HOWTO: Retrieving an Optimal DIB Format for a Device

/**********************************************************

GetOptimalDIBFormat

  Purpose:
   Retrieves the optimal DIB format for a display 
   device. The optimal DIB format is the format that 
   exactly matches the format of the target device. 
   Obtaining this is very important when dealing with 
   16bpp modes because you need to know what bitfields 
   value to use (555 or 565 for example).

   You normally use this function to get the best
   format to pass to CreateDIBSection() in order to
   maximize blt'ing performance.

  Input:
   hdc - Device to get the optimal format for.
   pbi - Pointer to a BITMAPINFO + color table
         (room for 256 colors is assumed).

  Output:
   pbi - Contains the optimal DIB format. In the 
         <= 8bpp case, the color table will contain the 
         system palette. In the >=16bpp case, the "color 
         table" will contain the correct bit fields (see 
         BI_BITFIELDS in the Platform SDK documentation 
         for more information).

  Notes:
   If you are going to use this function on a 8bpp device
   you should make sure the color table contains a identity
   palette for optimal blt'ing.

**********************************************************/ 
BOOL GetOptimalDIBFormat(HDC hdc, BITMAPINFOHEADER *pbi)
{
    HBITMAP hbm;
    BOOL bRet = TRUE;
    
    // Create a memory bitmap that is compatible with the
    // format of the target device.
    hbm = CreateCompatibleBitmap(hdc, 1, 1);
    if (!hbm)
        return FALSE;
    
    // Initialize the header.
    ZeroMemory(pbi, sizeof(BITMAPINFOHEADER));
    pbi->biSize = sizeof(BITMAPINFOHEADER);

    // First call to GetDIBits will fill in the optimal biBitCount.
    bRet = GetDIBits(hdc, hbm, 0, 1, NULL, (BITMAPINFO*)pbi, DIB_RGB_COLORS);
    
    // Second call to GetDIBits will get the optimal color table, o
    // or the optimal bitfields values.
    if (bRet)
        bRet = GetDIBits(hdc, hbm, 0, 1, NULL, (BITMAPINFO*)pbi, DIB_RGB_COLORS);
    
    // Clean up.
    DeleteObject(hbm);

    return bRet;
}

// Counts the number of set bits in a DWORD.
BYTE CountBits(DWORD dw)
{
    int iBits = 0;
    
    while (dw) {
        iBits += (dw & 1);
        dw >>= 1;
    }
    
    return iBits;
}

/**********************************************************

GetRGBBitsPerPixel

  Purpose:
   Retrieves the number of bits of color resolution for each 
   color channel of a specified.

  Input:
   hdc - Device to get the color information for.
   
  Output:
   pRed   - Number of distinct red levels the device can display.
   pGreen - Number of distinct green levels the device can display.
   pBlue  - Number of distinct blue levels the device can display.

  Notes:
   This function does not return any meaningful information for
   palette-based devices.

**********************************************************/ 
BOOL GetRGBBitsPerPixel(HDC hdc, PINT pRed, PINT pGreen, PINT pBlue)
{
    BITMAPINFOHEADER *pbi;
    LPDWORD lpdw;
    BOOL bRet = TRUE;

    // If the target device is palette-based, then bail because there is no
    // meaningful way to determine separate RGB bits per pixel.
    if (GetDeviceCaps(hdc, RASTERCAPS) & RC_PALETTE) 
        return FALSE;  

    // Shortcut for handling 24bpp cases.
    if (GetDeviceCaps(hdc, PLANES) * GetDeviceCaps(hdc, BITSPIXEL) == 24) {
        *pRed = *pGreen = *pBlue = 8;
        return TRUE;
    }
   
    // Allocate room for a header and a color table.
    pbi = (BITMAPINFOHEADER *)GlobalAlloc(GPTR, sizeof(BITMAPINFOHEADER) + 
                                                sizeof(RGBQUAD)*256);
    if (!pbi)
        return FALSE;

    // Retrieve a description of the device surface.
    if (GetOptimalDIBFormat(hdc, pbi)) {
        // Get a pointer to the bitfields.
        lpdw = (LPDWORD)((LPBYTE)pbi + sizeof(BITMAPINFOHEADER));

        *pRed   = CountBits(lpdw[0]);
        *pGreen = CountBits(lpdw[1]);
        *pBlue  = CountBits(lpdw[2]);
    } else
        bRet = FALSE;
    
    // Clean up.
    GlobalFree(pbi);

    return bRet;
}