How to draw on a full screen but show drawing on a specific rectangle/window? (C++)

Question

Hi,

I have a window. I want to draw inside the window (with a pen, like in paint), but I don't want to have the drawing to be attached to the window, i.e. I don't want the drawing to move with the window, when the window is moved. I would like the drawing to be attached to its screen position, and I would like it to disappear when it goes outside of the window area (i.e. when the window is moved). And when the window is moved back to its original position, drawing should be shown again, i.e. should be there. This can be achieved if it is possible to draw on a full screen, but only show drawing on a specific rectangle. In that case I can regularly update the rectangle when my window is moved. Is this possible in winapi?

Thanks

Answer 1

Hello @Davit ,

This can be achieved if it is possible to draw on a full screen, but only show drawing on a specific rectangle. In that case I can regularly update the rectangle when my window is moved. Is this possible in winapi?

You can use CreateCompatibleDC to create a memory device context, and draw in memdc. Then use the StretchBlt function to copy the graphics to the drawing window.

Some code for reference:

    HDC memdc = CreateCompatibleDC(hdc);  
    HDC child_hdc = GetDC(Child_hwnd);  
    HBITMAP membitmap = CreateCompatibleBitmap(hdc, 800, 600);  
    SelectObject(memdc, membitmap);  
    BitBlt(memdc, 0, 0, 800, 600, hdc, 0, 0, SRCCOPY);  
    MoveToEx(memdc, 10, 10, (LPPOINT)NULL);  
    LineTo(memdc, 100, 100);  
    MoveToEx(memdc, 200, 200, (LPPOINT)NULL);  
    LineTo(memdc, 300, 300);  
     
    StretchBlt(child_hdc,0,0,400,300, memdc,0,0,800,600, SRCCOPY);  

Like this:

When you move the main window, you will receive a WM_MOVE message. Use the InvalidateRect function in WM_MOVE to update the rectangle.

I would like the drawing to be attached to its screen position, and I would like it to disappear when it goes outside of the window area (i.e. when the window is moved).

PtInRect is what you need.

Determine whether the four coordinates of the drawing window are in the main window, if not, use ShowWindow to hide the drawing window.

ShowWindow(Child_hwnd, SW_HIDE);  

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