Scan Converting Circles - The Mid-Point Technique

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Hi, just wondering if theres any graphics people out there who can scan convert circles in excel.
i'm trying to do it but can't remember how to make a spreadsheet generalised to any circle (with an origin => -10 and =< 10) and radius =<20....been trying for ages but can't for the life of me get what i've got (radius 16, origin 0,0) to be 'general'

 

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An alternative is to solve it for the radius 16 origin 0,0 option and then use transformations to apply it to other circles, instead of transforming the circle, you do the inverse transformation on the solution to the test.

that sounds like a good option...i just havent a clue how to implement it...anyway - i could do with remembering how you use an if statement to stop a line plotting (i need to stop a section before it runs into the path of the next section.)