Graphics.GetHalftonePalette Method

The purpose of the GetHalftonePalette method is to enable GDI+ to produce a better quality halftone when the display uses 8 bits per pixel. To display an image using the halftone palette, use the following procedure.

The following code example is designed for use with Windows Forms, and it requires PaintEventArgs e, which is a parameter of the Paint event handler. The code performs the following actions:

• Defines interoperability DllImportAttribute attributes for the Windows DLL file gdi32.dll, which contains the necessary GDI functions.

• Defines the SelectPalette and RealizePalette functions in that DLL as external.

• Creates an image from an existing image file SampImag.jpg (which must be in the same folder as the example code file) and draws the image to the screen.

• Creates internal pointer type variables and sets their values to the handle to the graphics object and to the current Windows halftone palette, respectively.

• Selects and realizes the halftone palette.

• Creates a new graphics object using the hdc parameter.

• Draws the image again.

• Releases the handle to the device context.

The result is two renderings of the sample image: one with the 16-bit palette and one with the 8-bit palette.

[System.Runtime.InteropServices.DllImportAttribute("gdi32.dll")]
private static extern IntPtr SelectPalette(
    IntPtr hdc,
    IntPtr htPalette,
    bool bForceBackground);
[System.Runtime.InteropServices.DllImportAttribute("gdi32.dll")]
private static extern int RealizePalette(IntPtr hdc);

[System.Security.Permissions.SecurityPermission(
System.Security.Permissions.SecurityAction.LinkDemand, Flags = 
System.Security.Permissions.SecurityPermissionFlag.UnmanagedCode)]            
private void GetHalftonePaletteVoid(PaintEventArgs e)
{

    // Create and draw image.
    Image imageFile = Image.FromFile("SampImag.jpg");
    e.Graphics.DrawImage(imageFile, new Point(0, 0));

    // Get handle to device context.
    IntPtr hdc = e.Graphics.GetHdc();

    // Get handle to halftone palette.
    IntPtr htPalette = Graphics.GetHalftonePalette();

    // Select and realize new palette.
    SelectPalette(hdc, htPalette, true);
    RealizePalette(hdc);

    // Create new graphics object.
    Graphics newGraphics = Graphics.FromHdc(hdc);

    // Draw image with new palette.
    newGraphics.DrawImage(imageFile, 300, 0);

    // Release handle to device context.
    e.Graphics.ReleaseHdc(hdc);
}

.NET Framework

.NET Framework Client Profile

Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Reference