如何在 MFC C++ 应用程序中保存到 bitmap？

Question

I am just starting with MFC so please be tolerant;). I have wrote (it was mostly generated to be honest) a simple application which should do the Paint chores: drawing lines, rectangulars, ellipses, changing a color of object to be drawn etc.

I need to save what has been drawn on the screen into a bmp file. Any ideas how can I achieve this?

I do not know if that's relevant but I am drawing objects on the screen without the use of any CBitmaps or things like that. Here is a part of code responsible for drawing:

CPaintDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
Anchor.x=point.x;
Anchor.y=point.y;
OldPoint.x=Anchor.x;
OldPoint.y=Anchor.y;
if(pDoc->shapeCount>=MAX_SHAPES) return;
pDoc->shapeCount++;

if(bFreehand)
    {
    pDoc->m_shape[pDoc->shapeCount-1] = new Shape;
    pDoc->m_shape[pDoc->shapeCount-1]->shape = ePoint;
    }
if(bLine)
    {
    pDoc->m_shape[pDoc->shapeCount-1] = new CLine;
    pDoc->m_shape[pDoc->shapeCount-1]->shape = eLine;
    }
if(bRectangle)
    {
    pDoc->m_shape[pDoc->shapeCount-1] = new CRectangle;
    pDoc->m_shape[pDoc->shapeCount-1]->shape = eRectangle;
    }
if(bEllipse)
    {
    pDoc->m_shape[pDoc->shapeCount-1] = new CEllipse;
    pDoc->m_shape[pDoc->shapeCount-1]->shape=eEllipse;
    }
pDoc->m_shape[pDoc->shapeCount-1]->x=point.x;
pDoc->m_shape[pDoc->shapeCount-1]->y=point.y;
pDoc->m_shape[pDoc->shapeCount-1]->x2=point.x;
pDoc->m_shape[pDoc->shapeCount-1]->y2=point.y;
pDoc->m_shape[pDoc->shapeCount-1]->Pen=CurrentPen;
pDoc->m_shape[pDoc->shapeCount-1]->Brush=CurrentBrush;
bButtonDown=true;
SetCapture();

I have found this way to do it but I don't know how to obtain screen width and height to fill it in the CreateBitmap parameter's list

        CBitmap *bitmap;
    bitmap.CreateBitmap(desktopW, desktopH, 1, 32, rgbData);
    CImage image;
    image.Attach(bitmap);
    image.Save(_T("C:\\test.bmp"), Gdiplus::ImageFormatBMP);

Answer 1

The CreateBitmap call only requires the desktop width and height if the image you wish to save is actually the entire size of the screen. If that's indeed your intent, you can use CWnd::GetDesktopWindow() to get a CWnd object that you can query for its width and height:

http://msdn.microsoft.com/en-us/library/bkxb36k8(v=VS.80).aspx

That gets dodgy in general...if for no other reason than multi-monitor scenarios...so I'd recommend against it unless you really feel like writing a screen capture app.

What you probably want to do isn't to take a full screen shot, but just save the contents of your program's window. Typically you'd do this by breaking out the drawing logic of your program so that in the paint method you call a helper function that is written to take a CDC device context. Then you can either call that function on the window-based DC you get in the paint call or on a DC you create from the bitmap to do your save. Note that you can use a CBitmap in CDC::SelectObject :

http://msdn.microsoft.com/en-us/library/432f18e2(v=VS.71).aspx

(Though let me pitch you on not using MFC. Try Qt instead. Way better.)