Применение Windows API

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.

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.

The two pictures below are that of a sprite (my personal pug, Fanny) and its mask. The mask is a monochrome bitmap that has black areas where we want the sprite to be transparent. The sprite, on the other hand, must be white in these areas. What we want is to be able to see the background through these areas.

The trick is to first blit the background, then blit the mask using logical OR, and then blit the sprite over it using logical AND.

ORing a black mask pixel (all zero bits) with a background pixel will give back the background pixel. ORing a white mask pixel (all one bits) with a background will give a white pixel. So after blitting the mask, we'll have a white ghost in the shape of our sprite floating over the background.

ANDing a white sprite pixel (all ones) with a background pixel will give back the background pixel. ANDing a non-white sprite pixel with the white (all ones) background (the ghost from previous step) will give the sprite pixel. We'll end up with the sprite superimposed on the background.