A paletteis a collection that contains the colors that can be displayed on an output device. Palettes are used by devices that can display only a subset of their potential colors at any specified time.

Windows CE has no standard color palette and creates a default palette each time you create a device context. Windows CE bases this palette on the device capabilities. Most devices have 256 colors. Display devices, for example, often use the 16 standard video graphics adapter (VGA) colors and 4 other Windows CE–defined colors. Printer devices might use other default colors. If you specify a pen or text color not in the default palette, Windows CE approximates the display color with the closest color in the palette. When displaying bitmaps, Windows CE assigns colors to a bitmap based on the bitmap's associated color table. If an image has no color table, Windows CE uses the color palette in the currently selected device context.

You cannot change the entries in the default palette. However, you can create your own logical palette and select the palette into a device context in place of the default palette. You can use logical palettes to define and use colors that meet your specific needs. Windows CE enables you to create multiple logical palettes. You can attach each logical palette to a unique device context or you can switch between multiple logical palettes in a single device context.

Windows CE supports both palettized and non-palettized color display devices. Palettized devices have a color palette coded directly into their display card. Non-palettized devices use pixel bit values in the video memory to directly define colors in terms of their RGB values. You can use the GetDeviceCapsfunction to determine if a device supports color palettes.

To create a logical color palette

1. Assign values to the members of a LOGPALETTEstructure and pass a pointer to the structure to the CreatePalettefunction.

   The function returns a handle to a logical palette with the values specified in the LOGPALETEstructure.

2. Call the SelectPalettefunction to select the palette into the current device context.
3. Call the RealizePalettefunction to make the system palette the same as the palette in the current device context.

   Your logical palette should have just enough entries to represent the colors you need. You can call the GetDeviceCapsfunction with the SIZEPALETTEindex to retrieve the maximum palette size associated with a device.

   When working with palettes, you can change the colors in an existing logical palette as well as retrieve color values. The following table shows how to modify the color palette.

   To	Call
   Change the colors in an existing logical palette.	SetPaletteEntries
   Update the display after changing or creating a palette.	RealizePalette
   Retrieve the color values for a logical palette.	GetPaletteEntries
   Retrieve the value in a specified logical palette that most closely matches a specified color value.	GetNearestPaletteIndex
   Delete a logical palette. Be sure that the logical palette is not selected into a device context when you delete it.	DeleteObject

   Note   The GetSystemPaletteEntriesand RealizePalettefunctions will fail if the device associated with the selected device index does not have a palette that you can set. Call GetDeviceCapsto determine if the device has a palette that you can set.

   Windows CE does not arbitrate between the palettes of the background and foreground applications. The application running in the foreground controls the system palette. Applications that use colors other than standard Windows colors might not display properly when they run in the background. Windows CE does not perform any color matching operations between the foreground and background applications; therefore, background applications cannot successfully call RealizePalette.

   The following code example shows how to create color palettes.

   Note   To make the following code example easier to read, error checking is not included. This code example should not be used in a release configuration unless it has been modified to include secure error handling.

   HPALETTE CreateScalePalette (HDC hDC, int iColor)
   { HPALETTE hPalette = NULL; // Handle of the palette to be created
   LPLOGPALETTE lpMem = NULL; // Buffer for the LOGPALETTE structure
   // which defines the palette int index, // An integer iReserved, //
   Number of reserved entries in the // system palette iRasterCaps; //
   Raster capabilities of the display // device context // Retrieve
   the raster capabilities of the display device context. // Check if
   it is capable of specifying a palette-based device, then //
   determine the number of entries in the logical color palette.
   iRasterCaps = GetDeviceCaps (hDC, RASTERCAPS); iRasterCaps =
   (iRasterCaps & RC_PALETTE) ? TRUE : FALSE; if (iRasterCaps) {
   iReserved = GetDeviceCaps (hDC, NUMRESERVED); iPalSize =
   GetDeviceCaps (hDC, SIZEPALETTE) - iReserved; } else iPalSize =
   GetDeviceCaps (hDC, NUMCOLORS); // If there cannot be any entries
   in the logical color palette, exit. if (iPalSize <= 0) {
   MessageBox (g_hwndMain, TEXT("Palette can't be created, there can't
   be any") TEXT("entries in it."), TEXT("Info"), MB_OK); goto exit; }
   // Allocate a buffer for the LOGPALETTE structure. if (!(lpMem =
   (LOGPALETTE *) LocalAlloc (LMEM_FIXED, sizeof(LOGPALETTE) +
   sizeof(PALETTEENTRY) * iPalSize))) goto exit;
   lpMem->palNumEntries = (WORD) iPalSize; lpMem->palVersion =
   (WORD) 0x0300; switch(iColor) { case 0: // Red color component only
   for (index = 0; index < iPalSize; index++) {
   lpMem->palPalEntry[index].peRed = (BYTE) index;
   lpMem->palPalEntry[index].peGreen = 0;
   lpMem->palPalEntry[index].peBlue = 0;
   lpMem->palPalEntry[index].peFlags = 0; } break; case 1: // Green
   color component only for (index = 0; index < iPalSize; index++)
   { lpMem->palPalEntry[index].peRed = 0;
   lpMem->palPalEntry[index].peGreen = (BYTE) index;
   lpMem->palPalEntry[index].peBlue = 0;
   lpMem->palPalEntry[index].peFlags = 0; } break; case 2: // Blue
   color component only for (index = 0; index < iPalSize; index++)
   { lpMem->palPalEntry[index].peRed = 0;
   lpMem->palPalEntry[index].peGreen = 0;
   lpMem->palPalEntry[index].peBlue = (BYTE) index;
   lpMem->palPalEntry[index].peFlags = 0; } break; case 3: //
   Grayscale palette default: for (index = 0; index < iPalSize;
   index++) { lpMem->palPalEntry[index].peRed = (BYTE) index;
   lpMem->palPalEntry[index].peGreen = (BYTE) index;
   lpMem->palPalEntry[index].peBlue = (BYTE) index;
   lpMem->palPalEntry[index].peFlags = 0; } break; } // Create the
   palette. hPalette = CreatePalette (lpMem); // Free the memory
   object lpMem. LocalFree ((HLOCAL) lpMem); exit: return hPalette;
   }

   
   

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    Last updated on Tuesday, May 18, 2004

   © 2004 Microsoft Corporation. All rights reserved.