Results 1 to 5 of 5

Math Help - eight way symmetry of circle

  1. #1
    Newbie
    Joined
    Jan 2011
    Posts
    2

    eight way symmetry of circle

    Hi guys
    Please explain to me the eight way symmetry of a circle.I am learning computer graphics and it is an important concept to draw circles.I have tried searching in google but I could not grasp the concept.
    thank you
    Follow Math Help Forum on Facebook and Google+

  2. #2
    A Plied Mathematician
    Joined
    Jun 2010
    From
    CT, USA
    Posts
    6,318
    Thanks
    4
    Awards
    2
    I'm aware of the following symmetries of the circle:

    1. Rotation about its center through any angle whatsoever. Doing this just gives your circle back again.
    2. Reflection about any diameter of the circle. Again, you'll just get the circle back again.

    The phrase "eight way symmetry" is therefore puzzling to me.
    Follow Math Help Forum on Facebook and Google+

  3. #3
    MHF Contributor
    Joined
    Oct 2008
    Posts
    1,035
    Thanks
    49
    Quote Originally Posted by exert View Post
    Hi guys
    Please explain to me the eight way symmetry of a circle.I am learning computer graphics and it is an important concept to draw circles.I have tried searching in google but I could not grasp the concept.
    thank you
    What's special about relections in four particular of all the possible diameters, and then four more from the resulting images, is that they capitalise on the coordinate system to save work. Reflection in the x-axis is achieved by trading the y coordinate of each point for its negative. Reflection in the line y=x by swapping x and y values for each other. And similarly, see what does for reflecting in the lines x=0 and y = -x.

    So, if you draw a circle by specifying, say, 80 points/pixels as pairs of (x,y) coordinates so that the circle is centred at the origin (0,0), then happily you only have to do any real work to specify the 10 of them that define one eighth of the circle - say, the arc between the x axis and the line y=x. Then for each of these ten you'll get 7 more in corresponding positions on the grid, by trading and swapping. Good illustration here, Computer Graphics : Circle drawing : 5 / 8 : 8-way symmetry algorithm.
    Last edited by tom@ballooncalculus; January 29th 2011 at 10:20 AM.
    Follow Math Help Forum on Facebook and Google+

  4. #4
    Newbie
    Joined
    Jan 2011
    Posts
    2
    If between x-axis and y=x the point is (x,y) then between the lines y=x and the y-axis the coordinate of a point is (y,x).Continuing in the anticlock direction in the next octant the point is (-y,x) and then (-x,y),(-x,-y),(-y,-x),(y,-x),(x,-y).I want to know how these points have been found because in basic coordinate geometry any point in the first quadrant has coordinate (x,y),2nd quad (-x,y),3rd quad (-x,-y) and fourth quad (x,-y).Is there some different rule for octants.I hope my question is clearer now.Please help me.
    THANK YOU
    Follow Math Help Forum on Facebook and Google+

  5. #5
    Junior Member
    Joined
    Sep 2010
    Posts
    40
    Quote Originally Posted by exert View Post
    If between x-axis and y=x the point is (x,y) then between the lines y=x and the y-axis the coordinate of a point is (y,x).

    (x,y) changes into (y,x) by reflection in y=x, so now the x coordinate of the point is y and the y coordinate is x. For example (2,1) is transformed into (1,2) by reflection in the line y=x.


    Quote Originally Posted by exert View Post
    Continuing in the anticlock direction in the next octant the point is (-y,x)
    Now we take the starting point as being (y, x) which by reflection in the y axis becomes (-y, x). That is, the point (1,2) is transformed into (-1,2) by reflection in the y-axis.

    etc.
    Follow Math Help Forum on Facebook and Google+

Similar Math Help Forum Discussions

  1. Symmetry
    Posted in the Pre-Calculus Forum
    Replies: 3
    Last Post: September 2nd 2009, 04:57 PM
  2. Symmetry
    Posted in the Pre-Calculus Forum
    Replies: 5
    Last Post: December 11th 2007, 05:32 PM
  3. Symmetry
    Posted in the Geometry Forum
    Replies: 2
    Last Post: May 8th 2006, 02:31 PM
  4. Symmetry
    Posted in the Geometry Forum
    Replies: 2
    Last Post: May 8th 2006, 02:31 PM
  5. Symmetry
    Posted in the Geometry Forum
    Replies: 2
    Last Post: May 8th 2006, 02:31 PM

Search Tags


/mathhelpforum @mathhelpforum