title: SVG drawing not exactly the same circle with the same path but different color
body: <p>I am using pycairo, below is the full snippet: </p>
  
  <pre><code>from math import pi
  from cairo import PDFSurface, Context, Matrix
  WIDTH = 6 * 72
  HEIGHT = 4 * 72
  s = PDFSurface('example1.pdf', WIDTH, HEIGHT)
  c = Context(s)
  # Draw a circle
  c.save()
  c.set_line_width(0)
  for i in range(1000): 
      c.arc(1 * 72, 3 * 72, 0.5 * 72, 0, 2 * pi)
      c.set_source_rgb(1.0, 0, 0)
      c.fill()
      c.arc(1 * 72, 3 * 72, 0.5 * 72, 0, 2 * pi)
      c.set_source_rgb(1.0, 1.0, 1.0)
      c.fill()
  s.finish()
  </code></pre>
  
  <p>I expected the resulted pdf to be blank. But there's a red-circle as you can see. I know it's caused by anti-aliasing, and the same question was asked long time ago here:
  <a href="https://stackoverflow.com/questions/30002698/two-exactly-same-svg-paths-not-drawn-exactly-the-same-way">Two exactly same SVG paths not drawn exactly the same way</a>
  But the answer didn't explain how the anti-aliasing caused that effect. </p>
  
  <p>It will be appreciated if somebody could tell me <em>how</em> it happened. </p>
  
  <p>Thanks! </p>