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 System.Drawing
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
has not modified.Understanding .gif Files That Are Written By System.Drawing and GDI+
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 frequently requested task. To affect an optimized color table, you must be able to set any arbitrary custom color table in a .gif file.
namespace is primarily a wrapper around GDI+, therefore this article refers to the namespace as GDI+ unless behavior that is specific to the System.Drawing
namespace is discussed. In this case, the term System.Drawing
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
bits of the object 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 GDI+ 32-BPP graphics engine.
Although GDI+ supports the creation of Images
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
that is not
modified by GDI+ retains its original pixel format and can be written to a file 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.back to the top Writing 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
. This 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:
back to the top Using the Sample Code
- Create a duplicate Bitmap object that is the same size as the source Image.
- Set the custom color table of the Bitmap object to the color table that you want.
- Use the LockBits method to gain write access to the image bits of the copy.
- 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.
- Use UnLockBits to release the image bits.
- Use the Bitmap copy with the custom color table to save the Image to a file by using Save and the GIF encoder.
- Release the Bitmap copy of the Image.
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 sample function takes the following four parameters:
- Any GDI+ Image object.
- The file name for the destination file.
- The number of colors for the .gif file.
- A flag that indicates whether a transparent color is needed.
The function first creates a Bitmap
object that has the pixel format of PixelFormat.Format8BPPIndexed
because that is the object that is saved to create the .gif file with nColors
. Next, 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
. 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()
You can use other techniques to access the pixels, such as direct access to the pixels by using the LockBits()
method or interop with native unmanaged code by using 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.
Because the Visual Basic .NET Language cannot access memory buffers directly by means of a pointer as in other languages, an intermediate step is necessary. This code creates a working Byte
buffer in which are placed the color indexes by the color conversion loops. This buffer is a managed data structure whose elements can be manipulated by Visual Basic .NET code. After the image color index defitions are placed in this buffer, interop services are used to call the RtlMoveMemory
Operating System function to transfer the contents of this Byte
buffer to the memory buffer that is referenced by the Scan0 IntPtr
of the BitmapData
class that is defined in the sample code demonstrates how to define and use an unmanaged operating system function by using .NET Framework interop services. RtlMoveMemory
is declared by this class to be the CopyArrayTo
method. This method is defined to marshal the appropriate Visual Basic .NET data types into the appropriate parameters for RtlMoveMemory
therefore takes the source Byte
array in the second parameter and copies the contents of the Byte
array to the memory that is pointed at by the value in the first parameter that is defined as an Int32
. The first parameter to CopyArrayTo
is an System.Int32
type because that is the value type that can be obtained from the IntPtr
After you create the Bitmap
copy, use the Save
method with the ImageFormat.Gif
object to write the bitmap to the destination file.back to the top 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 use 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 Palette.Count
For more information about the GIF codec, see the "References
" section of this article.
Unfortunately, the ColorPalette
class of the System.Drawing
namespace in the .NET Framework cannot be instantiated independent of a Bitmap
object. This is a restriction that only the System.Drawing.Bitmap
class imposes in the .NET Framework; however, to use the approach in this article, the Bitmap
object must have a new ColorPalette
object that contains fewer colors than the default 256 ColorPalette
To achieve this, the sample code defines a function named GetColorPalette
. This function creates a temporary Bitmap
object that has a color depth close to the requested number of colors. The function then references the Palette
property and returns it to the caller. This creates a new ColorPalette
with one of several possible color counts: 256 colors, 16 colors, or two colors (monochrome). Although you can create color tables in .gif files that are smaller than 256 colors, color tables are limited to sizes that are a power of two.
When you limit color table sizes to a power of two, you minimize wasted space. The resulting color table in this example is 8 colors (2x2x2). With the sample code, the .gif file would be created with a color table of 16 colors because that is the smallest PixelFormat
for a Bitmap
that accomodates six colors.
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.back to the top 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 completely 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.back to the top 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 must 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:
193543back to the top Sample Code
INFO: Unisys GIF and LZW Technology License Information
