Stupid Question.........

Code:

const int BITMAPHEIGHT=32;
const int BITMAPWIDTH=64;

//////////////////////////////////////////////////////////////////////////////
//PROTOTYPES
//////////////////////////////////////////////////////////////////////////////
bool Prog_Init();//game data initalizer
void Prog_Loop();//main game loop
void Prog_Done();//game clean up

//////////////////////////////////////////////////////////////////////////////
//GLOBALS
//////////////////////////////////////////////////////////////////////////////
HINSTANCE hInstMain=NULL;//main application handle
HWND hWndMain=NULL;//handle to our main window
//memory dc
HDC hdcMem=NULL;
//bitmaps
HBITMAP hbmNew=NULL;
HBITMAP hbmOld=NULL;

//////////////////////////////////////////////////////////////////////////////
//WINDOWPROC
//////////////////////////////////////////////////////////////////////////////
LRESULT CALLBACK TheWindowProc(HWND hwnd,UINT uMsg,WPARAM wParam,LPARAM lParam)
{
	//which message did we get?
	switch(uMsg)
	{
	case WM_LBUTTONDOWN:
		{
			//borrow dc from main window
			HDC hdc=GetDC(hWndMain);

			//blit from the memory dc to the window's dc
			BitBlt(hdc,LOWORD(lParam)-BITMAPWIDTH/2,HIWORD(lParam)-BITMAPHEIGHT/2,BITMAPWIDTH,BITMAPHEIGHT,hdcMem,0,0,SRCCOPY);

			//return the borrowed dc to the system
			ReleaseDC(hWndMain,hdc);

			//handled, so return 0
			return(0);
		}break;

Ok this is a problem I am having with the BitBlt Function.
Warning!!! Some questions might sound stupid, but the problem is that I am really a newbie......

const int BITMAPHEIGHT=32; //Why is this initialized as the size?
const int BITMAPWIDTH=64;//same here

BitBlt(hdc,LOWORD(lParam)-BITMAPWIDTH/2,HIWORD(lParam)-BITMAPHEIGHT/2,BITMAPWIDTH,BITMAPHEIGHT,hdcMem,0,0,SRCCOPY);


Ok, the book says that the second and third parameter are the destion of the x and y coordinate of the bitmap respectively........why is the height of the bitmap divided by two? Warning stupid question coming.........and what does it have that little "-" Sign before the BITMAPWDTH parameter.........

Please help...I need to know this before I continue......

From MSDN:

BOOL BitBlt(
HDC hdcDest, // handle to destination device context
int nXDest, // x-coordinate of destination rectangle's upper-left
// corner
int nYDest, // y-coordinate of destination rectangle's upper-left
// corner
int nWidth, // width of destination rectangle
int nHeight, // height of destination rectangle
HDC hdcSrc, // handle to source device context
int nXSrc, // x-coordinate of source rectangle's upper-left
// corner
int nYSrc, // y-coordinate of source rectangle's upper-left
// corner
DWORD dwRop // raster operation code
);

When the msg is a mouse message (ie: WM_LBUTTONDOWN)
(LOWORD)lparam is the current mouse x coord.
(HIWORD)lparam is the current mouse y coord.



By subtracting half of our Bitmap width from the x coord we blit our bitmap positioned on the screen with the current mouse x coord as it's center (same goes for the y and Bitmap height).

The "-" is the subtraction operator. Used to get the proper left/top coords to "center" our blitted bitmap based on the current mouse coord.

(LOWORD)lparam is the current mouse x coord.

(HIWORD)lparam is the current mouse y coord.


Thank you, why couldn't the book had just said so....God

By subtracting half of our Bitmap width from the x coord we blit our bitmap positioned on the screen with the current mouse x coord as it's center (same goes for the y and Bitmap height).

not quite sure I follow......sorry.........



const int BITMAPHEIGHT=32;
const int BITMAPWIDTH=64;

is this the size of the window or the bitmap? If it is of the bitmap how do we know this is the size of the bitmap?


Thank you once again.

BitBlt(hdc,LOWORD(lParam)-BITMAPWIDTH/2,HIWORD(lParam)-BITMAPHEIGHT/2,BITMAPWIDTH,BITMAPHEIGHT,hdcMem,0,0,SRCCOPY);


I think I got it........this makes the mouse insert the image while pointing to the center of the bitmap...........32/2=center?

Originally posted by elchulo2002
not quite sure I follow......sorry.........

In that particular example you want to draw the bitmap onto the screen each time you click. You want to draw it with it's center based on wherever the mouse was when you clicked.

You clicked at coord (10,10)
Your bitmap is 20x20
(LOWORD)lparam-BITMAPWIDTH/2 = 20 - (20/2) *which* = 10
(HIWORD)lparam-BITMAPHEIGHT/2 = 20 - (20/2) *which* = 10

so your bitmap with be drawn in the upper left hand corner.

Once you compile the example notice that where ever you click the bitmap is drawn "centered" on you mouse.

const int BITMAPHEIGHT=32;
const int BITMAPWIDTH=64;

Originally posted by elchulo2002
is this the size of the window or the bitmap? If it is of the bitmap how do we know this is the size of the bitmap?

This is the size of the bitmap.

Normally you would grab the bitmap width/height with GetObject. The author just hard coded it here for simplicities sake.

Originally posted by elchulo2002
BitBlt(hdc,LOWORD(lParam)-BITMAPWIDTH/2,HIWORD(lParam)-BITMAPHEIGHT/2,BITMAPWIDTH,BITMAPHEIGHT,hdcMem,0,0,SRCCOPY);


I think I got it........this makes the mouse insert the image while pointing to the center of the bitmap...........32/2=center?

I was typing my last reply when you posted this.

Yes. That's the way it works.

Thank you very much Jdinger for your time, and patience.