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А. И. Легалов
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
The simplest program that loads and displays a bitmap might look something like this: There is a View object that contains a bitmap (I assume that the file "picture.bmp" is located in the current directory). It blits it to screen in the Paint method.
class View {
public:
View() {
_background.Load("picture.bmp");
}
void Paint(Canvas& canvas) {
Blitter blitter(_background);
blitter.BlitTo(canvas);
}
private:
Bitmap _background;
};
A sprite is an animated bitmap that moves over some background. We already know how to display bitmaps — we could blit the background first and then blit the sprite bitmap over it. This will work as long as the sprite is rectangular. If you want to be more sophisticated and use a non-rectangular sprite, you need a mask.
