Advanced MFC Programming

Chapter 11. Sample: Simple Paint
However, the problem is not that simple. Because the image size can be different from time to time,
also the image could be displayed in various zoomed ratio, it is difficult to decide the dimension of the
memory bitmap. To avoid creating and destroying memory bitmaps whenever the size (or ratio) of the
output image changes, we can create a bitmap with fixed size for painting the client window. If the actual
output needs a larger area, we can use a loop to update the whole client area bit by bit: within any loop we
can copy a portion of the source image to the memory bitmap, add the grid, then output it to the client
window. Because CDC::BitBlt(…) is a relatively fast function, and the bitmap drawing will be
implemented directly by the hardware, calling this function several times will not cause obvious delay.
In the sample application, some new variables are added for this purpose. We know that in order to
prepare a memory bitmap, we also need to prepare a memory DC that will be used to select the bitmap. In
class CGDIView, a CBitmap type variable m_bmpBKStore and a CDC type variable m_dcBKMem are declared.
Also, since the DC will select the bitmap and logical palette, two other pointers CGDIView::m_pBmpBKOld
and CGDIView::m_pPalBKOld are also declared. The two pointers are initialized to NULL in the constructor,
and the memory bitmap CGDIView::m_bmpBKStore is created in function
CGDIView::OnInitialUpdate()when an image is first loaded. The reverse procedure is done in function
CDC::OnDestroy(), where the memory bitmap and the palette are selected out of the memory DC.
In function CGDIView::OnDraw(…), the drawing procedure is modified. First the zoomed image is
divided horizontally and vertically into small portions, all of which can fit into the memory bitmap. Then
each portion of the image is copied to the memory bitmap, and the grid is added if necessary. Next the
image contained in the memory bitmap is copied to the client window at the corresponding position. Here
we need to calculate the origin and dimension of the output image within each loop. The following portion
of function CGDIView::OnDraw(…) shows how the zoomed source image is divided into several portions
and copied to the memory bitmap:
……
……
size.cx=BMP_BKSTORE_SIZE_X/nRatio;
size.cy=BMP_BKSTORE_SIZE_Y/nRatio;
nRepX=(bm.bmWidth-1)/size.cx+1;
nRepY=(bm.bmHeight-1)/size.cy+1;
for(j=0; j