Retrieving Palette Information from a Bitmap Resource

• Microsoft Windows Software Development Kit (SDK) for Windows version 3.1
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
      - Microsoft Win32s versions 1.2, 1.25a, and 1.3
  

SUMMARY

You may sometime need to create a logical palette from a bitmap resource in order to display the bitmap with the maximum number of available colors. For example, on an 8 bit-per-pixel display, a logical palette is necessary to draw a 256-color bitmap on a device context for that display. The LoadBitmap function does not return or take a palette as one of its parameters; thus, for example, there is no way to incorporate a palette with a 256-color bitmap loaded with LoadBitmap. Therefore, an application must load the resource as a device-independent bitmap (DIB), rather than a device-dependent bitmap (DDB), in order to retrieve the bitmap's color information. An application can use the FindResource, LoadResource, and LockResource functions to do this.

The solution differs slightly for Win32s.

MORE INFORMATION

Windows and Windows NT

A bitmap (.BMP file) is stored in an application's resources as a (DIB), along with a color table if one exists. When a DIB is loaded from an application's resources with the LoadBitmap function, a DDB is returned. This DDB is a bitmap compatible with the screen. Routines such as CreateDIBitmap and SetDIBits that convert DIBs to DDBs take a handle to a device context as their first parameter. This tells the routine what kind of DDB to create. If this device context currently has a palette selected into it, then CreateDIBitmap or SetDIBits can use this palette to create the DDB. Without a palette, the routines are restricted to system colors when matching the DIB's colors to the DDB's colors. For example, on an 8 bit-per-pixel display, the resulting DDB can have only up to 20 different colors. With a logical palette, the resulting bitmap could have had up to 256 different colors.

If the bitmap is loaded as a DIB from the resource, then an application can query the DIB's color table and create a logical palette for the DIB. Then, it can call either CreateDIBitmap or SetDIBits, along with a device context with that palette selected, to obtain a DDB compatible with that palette. To load a bitmap from a resource as a DIB, you can use the FindResource function with the RT_BITMAP flag set and then use the LoadResource function to load it. You can lock the resource with the LockResource function.

The following code demonstrates how to use the above technique to load a DIB from an application's resources, create a palette for it, and then create a DDB out of it. The LoadResourceBitmap function below can be used in place of the LoadBitmap function. The only additional parameter needed is the address of a logical palette handle. The palette handle referenced will contain a handle to a logical palette after the function is called.

HBITMAP LoadResourceBitmap(HINSTANCE hInstance, LPSTR lpString,

                           HPALETTE FAR* lphPalette)
{
    HRSRC  hRsrc;
    HGLOBAL hGlobal;
    HBITMAP hBitmapFinal = NULL;
    LPBITMAPINFOHEADER  lpbi;
    HDC hdc;
    int iNumColors;

    if (hRsrc = FindResource(hInstance, lpString, RT_BITMAP))
       {
       hGlobal = LoadResource(hInstance, hRsrc);
       lpbi = (LPBITMAPINFOHEADER)LockResource(hGlobal);

       hdc = GetDC(NULL);
       *lphPalette =  CreateDIBPalette ((LPBITMAPINFO)lpbi, &iNumColors);
       if (*lphPalette)
          {
          SelectPalette(hdc,*lphPalette,FALSE);
          RealizePalette(hdc);
          }

       hBitmapFinal = CreateDIBitmap(hdc,
                   (LPBITMAPINFOHEADER)lpbi,
                   (LONG)CBM_INIT,
                   (LPSTR)lpbi + lpbi->biSize + iNumColors *

                   (LPBITMAPINFO)lpbi,
                   DIB_RGB_COLORS );

       ReleaseDC(NULL,hdc);
       UnlockResource(hGlobal);
       FreeResource(hGlobal);
       }
    return (hBitmapFinal);

