Determining Available RGB Values of an Output Device

• Microsoft Windows Software Development Kit (SDK) for Windows versions 3.1 and 3.0
• Microsoft Win32 Application Programming Interface (API) included with:

      - Microsoft Windows NT versions 3.5 and 3.51
      - Microsoft Windows 95 version 4.0
  

SUMMARY

The best way to determine the colors supported by a device is to enumerate the solid pens from a device context (DC) associated with that device. The EnumObjects function enumerates the pens supported by a specified device and calls a callback function for each pen. To do so, EnumObjects requires three parameters: a device context associated with the desired device as the hDC parameter, OBJ_PEN as the value of the nOjbectType parameter, and the procedure-instance address of a callback function as the lpObjectFunc parameter.

MORE INFORMATION

The first parameter for the callback function, lpLogObject, points to a LOGPEN data structure that describes each enumerated pen. When the lopnStyle field of the LOGPEN structure contains the PS_SOLID value, the application can retrieve and store the value of the lopnColor field.

NOTE: For true color devices (devices that support more than 8 bits- per-pixel of color resolution), Windows enumerates only a subset of the supported pens. These devices support almost any color.

The following code demonstrates the process described above:

int cMaxRGB, nCnt, nSolid; // Global variables for system RGB colors

void GetColorList(HWND hWnd)

   HDC hdc;
   FARPROC lpProcCallback;
   HANDLE hmem;
   BYTE FAR *lpmem;

   nCnt = nSolid = 0;
   hdc = GetDC(hWnd);
   cMaxRGB = GetDeviceCaps(hdc, NUMCOLORS);
   if (cMaxRGB >= 0x7FFF)
      return; // All colors available. Enumeration not required.

   lpProcCallback = MakeProcInstance(Callback, hInst);

   // Allocate space for color table
   hmem = GlobalAlloc(GHND, sizeof(COLORREF) * cMaxRGB);
   lpmem = GlobalLock(hmem);

   EnumObjects(hdc, OBJ_PEN, lpProcCallback, lpmem);

   FreeProcInstance(lpProcCallback);

   // Use the color table stored in the first nSolid entries of a
   // COLORREF array stored in lpmem.

   GlobalUnlock(hmem);
   GlobalFree(hmem);
   ReleaseDC(hWnd, hdc);
   return;

The source for the enumeration callback function is below. The callback function must be listed in the EXPORTS section of the module definition (DEF) file.

int FAR PASCAL Callback(LPLOGPEN lpLogPen, LPSTR lpData)

   nCnt++;
   if (lpLogPen->lopnStyle == PS_SOLID) // solid pen
      {
      COLORREF FAR *lpDest = (COLORREF FAR *)lpData + nSolid++;

      *lpDest = lpLogPen->lopnColor; // remember the solid color
      }

   if (nCnt >= cMaxRGB)
      return 0;
   return 1;