CDC::CreateCompatibleDC

CDC::CreateCompatibleDC

virtual BOOL CreateCompatibleDC( CDC* pDC );

Return Value
Nonzero if the function is successful; otherwise 0.

Parameters
pDC
A pointer to a device context. If pDC is NULL, the function creates a memory device context that is compatible with the system display.
Remarks
Creates a memory device context that is compatible with the device specified by pDC. A memory device context is a block of memory that represents a display surface. It can be used to prepare images in memory before copying them to the actual device surface of the compatible device.

When a memory device context is created, GDI automatically selects a 1-by-1 monochrome stock bitmap for it. GDI output functions can be used with a memory device context only if a bitmap has been created and selected into that context.

This function can only be used to create compatible device contexts for devices that support raster operations. See the CDC::BitBlt member function for information regarding bit-block transfers between device contexts. To determine whether a device context supports raster operations, see the RC_BITBLT raster capability in the member function CDC::GetDeviceCaps.

CDC::StretchBlt

BOOL StretchBlt( int x, int y, int nWidth, int nHeight, CDC* pSrcDC, int xSrc, int ySrc, int nSrcWidth, int nSrcHeight, DWORD dwRop );

Return Value
Nonzero if the bitmap is drawn; otherwise 0.

Parameters
x
Specifies the x-coordinate (in logical units) of the upper-left corner of the destination rectangle.
y
Specifies the y-coordinate (in logical units) of the upper-left corner of the destination rectangle.
nWidth
Specifies the width (in logical units) of the destination rectangle.
nHeight
Specifies the height (in logical units) of the destination rectangle.
pSrcDC
Specifies the source device context.
xSrc
Specifies the x-coordinate (in logical units) of the upper-left corner of the source rectangle.
ySrc
Specifies the x-coordinate (in logical units) of the upper-left corner of the source rectangle.
nSrcWidth
Specifies the width (in logical units) of the source rectangle.
nSrcHeight
Specifies the height (in logical units) of the source rectangle.
dwRop
Specifies the raster operation to be performed. Raster operation codes define how GDI combines colors in output operations that involve a current brush, a possible source bitmap, and a destination bitmap. This parameter may be one of the following values:

BLACKNESS Turns all output black.

DSTINVERT Inverts the destination bitmap.

MERGECOPY Combines the pattern and the source bitmap using the Boolean AND operator.

MERGEPAINT Combines the inverted source bitmap with the destination bitmap using the Boolean OR operator.

NOTSRCCOPY Copies the inverted source bitmap to the destination.

NOTSRCERASE Inverts the result of combining the destination and source bitmaps using the Boolean OR operator.

PATCOPY Copies the pattern to the destination bitmap.

PATINVERT Combines the destination bitmap with the pattern using the Boolean XOR operator.

PATPAINT Combines the inverted source bitmap with the pattern using the Boolean OR operator. Combines the result of this operation with the destination bitmap using the Boolean OR operator.

SRCAND Combines pixels of the destination and source bitmaps using the Boolean AND operator.

SRCCOPY Copies the source bitmap to the destination bitmap.

SRCERASE Inverts the destination bitmap and combines the result with the source bitmap using the Boolean AND operator.

SRCINVERT Combines pixels of the destination and source bitmaps using the Boolean XOR operator.

SRCPAINT Combines pixels of the destination and source bitmaps using the Boolean OR operator.

WHITENESS Turns all output white.

Remarks
Copies a bitmap from a source rectangle into a destination rectangle, stretching or compressing the bitmap if necessary to fit the dimensions of the destination rectangle. The function uses the stretching mode of the destination device context (set by SetStretchBltMode) to determine how to stretch or compress the bitmap.

The StretchBlt function moves the bitmap from the source device given by pSrcDC to the destination device represented by the device-context object whose member function is being called. The xSrc, ySrc, nSrcWidth, and nSrcHeight parameters define the upper-left corner and dimensions of the source rectangle. The x, y, nWidth, and nHeight parameters give the upper-left corner and dimensions of the destination rectangle. The raster operation specified by dwRop defines how the source bitmap and the bits already on the destination device are combined.

The StretchBlt function creates a mirror image of a bitmap if the signs of the nSrcWidth and nWidth or nSrcHeight and nHeight parameters differ. If nSrcWidth and nWidth have different signs, the function creates a mirror image of the bitmap along the x-axis. If nSrcHeight and nHeight have different signs, the function creates a mirror image of the bitmap along the y-axis.

The StretchBlt function stretches or compresses the source bitmap in memory and then copies the result to the destination. If a pattern is to be merged with the result, it is not merged until the stretched source bitmap is copied to the destination. If a brush is used, it is the selected brush in the destination device context. The destination coordinates are transformed according to the destination device context; the source coordinates are transformed according to the source device context.

If the destination, source, and pattern bitmaps do not have the same color format, StretchBlt converts the source and pattern bitmaps to match the destination bitmaps. The foreground and background colors of the destination device context are used in the conversion.

If StretchBlt must convert a monochrome bitmap to color, it sets white bits (1) to the background color and black bits (0) to the foreground color. To convert color to monochrome, it sets pixels that match the background color to white (1) and sets all other pixels to black (0). The foreground and background colors of the device context with color are used.

Not all devices support the StretchBlt function. To determine whether a device supports StretchBlt, call the GetDeviceCaps member function with the RASTERCAPS index and check the return value for the RC_STRETCHBLT flag.

StretchBlt() 把一个DC中的位图自制到另一个DC中但可以改变位图的长宽比

举例代码:

void CResumeView::OnDraw(CDC* pDC)
{
CResumeDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
// TODO: add draw code for native data here
CRect ClientRect;
GetClientRect(&ClientRect);
CDC MemDC;
MemDC.CreateCompatibleDC(NULL);
MemDC.SelectObject(pDoc->m_Bitmap);
pDC->StretchBlt(0,0,ClientRect.Width(),ClientRect.Height(),&MemDC,0,0,pDoc->m_nWidth,pDoc->m_nHeight,SRCCOPY);
}