penWidth in frameOval

1
Hi everyone,

I was wondering how the penWidth in frameOval was calculated, If I have a framed oval
within a 600 pixel square and I chose the penWidth to be 100, why does it appear to be a
filled oval? The pen width of 100 pixels should not fill up the entire oval

screenNumber = max(Screen('Screens'));
[window,screenRect] = Screen('OpenWindow', screenNumber, 0, []);

positionOfMainCircle = [250 250 850 850] ;
Screen('FrameOval', window,[0 0 255],positionOfMainCircle,100);

any explanations?