Capture only part of the screen with Bitmap (C)

Question

I have a problem using BitBlt() trying to capture a full width and 1 pixel tall image that sits in the exact center of the screen.

HDC hdcTemp;
BYTE* bitPointer;
int x, y;
int red, green, blue, alpha;

HDC desktopWindow = GetDC(HWND_DESKTOP);
//GetWindowRect(hWND_Desktop, &rect);
int screenWidth = GetSystemMetrics(SM_CXSCREEN);
int screenHeight = GetSystemMetrics(SM_CYSCREEN);
int wPos = screenWidth / 2;
int hPos = screenHeight / 2;
int screenShotWidth = screenWidth;
int screenShotHeight = 1;

while (true)
{

    hdcTemp = CreateCompatibleDC(desktopWindow);
    BITMAPINFO bitmap;
    bitmap.bmiHeader.biSize = sizeof(bitmap.bmiHeader);
    bitmap.bmiHeader.biWidth = screenShotWidth;
    bitmap.bmiHeader.biHeight = screenShotHeight;
    bitmap.bmiHeader.biPlanes = 1;
    bitmap.bmiHeader.biBitCount = 24;
    bitmap.bmiHeader.biCompression = BI_RGB;
    bitmap.bmiHeader.biSizeImage = screenShotWidth * 4 * screenShotHeight;
    bitmap.bmiHeader.biClrUsed = 0;
    bitmap.bmiHeader.biClrImportant = 0;
    HBITMAP hBitmap2 = CreateDIBSection(hdcTemp, &bitmap, DIB_RGB_COLORS, (void**)(&bitPointer), NULL, NULL);
    SelectObject(hdcTemp, hBitmap2);
    BitBlt(hdcTemp, 0, 0, screenShotWidth, screenShotHeight, desktopWindow, 0, hPos-1 ,SRCCOPY);

    for (int i = 0; i < (screenShotWidth * 4 * screenShotHeight); i += 4)
    {
        red = (int)bitPointer[i];
        green = (int)bitPointer[i + 1];
        blue = (int)bitPointer[i + 2];

        x = i / (4 * screenShotHeight);
        y = i / (4 * screenShotWidth);
        std::cout << "r : " << red << " x: " << x <<  " y: " << y <<  "\n";
    }
    std::this_thread::sleep_for(std::chrono::milliseconds(2));
}

I know when taking a fullscreen caputre BitBlt() needs to look like this:

BitBlt(hdcTemp, 0, 0, screenWidth, screenHeight, desktopWindow, 0, 0 ,SRCCOPY);

I just dont really get what to change to make it capture only the part i want to capture sitting in the middle of the screen...

Answer 1

bitmap.bmiHeader.biSizeImage = screenShotWidth * 4 * screenShotHeight;

For a 24 bit bitmap size should be calculated as follows:

int size = ((screenShotWidth * 24 + 31) / 32) * 4 * screenShotHeight;

Note that 24 bit bitmap doesn't have alpha channel and it is 3 bytes per pixel. Change the increment to the following:

for(int i = 0; i < (screenShotWidth * 3 * screenShotHeight); i += 3)
{
    red = (int)bitPointer[i];
    green = (int)bitPointer[i + 1];
    blue = (int)bitPointer[i + 2];
    ...
}

To make things easier you can also use a 32 bit bitmap.