in Vsual Studio C++ OffsetRgn not working> - c++

Question

void CBallBounceView::OnDraw(CDC* pDC)
{
CBallBounceDoc* pDoc = GetDocument();
ASSERT_VALID(pDoc);
if (!pDoc)
return;
CString str;
str.Format(_T("Counter = %d"), counter);
CRgn rgnA,copyeed;
VERIFY(rgnA.CreateEllipticRgn(50, 50, 150, 150));
CBrush brA;
VERIFY(brA.CreateSolidBrush(RGB(255, 0, 0)));
VERIFY(pDC->FillRgn(&rgnA, &brA)); // rgnA Red Filled
int nOffsetResult = rgnA.OffsetRgn(500, 100);
ASSERT(nOffsetResult != ERROR && nOffsetResult != NULLREGION);
}
I am trying to offset the ellipse region but it is not offsetting. I am new in visual studio C++;
I am using Visual studio 2019.

Answer 1

The problem is that, once output has been rendered to the given device context (as by your pDC->FillRgn(&rgnA, &brA) call), any subsequent changes to that region will not change the displayed output.
In order to do this, you will need to perform a cycle of operations:
Draw the object in its original position.
Then, when moved, erase that original object; and...
Draw the object in its new position.
Repeat each time you move the object.
You typically perform the 'erase' operation by drawing the object each time using the XOR (exclusive or) 'mix mode' on the target device context. Thus, the first time you draw this, it will paint the object over the background, and the second time, it will restore that background. You can use the CDC::SetROP2(R2_XORPEN) function to set that mode.
Here's a 'scruffy' (but runnable) program demonstrating the use of this XOR drawing technique. It uses 'raw' Windows GDI calls (rather than MFC) and is a Console-Mode program, but it will hopefully show the basic principles of the technique:
#include <Windows.h>
#define BLOBSIZE 20
#define MOVESTEP 5
int main()
{
SetProcessDPIAware(); // Comment this line out if using 'older' versions of Windows.
HWND hWnd = GetConsoleWindow();
HDC hDC = GetDC(hWnd);
RECT rc; GetClientRect(hWnd, &rc);
int x = 50, y = 30;
int dx = MOVESTEP, dy = MOVESTEP;
SetROP2(hDC, R2_XORPEN);
SelectObject(hDC, GetStockObject(WHITE_BRUSH));
SelectObject(hDC, GetStockObject(NULL_PEN));
do {
Ellipse(hDC, x, y, x + BLOBSIZE, y + BLOBSIZE); // First call: Draws the object
Sleep(10); // Here, we just wait, but you can do whatever else you want between moves
Ellipse(hDC, x, y, x + BLOBSIZE, y + BLOBSIZE); // Second call: erase it
x += dx;
if ((dx > 0 && x >= rc.right - BLOBSIZE) || (dx < 0 && x == 0)) {
Beep(1000, 10);
dx *= -1;
}
y += dy;
if ((dy > 0 && y >= rc.bottom - BLOBSIZE) || (dy < 0 && y == 0)) {
Beep(1000, 10);
dy *= -1;
}
} while (!(GetAsyncKeyState(VK_SPACE) & 0x8000)); // Press spacebar to quit!
ReleaseDC(hWnd, hDC);
return 0;
}