C++ Window capture output isn't the same size as said window

Question

I have a short program in C++ to capture the pixels from this BlueStacks emulator window, and then I'll be manipulating the pixels using OpenCV and then sending mouse input (win32api) based on some decisions here and there. I'm working in Visual Studio Enterprise 2017.

Only problem is, the function seems to be capturing a smaller region of pixels compared to the full window.

Here's an example image. Original window is on the left, mirrored output is on the right.

How can I fix this? I've already enabled High DPI awareness in my project settings, not sure what other steps to take other than adding random magic numbers into the function.

I've done this program already in Python but I'd like to redo this in C++ for the performance boost.

Here's a video (warning: noise). And another one.

Here's my current code:

#include "stdafx.h"
#include "opencv2/imgproc.hpp"
#include "opencv2/highgui.hpp"
#include <Windows.h>
#include <iostream>

using namespace std;
using namespace cv;

Mat hwnd2mat(HWND hwnd);

int main(int argc, char **argv)
{
    printf("Hello, world!\n");
    Sleep(1000);

    HWND hwndDesktop;

    hwndDesktop = GetForegroundWindow();
    // namedWindow("output", WINDOW_NORMAL);
    int key = 0;
    Mat src;

    while (key != 27)
    {
        src = hwnd2mat(hwndDesktop);
        // you can do some image processing here
        imshow("output", src);
        key = waitKey(1); // you can change wait time
    }

}

Mat hwnd2mat(HWND hwnd)
{
    HDC hwindowDC, hwindowCompatibleDC;

    int height, width, srcheight, srcwidth;
    HBITMAP hbwindow;
    Mat src;
    BITMAPINFOHEADER  bi;

    hwindowDC = GetDC(hwnd);
    hwindowCompatibleDC = CreateCompatibleDC(hwindowDC);
    SetStretchBltMode(hwindowCompatibleDC, COLORONCOLOR);

    RECT windowsize;    // get the height and width of the screen
    GetClientRect(hwnd, &windowsize);

    srcheight = windowsize.bottom;
    srcwidth = windowsize.right;
    height = windowsize.bottom / 1;  //change this to whatever size you want to resize to
    width = windowsize.right / 1;

    src.create(height, width, CV_8UC4);

    // create a bitmap
    hbwindow = CreateCompatibleBitmap(hwindowDC, width, height);
    bi.biSize = sizeof(BITMAPINFOHEADER);    //http://msdn.microsoft.com/en-us/library/windows/window/dd183402%28v=vs.85%29.aspx
    bi.biWidth = width;
    bi.biHeight = -height;  //this is the line that makes it draw upside down or not
    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;

    // use the previously created device context with the bitmap
    SelectObject(hwindowCompatibleDC, hbwindow);
    // copy from the window device context to the bitmap device context
    StretchBlt(hwindowCompatibleDC, 0, 0, width, height, hwindowDC, 0, 0, srcwidth, srcheight, SRCCOPY); //change SRCCOPY to NOTSRCCOPY for wacky colors !
    GetDIBits(hwindowCompatibleDC, hbwindow, 0, height, src.data, (BITMAPINFO *)&bi, DIB_RGB_COLORS);  //copy from hwindowCompatibleDC to hbwindow

                                                                                                       // avoid memory leak
    DeleteObject(hbwindow);
    DeleteDC(hwindowCompatibleDC);
    ReleaseDC(hwnd, hwindowDC);

    return src;
}

Answer 1

The stretching is due to DPI scaling. Your own program is not DPI aware, the other program seems to be DPI aware. The easiest way to make your program DPI aware is by calling SetProcessDPIAware(); at the start of the program.

Aside note, HBITMAP handle should not be selected in a device context when calling GetDIBits. You can rewrite your code as

SetProcessDPIAware();
...

Mat hwnd2mat(HWND hwnd)
{
    RECT rc;
    GetClientRect(hwnd, &rc);
    int width = rc.right;
    int height = rc.bottom;

    Mat src;
    src.create(height, width, CV_8UC4);

    HDC hdc = GetDC(hwnd);
    HDC memdc = CreateCompatibleDC(hdc);
    HBITMAP hbitmap = CreateCompatibleBitmap(hdc, width, height);
    HBITMAP oldbmp = (HBITMAP)SelectObject(memdc, hbitmap);

    BitBlt(memdc, 0, 0, width, height, hdc, 0, 0, SRCCOPY);
    SelectObject(memdc, oldbmp);

    BITMAPINFOHEADER  bi = { sizeof(BITMAPINFOHEADER), width, -height, 1, 32, BI_RGB };
    GetDIBits(hdc, hbitmap, 0, height, src.data, (BITMAPINFO*)&bi, DIB_RGB_COLORS);

    DeleteObject(hbitmap);
    DeleteDC(memdc);
    ReleaseDC(hwnd, hdc);

    return src;
}

This code will only work for native Win32 programs. For other programs like Chrome, WPF, Qt apps, this will show blank screen. You would need to take screen shot of desktop window instead.

Answer 2

I think you want GetActiveWindow() instead of GetForegroundWindow()

See this SO answer