Advanced MFC Programming

Chapter 12. Screen Capturing & Printing
will be called before CView::BeginPrinting(…) is called when the printing DC needs to be prepared.
Please note that CView::OnPrepareDC(…) will also be called for preparing display DC, to distinguish
between the two situations, we can check if parameter pInfo is NULL, if not, it is called for the printing
job.
The number of required pages can be calculated by retrieving the device resolution (need to be
converted to logical unit) and comparing it with the image size. If the image size is greater than the device
resolution, we can print one portion at a time until the whole image is output to the target device.
Sample 12.5-2\GDI is based on sample 12.4\GDI, it demonstrates how to print the captured image
using this method. In the sample, first function CGDIView::OnPrepareDC(…) is overridden, within which the
number of required pages is calculated as follows:
……
nMapMode=pDC->SetMapMode(MM_LOMETRIC);
CGDIDoc* pDoc=GetDocument();
ASSERT_VALID(pDoc);
……
hDib=pDoc->GetHDib();
if(hDib != NULL)
{
lpBi=(LPBITMAPINFO)::GlobalLock(hDib);
ASSERT(lpBi);
size.cx=lpBi->bmiHeader.biWidth;
size.cy=lpBi->bmiHeader.biHeight;
rect.left=rect.top=0;
rect.right=pDC->GetDeviceCaps(HORZRES);
rect.bottom=pDC->GetDeviceCaps(VERTRES);
pDC->DPtoLP(rect);
nNumPages=((size.cx-1)/abs(rect.Width())+1)*((size.cy-1)/abs(rect.Height())+1);
pInfo->SetMaxPage(nNumPages);
::GlobalUnlock(hDib);
}
Here, the device resolution is retrieved by calling function CDC::GetDeviceCaps(…). Because the
device mapping mode is MM_LOMETRIC, which may cause the values returned by this function to be negative
for vertical dimensions, we need to use absolute value when doing the calculation.
Within function CGDIView::OnPrint(…), we print the corresponding portion of the image according to
the current page number. This procedure can be illustrated in Figure 12-7.
The shaded area represents the image. It is divided into horizontal and vertical cells, each cell has a
dimension that is the same with target device resolution. To draw the image, we need nine pages, each page
print one cell that is labeled (v, u). First we need to calculate the cell label from the page number:
v = (page number - 1)/(number of horizontal cells)
u = (page number -1)%(number of horizontal cells)
Here the page number starts from 1. The next step is to calculate the position and the dimension of the
cell. Obviously, the origin of a cell rectangle can be calculated as follows:
(0, 0)
(0, 1)
(0, 2)
Image
(1, 0)
(1, 1)
(1, 2)
(v, u)
(2, 0)
(2, 1)
(2, 2)
One cell
Figure 12-7. Print a portion of the image according to the current page number
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