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

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

```Tux Paint has a magic tool called "Rainbow."  It's not really, it's just
a color-cycling circle-shaped paintbrush.  A few years ago, a couple of
the other TP developers joked that we need a tool to draw real rainbows.
(Draw an arc, and the rainbow gradient appears over it.)

So I've decided it might be fun to take a stab at it.  My main problem is
I'm rusty with math (trig., geometry, etc. ... you don't use those much in PHP
or MySQL. ;) Heck, you don't use them much in most of the kinds of games I've
worked on over the years.)

Here's how I imagine the UI working.  Click mouse to select one endpoint
of the rainbow.  Drag to where you want the other endpoint, and release
mouse.  Rainbow gets drawn.

The rainbow would be an upside-down-"U" shape (with a 1:1 aspect, e.g.
it'd be a portion of a perfect circle).  I think for artistic purposes,
it shouldn't always be a complete 180-degree arc, though.  So here's
how I complicate the UI :)

P1 (aka x1,y1) is where you click.  P2 (aka x2,y2) is where you release.

0,0 is the top left of the screen.  Angles go from 0 (bottom/right of arc)
thru 90 (top/center of arc) to 180 (bottom/left of arc).

If y1 >= y2, then P1 represents the 180th degree of the arc
(going counter-clockwise from the far right of the circle), and
P2 represents the Nth degree of the arc (where 0 <= N < 180)

If y1 < y2, then P1 represents the Nth degree of the arc,
and P2 represents the 0th degree.

/==P2
/
|
P1

as well as the full semi-circular arc:

/==\
/    \
|    |
P1    P2

and of course, if P2 is lower, then:

/==\
P1   \
|
P2

In both cases, there's an invisible P3, which is at 0 degrees or 180 deg.,
depending on which point ended up being lower.  The center of the circle
is not necessarily exactly at (x1+((x2-x1)/2) , max(y1,y2)) which is where
I'm getting stuck.

My initial though was to find the vector V between P1 and P2.
Going off at the normal of V, from its center, I believe I get a line
that strikes the center of my would-be circle, when it crosses the
max(y1,y2) height down the screen.

intersect tests, and starting to think there's a simpler way.

So, given two points on an arc of a circle, P1 and P2, how do I find
the center and radius of the circle?  (Then all I need to do is determine
the angle (N) of one of the points (P1 or P2), while I know that the
angle of the other (P2 or P1) is 0 or 180, depending on my UI rules above.

Whew.  Did I make ANY sense? :)

--
-bill!
"Tux Paint" - free children's drawing software for Windows / Mac OS X / Linux!