Imports System.DrawingImports System.Drawing.ImagingImports System.Runtime.InteropServicesPublic Class Form1 Inherits System.Windows.Forms.Form Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load Dim pic As Image = Image.FromFile("test.jpg") SaveGIFWithNewColorTable(pic, "test.gif", 16, True) End Sub Class Win32API <DllImport("KERNEL32.DLL", EntryPoint:="RtlMoveMemory", _ SetLastError:=True, CharSet:=CharSet.Auto, _ ExactSpelling:=True, _ CallingConvention:=CallingConvention.StdCall)> _ Public Shared Sub CopyArrayTo(<[In](), MarshalAs(UnmanagedType.I4)> ByVal hpvDest As Int32, <[In](), Out()> ByVal hpvSource() As Byte, ByVal cbCopy As Integer) ' Leave function empty - DLLImport attribute forwards calls to CopyArrayTo to ' RtlMoveMemory in KERNEL32.DLL. End Sub End Class Private Function GetColorPalette(ByVal nColors As Integer) As ColorPalette ' Assume monochrome image. Dim bitscolordepth As PixelFormat = PixelFormat.Format1BppIndexed Dim palette As ColorPalette 'The Palette we are stealing Dim bitmap As Bitmap 'The source of the stolen palette ' Determine number of colors. If nColors > 2 Then bitscolordepth = PixelFormat.Format4BppIndexed End If If (nColors > 16) Then bitscolordepth = PixelFormat.Format8BppIndexed End If ' Make a new Bitmap object to get its Palette. bitmap = New Bitmap(1, 1, bitscolordepth) palette = bitmap.Palette ' Grab the palette bitmap.Dispose() ' cleanup the source Bitmap Return palette ' Send the palette back End Function Private Sub SaveGIFWithNewColorTable(ByVal image As Image, ByVal filename As String, ByVal nColors As Integer, ByVal fTransparent As Boolean) ' GIF codec supports 256 colors maximum, monochrome minimum. If (nColors > 256) Then nColors = 256 End If If (nColors < 2) Then nColors = 2 End If ' Make a new 8-BPP indexed bitmap that is the same size as the source image. Dim Width As Integer = image.Width Dim Height As Integer = image.Height ' Always use PixelFormat8BppIndexed because that is the color ' table based interface to the GIF codec. Dim bitmap As Bitmap = New Bitmap(Width, Height, PixelFormat.Format8BppIndexed) ' Create a color palette big enough to hold the colors you want. Dim pal As ColorPalette = GetColorPalette(nColors) ' Initialize a new color table with entries that are determined ' by some optimal palette-finding algorithm; for demonstration ' purposes, use a grayscale. Dim i As Integer For i = 0 To nColors - 1 Dim Alpha As Integer = 255 ' Colors are opaque Dim Intensity As Double = CDbl(i) * 255 / (nColors - 1) ' even distribution ' The GIF encoder makes the first entry in the palette ' with a ZERO alpha the transparent color in the GIF. ' Pick the first one arbitrarily, for demonstration purposes. If (i = 0 And fTransparent) Then ' Make this color index... Alpha = 0 ' Transparent End If ' 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. pal.Entries(i) = Color.FromArgb(Alpha, Intensity, Intensity, Intensity) Next i ' Set the palette into the new Bitmap object. bitmap.Palette = pal ' 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. Next, make a new Bitmap that is the same ' size as the image that you want to export. Or, try to interpret ' the native pixel format of the image by using a LockBits ' call. Use PixelFormat32BppARGB so you can wrap a graphics ' around it. Dim BmpCopy As Bitmap = New Bitmap(Width, Height, PixelFormat.Format32BppArgb) Dim g As Graphics g = Graphics.FromImage(BmpCopy) g.PageUnit = GraphicsUnit.Pixel ' Transfer the Image to the Bitmap. g.DrawImage(image, 0, 0, Width, Height) ' Force g to release its resources, namely BmpCopy. g.Dispose() ' Lock a rectangular portion of the bitmap for writing. Dim bitmapData As BitmapData Dim rect As Rectangle = New Rectangle(0, 0, Width, Height) bitmapData = bitmap.LockBits(rect, ImageLockMode.WriteOnly, PixelFormat.Format8BppIndexed) ' Write to a temporary buffer, and then copy to the buffer that ' LockBits provides. Copy the pixels from the source image in this ' loop. Because you want an index, convert RGB to the appropriate ' palette index here. Dim pixels As IntPtr = bitmapData.Scan0 Dim bits As Byte() ' the working buffer ' Get the pointer to the image bits. Dim pBits As Int32 If (bitmapData.Stride > 0) Then pBits = pixels.ToInt32() Else ' If the Stide is negative, Scan0 points to the last ' scanline in the buffer. To normalize the loop, obtain ' a pointer to the front of the buffer that is located ' (Height-1) scanlines previous. pBits = pixels.ToInt32() + bitmapData.Stride * (Height - 1) End If Dim stride As Integer = Math.Abs(bitmapData.Stride) ReDim bits(Height * stride) ' Allocate the working buffer. Dim row As Integer Dim col As Integer For row = 0 To Height - 1 For col = 0 To Width - 1 ' Map palette indices for a gray scale. ' Put your favorite color reduction algorithm here. ' If you use some other technique to color convert. Dim pixel As Color ' The source pixel. ' The destination pixel. Dim i8BppPixel As Integer = row * stride + col pixel = BmpCopy.GetPixel(col, row) ' Use luminance/chrominance conversion to get grayscale. ' Basically, turn the image into black and white TV. ' Do not calculate Cr or Cb because you ' discard the color anyway. ' Y = Red * 0.299 + Green * 0.587 + Blue * 0.114 ' This expression should be integer math for performance; ' however, because GetPixel above is the slowest part of ' this loop, the expression is left as floating point ' for clarity. Dim luminance As Double = (pixel.R * 0.299) + _ (pixel.G * 0.587) + _ (pixel.B * 0.114) ' Gray scale is an intensity map from black to white. ' Compute the index to the grayscale entry that ' approximates the luminance, and then round the index. ' Also, constrain the index choices by the number of ' colors to do, and then set that pixel's index to the byte ' value. Dim colorIndex As Double = Math.Round((luminance * (nColors - 1) / 255)) bits(i8BppPixel) = CByte(colorIndex) ' /* end loop for col */ Next col ' /* end loop for row */ Next row ' Put the image bits definition into the bitmap. Win32API.CopyArrayTo(pBits, bits, Height * stride) ' To commit the changes, unlock the portion of the bitmap. bitmap.UnlockBits(bitmapData) bitmap.Save(filename, ImageFormat.Gif) ' Bitmap goes out of scope here and is also marked for ' garbage collection. ' Pal is referenced by bitmap and goes away. ' BmpCopy goes out of scope here and is marked for garbage ' collection. Force it, because it is probably quite large. ' The same applies for bitmap. BmpCopy.Dispose() bitmap.Dispose() End SubEnd Class
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;CNTACTMSback to the top Troubleshooting
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.back to the top