Применение Windows API
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Легалов А. И.
Шрифт:

 BITMAP bm;

 ::GetObject(_hBitmap, sizeof(bm), &bm);

 width = bm.bmWidth;

 height = bm.bmHeight;

}

Finally, you might want to create an empty bitmap and fill it with your own drawings programmatically. You have to specify the dimensions of the bitmap and you have to provide a device context (Canvas) for which the bitmap is targeted. Windows will create a different type of bitmap when your target is a monochrome monitor or printer, and different when it's a graphics card set to True Color. Windows will create a bitmap that is compatible with the target device.

Bitmap::Bitmap(Canvas& canvas, int dx, int dy) : _hBitmap (0) {

 CreateCompatible(canvas, dx, dy);

}

void Bitmap::CreateCompatible(Canvas& canvas, int width, int height) {

 Free;

 _hBitmap = ::CreateCompatibleBitmap(canvas, width, height);

}

How do you display the bitmap on screen? You have to blit it. Blit stands for "block bit transfer" or something like that. When you blit a bitmap, you have to specify a lot of parameters, so we'll just encapsulate the blitting request in a separate object, the blitter. This is a very handy object that sets the obvious defaults for blitting, but at the same time lets you override each and any of them.

A blitter transfers a rectangular area of the bitmap into a rectangular area of the screen. The meaning of various parameters is the following:

• Source position: pixel coordinates of the upper left corner of the bitmap area, to be transferred. The default is the upper left corner of the bitmap.

• Destination position: pixel coordinates within the target window of the upper left corner of the transferred area. The default is upper left corner of the window.

• Area dimensions: the dimensions of the rectangular area to be transferred. The default is the dimensions of the bitmap.

• Transfer mode. The way bitmap pixels are combined with existing window pixels. The default, SRCCOPY, copies the pixels over existing pixels. You may also specify more involved logical operations, like SRCAND (Boolean AND), SRCPAINT (Boolean OR), etc. (see your compiler's help on BitBlt).

class Blitter {

public:

 Blitter(Bitmap& bmp) : _bmp (bmp), _mode (SRCCOPY), _xDst (0), _yDst (0), _xSrc (0), _ySrc (0) {

bmp.GetSize(_width, _height);

 }

 void SetMode(DWORD mode) {

_mode = mode;

 }

 void SetDest(int x, int y) {

_xDst = x;

_yDst = y;

 }

 void SetSrc(int x, int y) {

_xSrc = x;

_ySrc = y;

 }

 void SetArea(int width, int height) {

_width = width;

_height = height;

 }

 // transfer bitmap to canvas

 void BlitTo(Canvas & canvas);

private:

 Bitmap& _bmp;

 int _xDst, _yDst;

 int _xSrc, _ySrc;

 int _width, _height;

 DWORD _mode;

};

The BlitTo method performs the transfer from the bitmap to the window (or printer) as described by its Canvas.

void Blitter::BlitTo(Canvas& canvas) {

 // Create canvas in memory using target canvas as template

 MemCanvas memCanvas (canvas);

 // Convert bitmap to the format appropriate for this canvas

 memCanvas.SelectObject(_bmp);

 // Transfer bitmap from memory canvas to target canvas

 ::BitBlt(canvas, _xDst, _yDst, _width, _height, memCanvas, _xSrc, _ySrc, _mode);

}

The API, BitBlt, transfers bits from one device to another. That's why we have to set up a fake source device. This "memory canvas" is based on the actual canvas--in this case we use target canvas as a template. So, for instance, if the target canvas describes a True Color device, our MemCanvas will also behave like a True Color device. In particular, when our bitmap is selected into it, it will be converted to True Color, even if initially it was a monochrome or a 256-color bitmap.