HPALETTE CreateDIBPalette (LPBITMAPINFO lpbmi, LPINT lpiNumColors) {

   LPBITMAPINFOHEADER  lpbi;
   LPLOGPALETTE     lpPal;
   HANDLE           hLogPal;
   HPALETTE         hPal = NULL;
   int              i;

   lpbi = (LPBITMAPINFOHEADER)lpbmi;
   if (lpbi->biBitCount <= 8)
       *lpiNumColors = (1 << lpbi->biBitCount);
   else
       *lpiNumColors = 0;  // No palette needed for 24 BPP DIB

   if (lpbi->biClrUsed > 0)
       *lpiNumColors = lpbi->biClrUsed;  // Use biClrUsed

   if (*lpiNumColors)
      {
      hLogPal = GlobalAlloc (GHND, sizeof (LOGPALETTE) +
                             sizeof (PALETTEENTRY) * (*lpiNumColors));
      lpPal = (LPLOGPALETTE) GlobalLock (hLogPal);
      lpPal->palVersion    = 0x300;
      lpPal->palNumEntries = *lpiNumColors;

      for (i = 0;  i < *lpiNumColors;  i++)
         {
         lpPal->palPalEntry[i].peRed   = lpbmi->bmiColors[i].rgbRed;
         lpPal->palPalEntry[i].peGreen = lpbmi->bmiColors[i].rgbGreen;
         lpPal->palPalEntry[i].peBlue  = lpbmi->bmiColors[i].rgbBlue;
         lpPal->palPalEntry[i].peFlags = 0;
         }
      hPal = CreatePalette (lpPal);
      GlobalUnlock (hLogPal);
      GlobalFree   (hLogPal);
   }
   return hPal;

Here is an example of how you might use the above function to load a bitmap from a resource and display it using a logical palette:

   {
   HBITMAP hBitmap,hOldBitmap;
   HPALETTE hPalette;
   HDC hMemDC, hdc;
   BITMAP bm;

   hBitmap = LoadResourceBitmap(hInst,"test", &hPalette);
   GetObject(hBitmap, sizeof(BITMAP), (LPSTR)&bm);
   hdc = GetDC(hWnd);
   hMemDC = CreateCompatibleDC(hdc);
   SelectPalette(hdc,hPalette,FALSE);
   RealizePalette(hdc);
   SelectPalette(hMemDC,hPalette,FALSE);
   RealizePalette(hMemDC);
   hOldBitmap = SelectObject(hMemDC,hBitmap);
   BitBlt(hdc,0,0,bm.bmWidth,bm.bmHeight,hMemDC,0,0,SRCCOPY);
   DeleteObject(SelectObject(hMemDC,hOldBitmap));
   DeleteDC(hMemDC);
   ReleaseDC(hWnd,hdc);
   DeleteObject(hPalette);
   }

Win32s

On Win32s, LoadResource does not return a true global memory handle. This causes CreateDIBitmap to fail. To work around this problem, use GlobalAlloc to create a global memory handle.

Here is what LoadResourceBitmap should be on Win32s:

HBITMAP LoadResourceBitmap(HINSTANCE hInstance, LPSTR lpString,

                           HPALETTE FAR* lphPalette)
{
    HRSRC  hRsrc;
    HGLOBAL hGlobal, hTemp;
    DWORD dwSize;
    HBITMAP hBitmapFinal = NULL;
    LPBITMAPINFOHEADER  lpbi;
    LPSTR lpRes, lpNew;
    HDC hdc;
    int iNumColors;

    if (hRsrc = FindResource(hInstance, lpString, RT_BITMAP))
    {
       hTemp = LoadResource(hInstance, hRsrc);
       dwSize = SizeofResource(hInstance, hRsrc);
       lpRes = LockResource(hTemp);

       hGlobal = GlobalAlloc(GHND, dwSize);
       lpNew = GlobalLock(hGlobal);
       memcpy(lpNew, lpRes, dwSize);
       UnlockResource(hTemp);
       FreeResource(hTemp);

       lpbi = (LPBITMAPINFOHEADER)lpNew;

       hdc = GetDC(NULL);
       *lphPalette =  CreateDIBPalette ((LPBITMAPINFO)lpbi, &iNumColors);
       if (*lphPalette)
       {
          SelectPalette(hdc,*lphPalette,FALSE);
          RealizePalette(hdc);
       }

       hBitmapFinal = CreateDIBitmap(hdc,
                 (LPBITMAPINFOHEADER)lpbi,
                 (LONG)CBM_INIT,
                 (LPSTR)lpbi + lpbi->biSize + iNumColors * sizeof(RGBQUAD),
                 (LPBITMAPINFO)lpbi,
                 DIB_RGB_COLORS );

       ReleaseDC(NULL,hdc);
       GlobalUnlock(hGlobal);
       GlobalFree(hGlobal);
    }
    return (hBitmapFinal);

REFERENCES

For more information on DIB-related functions, please review the Microsoft Windows SDK sample DIBVIEW.