Capturing a program window with BitBlt

[EDN Admin]

I wrote the following code (C++ Win32) to capture a game window screen and get pixel color array from the image. Function autoB() does the job.
Then I draw the result array into my window to visually check what I got.

The problem is that this program works only once after I start the computer, after the first time it "caches" the first screenshot
taken from the game and I always get the same array of pixels. Even if I close and start the program again I get the same
screenshot.

The game is not using DirectX to draw on the screen and Im always able to take the screenshots using Alt+PrtSc.

Any help in understanding why its happening this way is appreciated.


<pre class="prettyprint" style=" int getPixels(HDC *eClientHdcMem, HBITMAP *eClientBmp, unsigned char **lp) {

BITMAP bmpScreen;
BITMAPINFOHEADER bi;

GetObject(*eClientBmp, sizeof(BITMAP), &bmpScreen);

LONG bW = bmpScreen.bmWidth, bH = bmpScreen.bmHeight;

bi.biSize = sizeof(BITMAPINFOHEADER);
bi.biWidth = bW;
bi.biHeight = -bH;
bi.biPlanes = 1;
bi.biBitCount = 32;
bi.biCompression = BI_RGB;
bi.biSizeImage = 0;
bi.biXPelsPerMeter = 0;
bi.biYPelsPerMeter = 0;
bi.biClrUsed = 0;
bi.biClrImportant = 0;

DWORD dw = ((bW * bi.biBitCount + 31) / 32) * 4 * bH;
*lp = new unsigned char[dw];

return GetDIBits(*eClientHdcMem, *eClientBmp, 0, (UINT)bH, *lp, (BITMAPINFO *)&bi, DIB_RGB_COLORS);

}

void autoB() {
HWND hwnd;
HDC hDC0 = NULL, eClientHdcMem = NULL;
HBITMAP eClientBmp = NULL;
BITMAP bmp = {0};
unsigned char *lp = NULL, *sp = NULL;
WINDOWINFO wi;
wi.cbSize = sizeof(WINDOWINFO);
RECT vp;
int vpW, vpH;
long iW, iH;

if (!(hwnd = FindWindow(NULL,TEXT("Client")))) return;
if (!(hDC0 = GetDC(hwnd))) return;

GetWindowInfo(hwnd,&wi);
vp = wi.rcClient;
vpW = vp.right - vp.left;
vpH = vp.bottom - vp.top;

if (!(eClientBmp = CreateCompatibleBitmap(hDC0, vpW, vpH))) return;
if (!(eClientHdcMem = CreateCompatibleDC(hDC0))) return;
SelectObject(eClientHdcMem, eClientBmp);

BitBlt(eClientHdcMem, 0, 0, vpW, vpH, hDC0, 0, 0, SRCCOPY);

int res = getPixels(&eClientHdcMem, &eClientBmp, &lp);

DeleteObject(eClientBmp);
DeleteObject(eClientHdcMem);
// begin displaying results for testing
HDC sts = GetDC(hStats);
HBITMAP stsBmp = CreateCompatibleBitmap(sts, vpW, vpH);
HBITMAP stsBmpOld = (HBITMAP)SelectObject(sts, stsBmp);
unsigned char r,g,b;
for(unsigned int i=0;i<vpW;i++) {
for(unsigned int j=0;j<vpH;j++) {
r = lp[(vpW*j+i) * 4 + 2];
g = lp[(vpW*j+i) * 4 + 1];
b = lp[(vpW*j+i) * 4 + 0];
SetPixel(sts,i,j,RGB(r,g,b));
}
}
SelectObject(sts, stsBmpOld);
DeleteObject(stsBmp);
DeleteObject(stsBmpOld);
ReleaseDC(hStats,sts);
// end

DeleteDC(eClientHdcMem);
ReleaseDC(hwnd,hDC0);

delete [] lp;
lp = NULL;
delete [] sp;
sp = NULL;

} [/code]
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