Win32 GDI Drawing a circle? - c++

Question

I am trying to draw a circle and I am currently using the Ellipse() function.
I have the starting mouse coordinates - x1 and y1 and the ending coordinates x2 and y2. As you can see, I am forcing the y2(temp_shape.bottom) to be = y1+(x2-x1). This doesn't work as intended. I know the calculation is completely wrong but any ideas on what is right?
Code Below.
case WM_PAINT:
{
hdc = BeginPaint(hWnd, &ps);
// TODO: Add any drawing code here...
RECT rect;
GetClientRect(hWnd, &rect);
HDC backbuffDC = CreateCompatibleDC(hdc);
HBITMAP backbuffer = CreateCompatibleBitmap( hdc, rect.right, rect.bottom);
int savedDC = SaveDC(backbuffDC);
SelectObject( backbuffDC, backbuffer );
HBRUSH hBrush = CreateSolidBrush(RGB(255,255,255));
FillRect(backbuffDC,&rect,hBrush);
DeleteObject(hBrush);
//Brush and Pen colours
SelectObject(backbuffDC, GetStockObject(DC_BRUSH));
SetDCBrushColor(backbuffDC, RGB(255,0,0));
SelectObject(backbuffDC, GetStockObject(DC_PEN));
SetDCPenColor(backbuffDC, RGB(0,0,0));
//Shape Coordinates
temp_shape.left=x1;
temp_shape.top=y1;
temp_shape.right=x2;
temp_shape.bottom=y2;
//Draw Old Shapes
//Rectangles
for ( int i = 0; i < current_rect_count; i++ )
{
Rectangle(backbuffDC, rect_list[i].left, rect_list[i].top, rect_list[i].right, rect_list[i].bottom);
}
//Ellipses
for ( int i = 0; i < current_ellipse_count; i++ )
{
Ellipse(backbuffDC, ellipse_list[i].left, ellipse_list[i].top, ellipse_list[i].right, ellipse_list[i].bottom);
}
if(mouse_down)
{
if(drawCircle)
{
temp_shape.right=y1+(x2-x1);
Ellipse(backbuffDC, temp_shape.left, temp_shape.top, temp_shape.right, temp_shape.bottom);
}
if(drawRect)
{
Rectangle(backbuffDC, temp_shape.left, temp_shape.top, temp_shape.right, temp_shape.bottom);
}
if(drawEllipse)
{
Ellipse(backbuffDC, temp_shape.left, temp_shape.top, temp_shape.right, temp_shape.bottom);
}
}
BitBlt(hdc,0,0,rect.right,rect.bottom,backbuffDC,0,0,SRCCOPY);
RestoreDC(backbuffDC,savedDC);
DeleteObject(backbuffer);
DeleteDC(backbuffDC);
EndPaint(hWnd, &ps);
}
break;

Answer 1

If you want Ellipse() to draw a perfectly round circle, you need to give it coordinates for a perfectly square shape, not a rectangular shape.
Assuming x1,y1 are the starting coordinates of the dragging and x2,y2 are the current mouse coordinates, then try this:
//Shape Coordinates
temp_shape.left = min(x1, x2);
temp_shape.top = min(y1, y2);
temp_shape.right = max(x1, x2);
temp_shape.bottom = max(y1, y2);
...
if (drawCircle)
{
int length = min(abs(x2-x1), abs(y2-y1));
if (x2 < x1)
temp_shape.left = temp_shape.right - length;
else
temp_shape.right = temp_shape.left + length;
if (y2 < y1)
temp_shape.top = temp_shape.bottom - length;
else
temp_shape.bottom = temp_shape.top + length;
Ellipse(backbuffDC, temp_shape.left, temp_shape.top, temp_shape.right, temp_shape.bottom);
}

Answer 2

I have worked out a calculation which works better. Pasted below for anyone else wanting the same.
if(drawSquare)
{
int xdiff = abs(x2-x1);
int ydiff=abs(y2-y1);
if(xdiff>ydiff)
{
if(y2>y1)
temp_shape.bottom=y1+xdiff;
else
temp_shape.bottom=y1-xdiff;
}
else
{
if(x2>x1)
temp_shape.right=x1+ydiff;
else
temp_shape.right=x1-ydiff;
}
Rectangle(backbuffDC, temp_shape.left, temp_shape.top, temp_shape.right, temp_shape.bottom);
}

Answer 3

Your code is unnecessarily complex with temp DCs and back buffers and you are recreating GDI brushes in every WM_PAIT? But that's besides the point. Here is the question : why do you do this:
temp_shape.right=y1+(x2-x1); //basing horizontal coordinate on vertical?
What framework stack is this on top of? .NET, then why don't you use built-in double buffering?