Re: [Groff] Problems with arcs and angles

[Jones, Larry]

Sorry, I just noticed that your drawing canvas measures the angles clockwise 
rather than the traditional counterclockwise so you need to negate the y 
coordinates to atan2:

startAngle = Math.atan2(centreY - startY, startX - centreX)
endAngle = Math.atan2(centreY - endY, endX - centreX)

-Larry

> -----Original Message-----
> From: Jones, Larry (DF PL PDS EI US)
> Sent: Friday, April 28, 2017 10:27 AM
> To: 'John Gardner' <address@hidden>
> Subject: RE: [Groff] Problems with arcs and angles
> 
> startAngle = atan2(startY - centreY, startX - centreX)
> endAngle = atan2(endY - centreY, endX - centreX)
> radius = sqrt((startY - centreY) * (startY - centreY) + (startX - centreX) *
> (startX - centreX))
> 
> -Larry
> 
> > -----Original Message-----
> > From: Groff [mailto:groff-
> address@hidden
> > On Behalf Of John Gardner
> > Sent: Friday, April 28, 2017 1:32 AM
> > To: groff <address@hidden>
> > Subject: [Groff] Problems with arcs and angles
> >
> > Hello!
> >
> > Some time ago, I announced my intent to build a man-page previewer for a
> > text-editor named Atom <https://atom.io/>. Work is coming along nicely:
> > this page <https://cdn.rawgit.com/Alhadis/language-
> > roff/a7c07744a9d44adf32a546646f7a8d57d52e6e58/preview-tty.html>
> > was generated using a lightweight tokeniser
> > <https://github.com/Alhadis/language-
> roff/blob/renderer/lib/tokeniser.js>
> > written in JavaScript, which processes Groff's intermediate output
> language
> > into web-based formats. That's the nroff preview... I'm now halfway
> through
> > developing the troff previewer, which uses HTML5 canvas
> > <https://developer.mozilla.org/en-
> > US/docs/Web/API/CanvasRenderingContext2D>
> > technology to draw realtime previews of an opened Roff document.
> >
> > So far, so good!
> >
> <https://cloud.githubusercontent.com/assets/2346707/25514878/732e09ea-
> > 2c23-11e7-97ea-9674d3845dd1.png>
> > I'm
> > proud of how this is turning out: bear in mind, that preview updates as the
> > user types, so it'll feel like an efficient replacement for a word 
> > processing
> > program, haha.
> >
> > I'm currently stuck with drawing arcs
> > <https://developer.mozilla.org/en-
> > US/docs/Web/API/CanvasRenderingContext2D/arc>.
> > This
> > might be the wrong forum to ask for help with trigonometry, but I was
> > planning to announce progress eventually anyway.
> >
> > Now, this pic code:
> >
> > .PS
> > > "+" at 0,0
> > > arc -> from 0.5,0 to 0,0.5
> > > arc -> cw from 0,0 to 1,0.5
> > > arc -> cw from 0,0 to 2,0 rad 15
> > > .PE
> >
> >
> > ... should look like this
> >
> <https://cloud.githubusercontent.com/assets/2346707/25515071/4bc46870-
> > 2c25-11e7-9883-9248e4b4aa68.png>.
> > Instead, it, uh, looks like this
> >
> <https://cloud.githubusercontent.com/assets/2346707/25515084/5d84eed6-
> > 2c25-11e7-8cdd-bdb65f7804e9.png>
> > .
> >
> > Groff's output gives me these coordinates to go by:
> >
> >    - startX, startY - Coordinates of the arc's starting point
> >    - centreX, centreY - Coordinates of the arc's centre
> >    - endX, endY - Coordinates of the arc's terminal point
> >
> > But the canvas arc method I'm working with requires all of these:
> >
> >    - x - The x coordinate of the arc's centre.
> >    - y - The y coordinate of the arc's centre.
> >    - radius - The arc's radius.
> >    - startAngle - The angle at which the arc starts, measured clockwise
> >    from the positive x axis and expressed in radians.
> >    - endAngle - The angle at which the arc ends, measured clockwise from
> >    the positive x axis and expressed in radians.
> >
> > It's embarrassing to be stuck on something so obvious, but this is what's
> > blocking further progress...