Re: How can I change CDC's color?

From:

"Jonathan Wood" <jwood@softcircuits.com>

Newsgroups:

microsoft.public.vc.mfc

Date:

Wed, 13 Sep 2006 13:07:56 -0600

Message-ID:

<u$QDwe21GHA.2036@TK2MSFTNGP05.phx.gbl>

What color do you expect a bitmap to be that you haven't drawn to? In short,
the contents of an uninitialized bitmap is undefined until you draw it.

Why on Earth would it be an issue to draw a rectangle to fill the background
before doing anything else?

--
Jonathan Wood
SoftCircuits Programming
http://www.softcircuits.com

"Sam_2" <Sam2@discussions.microsoft.com> wrote in message
news:8B13B5F7-87E5-4F2E-B464-C9505B32CF59@microsoft.com...

After I created a Memory DC compatible with screen DC, I created a color
DIB
bitmap and selected into the memory DC. However, before I draw anything
onto
the memory DC, everything is black in color? What's wrong with my code?

/****************************************************/
CWnd* pTestWnd = AfxGetApp()->GetMainWnd();
CClientDC dcScreen(pTestWnd);
pTestWnd->GetClientRect(&rect);

// Create Memory DC and Bitmap associate with the Memory DC
CDC MemDC.CreateCompatibleDC(&dcScreen);

// Create a 32-bit color Bitmap using DIB section.
// Code from CodeGuru
BITMAPINFO bmInfo;
ZeroMemory( &bmInfo.bmiHeader, sizeof(BITMAPINFOHEADER) );

bmInfo.bmiHeader.biWidth= rect.Width(); // Set size same as
Screen.Width
bmInfo.bmiHeader.biHeight=rect.Height(); // Set size same as
Screen.Height
bmInfo.bmiHeader.biPlanes=1;
bmInfo.bmiHeader.biBitCount=32; // Color depth 8, 16, 32 bpp
bmInfo.bmiHeader.biSizeImage=0;
bmInfo.bmiHeader.biSize=sizeof(BITMAPINFOHEADER);
bmInfo.bmiHeader.biClrUsed= 0;
bmInfo.bmiHeader.biClrImportant= 0;
VOID *pvBits; // Not used

Bitmap bm; bmt.Attach(CreateDIBSection( m_MemDC,
&bmInfo, DIB_RGB_COLORS, &pvBits, NULL, 0 ));

// Select created bitmap as PrintOut bitmap
MemDC.SelectObject(bm);

// Set Map Mode to Default Logical Unit (Default is Twips)
m_MemDC.SetMapMode(DEFAULT_PAGE_UNIT);
m_MemDC.DPtoLP(&rect);

/* If I stop here and save the DC to a file, the bitmap file will be
completely black.
If I add the following line and draw an rectangle over the DC area, then
the screen become white in color.
*/

MemDC.Rectangle(rect);