImageLockModeWrite,
      PixelFormat8BppIndexed,
      &bitmapData);

    if (stat == Ok)
    {
        // Write to the temporary buffer provided by LockBits.
        // Copy the pixels from the source image in this loop.
        // Because you want an index, convert RGB to the appropriate
        // palette index here.
        BYTE *pixels;
        if (bitmapData.Stride > 0)
            pixels = (BYTE*) bitmapData.Scan0;
        else
            // If the Stride is negative, Scan0 points to the last             // scanline in the buffer.
            // To normalize the loop, obtain the start of the buffer,
            // which is located (Height-1) scanlines previous.
            pixels = (BYTE*) bitmapData.Scan0 + bitmapData.Stride*(dwHeight-1);
        UINT stride = abs(bitmapData.Stride);

        // Top-down and bottom-up is not relevant to this algorithm.

        for(UINT row = 0; row < dwHeight; ++row)
        {
          for(UINT col = 0; col < dwWidth; ++col)
          {
              // Map palette indexes for a grayscale.
              // If you use some other technique to color convert,
              // put your favorite color reduction algorithm here.
              Color     pixel;
              UINT      i8BppPixel = row * stride + col;

              BmpCopy.GetPixel(col, row, &pixel);

              // Use luminance/chrominance conversion to get grayscale.
              // Basically, turn the image into black and white TV: YCrCb.
              // You do not have to to calculate Cr or Cb because you 
              // throw away the color anyway.
              // Y = Red * 0.299 + Green * 0.587 + Blue * 0.114

              // This expression is best as integer math for performance,
              // however, because GetPixel listed earlier is the slowest 
              // part of this loop, the expression is left as 
              // floating point for clarity.
              double luminance = (pixel.GetRed() * 0.299) +
                                (pixel.GetGreen() * 0.587) +
                                (pixel.GetBlue() * 0.114);

              // Gray scale is an intensity map from black to white.
              // Compute the index to the gray scale entry that  
              // approximates the luminance, and then round the index.      
              // Also, constrain the index choices by the number of colors to do
              pixels[i8BppPixel] = (BYTE)(luminance * (nColors-1)/255 +0.5);
          }
        }
    // To commit the changes, unlock the portion of the bitmap.  
        stat = bitmap.UnlockBits(&bitmapData);
    }

    // If destination work was successful, see whether the source was successful.
    if (stat == Ok) stat = BmpCopy.GetLastStatus();

    // See whether the code has been successful to this point.
    if (stat == Ok)
    {
    // Write it out to the disk.
        stat =  bitmap.Save(filename, clsidEncoder, NULL);
    }

    // Clean up after yourself.
    delete [] (LPBYTE) ppal;
    ppal = NULL;
    // BmpCopy goes away on its own when it falls out of scope.

    return stat;
}
				

About Sample Code

Microsoft provides programming examples for illustration only, without warranty either expressed or implied, including, but not limited to, the implied warranties of merchantability and/or fitness for a particular purpose. This article assumes that you are familiar with the programming language being demonstrated and the tools used to create and debug procedures. Microsoft support professionals can help explain the functionality of a particular procedure, but they will not modify these examples to provide added functionality or construct procedures to meet your specific needs.
If you have limited programming experience, you may want to contact a Microsoft Certified Partner or Microsoft Advisory Services. For more information, visit these Microsoft Web sites:

Microsoft Certified Partners - https://partner.microsoft.com/global/30000104

Microsoft Advisory Services - http://support.microsoft.com/gp/advisoryservice

For more information about the support options that are available and about how to contact Microsoft, visit the following Microsoft Web site:http://support.microsoft.com/default.aspx?scid=fh;EN-US;CNTACTMS

REFERENCES

For additional information about GIF codec pixel formats, click the article number below to view the article in the Microsoft Knowledge Base:
318343 INFO: GDI+ GIF Files Are Saved Using the 8-BPP Format
For additional information about Image file sizes, click the article number below to view the article in the Microsoft Knowledge Base:
312119 PRB: Save Method of Bitmap Class Does Not Truncate File Size
Although the article that is referenced in this link discusses the use of the Bitmap class in the System.Drawing namespace of the .NET Framework, the topic applies to GDI+ because System.Drawing is implemented by GDI+.

Glossary

BPP
bits per pixel - the number of bits used to represent the color value of each pixel in a digitized image; describes the physical layout of each pixel's color definition in an image. Common and generically referenced pixel formats include 32 BPP, 24 BPP, 16 BPP, 8 BPP, 4 BPP, 1 BPP.
8 BPP
The image pixel format that is expressed as eight bits contained in one byte. The byte value is used as an index into a color table that contains the actual red-green-blue (RGB) color definitions. Because the index is one byte in size, the color table is limited to 256 colors.
GIF
Graphics Interchange Format - a streamable image file format that was created by CompuServe.
RGB
Red, green and blue - each commonly expressed as a byte, and resulting in a color 3-byte triplet.

Properties

Article ID: 315780 - Last Review: February 12, 2007 - Revision: 3.5
APPLIES TO
  • Microsoft GDI+ 1.0
  • Microsoft Windows XP Professional
  • the operating system: Microsoft Windows XP 64-Bit Edition
Keywords: 
kbdswgdi2003swept kbbitmap kbgdiplus kbhowto KB315780

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