What is the utility of NULL PEN?

As we all know a PEN GDI object has three general attributes -

• style of the line,
• width in pixel,
• color in RGB

When pen style is PS_NULL all other two attributes are not applicable and it is treated as hollow or transparent. GDI APIs which draws shapes in two steps

• Draw outer lines
• Fill inner area as per selected brush

With a style PS_NULL style it will draw a shape whose outer lines are invisible.

For example if this is a rectangle then outer 1 pixel will be transparent as background color and inner rectangle area will be filled with the color and style of selected brush.

Solid Style PEN Drawing:

  case WM_PAINT:
    hdc = BeginPaint(hWnd, &ps);
    hpen = CreatePen(PS_SOLID, 3, RGB(50,50,50));
    logbrush.lbColor = RGB(200,200,200);
    logbrush.lbStyle = BS_SOLID;
    logbrush.lbHatch = 0;
    hbrush = CreateBrushIndirect(&logbrush);
    old_hbrush = (HBRUSH) SelectObject(hdc, (HGDIOBJ)hbrush);
    old_hpen = (HPEN) SelectObject(hdc, (HGDIOBJ)hpen);
    Rectangle(hdc, 10, 10, 100, 100);
    SelectObject(hdc, (HGDIOBJ)old_hbrush);
    SelectObject(hdc, (HGDIOBJ)old_hpen);
    DeleteObject(hbrush);
    DeleteObject(hpen);
    EndPaint(hWnd, &ps);
    break;

NULL Style PEN Drawing:

  case WM_PAINT:
    hdc = BeginPaint(hWnd, &ps);
    hpen = CreatePen(PS_NULL, 3, RGB(50,50,50));
    logbrush.lbColor = RGB(200,200,200);
    logbrush.lbStyle = BS_SOLID;
    logbrush.lbHatch = 0;
    hbrush = CreateBrushIndirect(&logbrush);
    old_hbrush = (HBRUSH) SelectObject(hdc, (HGDIOBJ)hbrush);
    old_hpen = (HPEN) SelectObject(hdc, (HGDIOBJ)hpen);
    Rectangle(hdc, 10, 10, 100, 100);
    SelectObject(hdc, (HGDIOBJ)old_hbrush);
    SelectObject(hdc, (HGDIOBJ)old_hpen);
    DeleteObject(hbrush);
    DeleteObject(hpen);
    EndPaint(hWnd, &ps);
    break;

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