Capturing a specific window using C++ returns old data - c++

Question

I'm currently working on a project that requires to take a screenshot a specific window.
That's what I got so far:
Main function
int main() {
LPCSTR windowname = "Calculator";
HWND handle = FindWindowA(NULL, windowname);
while (!handle) {
std::cout << "Process not found..." << std::endl;
handle = FindWindowA(NULL, windowname);
Sleep(100);
}
Mat img = captureScreenMat(handle);
resetMat();
imwrite("test2.jpg", img);
showInMovedWindow("IMG", img);
waitKey(0);
destroyAllWindows();
return 0;
}
winCapture
Mat src;
void showInMovedWindow(string winname, Mat img) {
namedWindow(winname);
moveWindow(winname, 40, 30);
imshow(winname, img);
}
BITMAPINFOHEADER createBitmapHeader(int width, int height)
{
BITMAPINFOHEADER bi;
bi.biSize = sizeof(BITMAPINFOHEADER);
bi.biWidth = width;
bi.biHeight = -height;
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;
return bi;
}
int getHeight(HWND hwnd) {
RECT rect;
GetWindowRect(hwnd, &rect);
int height = rect.bottom - rect.top;
return height;
}
int getWidth(HWND hwnd) {
RECT rect;
GetWindowRect(hwnd, &rect);
int width = rect.right - rect.left;
return width;
}
Mat captureScreenMat(HWND hwnd)
{
HDC hwindowDC = GetDC(hwnd);
HDC hwindowCompatibleDC = CreateCompatibleDC(hwindowDC);
SetStretchBltMode(hwindowCompatibleDC, COLORONCOLOR);
int screenx = GetSystemMetrics(SM_XVIRTUALSCREEN);
int screeny = GetSystemMetrics(SM_YVIRTUALSCREEN);
int width = getWidth(hwnd);
int height = getHeight(hwnd);
src.create(height, width, CV_8UC4);
HBITMAP hbwindow = CreateCompatibleBitmap(hwindowDC, width, height);
BITMAPINFOHEADER bi = createBitmapHeader(width, height);
//DEBUG
cout << hbwindow << endl;
cout << hwindowCompatibleDC << endl;
cout << hwindowDC << endl;
SelectObject(hwindowCompatibleDC, hbwindow);
StretchBlt(hwindowCompatibleDC, 0, 0, width, height, hwindowDC, screenx, screeny, width, height, SRCCOPY);
GetDIBits(hwindowCompatibleDC, hbwindow, 0, height, src.data, (BITMAPINFO*)&bi, DIB_RGB_COLORS);
DeleteObject(hbwindow);
DeleteDC(hwindowCompatibleDC);
DeleteDC(hwindowDC);
ReleaseDC(NULL, hwindowDC);
return src;
}
void resetMat() {
src.release();
}
Most windows work really well with this approach, but there are some windows that are working the first time, I try to take a img of them, but every time I try to take another screenshot of the same process, it just gives me the first screenshot I took of it. It only works again after a restart of the process and even then It just works again for one screenshot and all after are the same.
I thought it would be some kind of memory leak, but I'm deleting all the objects and releasing the handle.
I think that something is wrong with the handle, but I couldn't figure out what.
I'm not familiar with working with the windowsAPI and hope someone knowns more than me.
Fixed: Process blocked creating handle.

Answer 1

When you call SelectObject you must save the previous-selected handle (available from the return value) and you MUST select it back before deleting or releasing the device context.
Right now you are breaking a bunch of rules.
Deleting a bitmap which is selected into a device context.
Deleting a DC gotten from GetDC.
Calling both DeleteDC and ReleaseDC on the same handle.
Passing NULL as the first parameter of ReleaseDC, which should be the same HWND passed to GetDC.
Deleting a DC without selecting the original bitmap back into it.
These bugs royally mess up the window DC. If it was a transient DC, the system probably cleans up your mess immediately. But if the window class has the CS_OWNDC or CS_CLASSDC flags, you do permanent damage to the window. That's why your method appears to work with some windows and not others.