[vox-tech] [OT] Math question: help me make a rainbow

[Jeff Newmiller]

Bill Kendrick wrote:
> On Tue, Mar 31, 2009 at 07:43:19PM -0700, Jeff Newmiller wrote:
>> You seem to be trying to implement an algorithm that needs
>> three points with only two inputs... you are bound to fail.
>> One method I have seen is to click-drag-release to get the
>> two endpoints and then interactively pick up the mouse position
>> to track the height of the arc until a second click finishes
>> the input. You would have to back out the actual radius of
>> the circle if you needed it, but two inputs don't magically
>> become three.
> 
> I think that third point is the center of the circle, based on
> the normal off the vector between P1,P2, where it intersects
> the segment (x1,(max(y1,y2))) and (x2,(max(y1,y2)))
> 
> maybe? ;)

Is this [1] what you are thinking of?

[1] http://home.comcast.net/~jdnewmil/puzzle/rainbow_draw.gnumeric

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