Microsoft KB Archive/124947

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INFO: Retrieving Palette Information from a Bitmap Resource

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Q124947

1.20 1.25a 1.30 3.10 4.00 | 3.50 3.51 WINDOWS | WINDOWS NT kbgraphic

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The information in this article applies to:


 * Microsoft Windows Software Development Kit (SDK) 3.1
 * Microsoft Win32 Application Programming Interface (API), used with:
 * Microsoft Win32s versions 1.2, 1.25a, 1.3

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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.

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 *

sizeof(RGBQUAD),

(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);

}