Windows API graphics function Ellipse circle anomaly

Question

I have used the Windows API function Ellipse for many years.

I have seen that when the Ellipse function draws a circle with an even number diameter,
the top side and right side have different pixel counts.

The function call is usually like this for a circle of diameter of 40 pixels;
HDC hdc; // Device context
hdc = GetDC( Windows handle );
Ellipse( hdc, 0, 0, 40, 40 );
ReleaseDC( Windows handle, hdc );

The first picture is an actual size screen capture.

The second picture is enlarged by a factor of eight to show the pixels better.

Note that there are 4 pixels at the top and 6 pixels on the right.

Since a circle is supposed to by symmetric this looks a little unusual.

Does anyone have an answer?

Am I calling the function Ellipse wrong?

Thank you.

Answer 1

Ellipse in GDI is not a real ellipse and uses PolyBezier

Direct2D Ellipse is better (with anti-aliasing)

A comparison GDI/Direct2D with the Ellipse sample you posted :

Original size :

Zoom :

Answer 2

I tried SetArcDirection( hdc, AD_CLOCKWISE ) and it seemed to work - but why?

Answer 3

As suggested I ran the program with SetArcDirection and size 46 pixels:

SetArcDirection( hdc, AD_CLOCKWISE );
Ellipse( hdc, 0, 0, 46, 46 );

The first picture is an actual size screen capture.

The second picture is enlarged by a factor of eight to show the pixels better.

Note that there are 6 pixels at the top and 4 pixels on the right.