c++ - 透明椭圆

Question

如何用GDI绘制一个透明的椭圆？我尝试了 SetBkMode() 但我仍然得到一个白色椭圆 bk。

case WM_PAINT:
{
    hdc = BeginPaint(hwnd, &ps);
    SetBkMode(hdc, TRANSPARENT); // doesnt work
    Ellipse(hdc, 0,0,500,500);
    EndPaint(hwnd, &ps);
    break;
}

Answer 1

借自Fill an ellipse in C++ :

The ellipse is outlined by using the current pen and is filled by using the current brush.

因此，需要设置一个透明的笔刷。为此，使用 GetStockObject(HOLLOW_BRUSH) 获取它并使用 SelectObject() 为给定的设备上下文激活它。所以你的代码可以是这样的:

case WM_PAINT:
{
    hdc = BeginPaint(hwnd, &ps);
    SelectObject(hdc, GetStockObject(HOLLOW_BRUSH));
    Ellipse(hdc, 0,0,500,500);
    EndPaint(hwnd, &ps);
    break;
}