Re: [R] to draw a smooth arc

From: Prof Brian Ripley <>
Date: Wed, 09 May 2007 05:54:17 +0100 (BST)

On Tue, 8 May 2007, Paulo Barata wrote:

> Prof. Ripley,
> Maybe the fact that few R plot regions have a 1:1 aspect ratio
> is not a major problem here. One has to deal with the same
> issue when drawing a circle parametrically. Depending on the
> window size, a (cos(t),sin(t)) plot appears as an ellipse.
> To get a circle parametrically, one has to resize the plot region,
> or define it (possibly by trial and error) with functions like
> windows() or win.graph() (package grDevices), to have a 1:1 aspect
> ratio.

You miss the point: those graphics languages which support circular arcs do so in device space, not user space.

> Maybe more important is to have some sure way to draw smooth arcs
> and arrows. Could it be done even if the plot region does not have
> a 1:1 aspect ratio, when the arc (intended to be an arc of a circle)
> would show as an arc of an ellipse? Would the smooth ellipse arc
> then turn into a smooth circle arc, when the plot region is resized?
> Paulo Barata
> (Rio de Janeiro - Brazil)
> -----------------------------------------------------------------------
> Prof Brian Ripley wrote:
>> There is now an xspline() function in R-devel, with an example showing how
>> to add arrows.
>> I thought a bit more about a 'circular arc' function, but there really is a
>> problem with that. Few R plot regions have a 1:1 aspect ratio including
>> some that are intended to do so (see the rw-FAQ). symbols() is designed to
>> draw circles in device coordinates, but attempting to specify circular arcs
>> by endpoints in user coordinates is fraught.
>> On Wed, 2 May 2007, Paul Murrell wrote:
>>> Hi
>>> Paulo Barata wrote:
>>>> Dr. Murrell and all,
>>>> One final suggestion: a future function arc() in package graphics,
>>>> with centre-radius-angle parameterisation, could also include an
>>>> option to draw arrows at either end of the arc, as one can find
>>>> in function arrows().
>>> ... and in grid.xspline() and grid.curve().
>>> Paul
>>>> Thank you.
>>>> Paulo Barata
>>>> -----------------------------------------------------------------------
>>>> Paul Murrell wrote:
>>>>> Hi
>>>>> Paulo Barata wrote:
>>>>>> Dr. Snow and Prof. Ripley,
>>>>>> Dr. Snow's suggestion, using clipplot (package TeachingDemos),
>>>>>> is maybe a partial solution to the problem of drawing an arc of
>>>>>> a circle (as long as the line width of the arc is not that large,
>>>>>> as pointed out by Prof. Ripley). If the arc is symmetrical around
>>>>>> a vertical line, then it is not so difficult to draw it that way.
>>>>>> But an arc that does not have this kind of symmetry would possibly
>>>>>> require some geometrical computations to find the proper rectangle
>>>>>> to be used for clipping.
>>>>>> I would like to suggest that in a future version of R some function
>>>>>> be included in the graphics package to draw smooth arcs with
>>>>>> given center, radius, initial and final angles. I suppose
>>>>>> that the basic ingredients are available in function "symbols"
>>>>>> (graphics).
>>>>> Just to back up a few previous posts ...
>>>>> There is something like this facility already available via the
>>>>> grid.xspline() function in the grid package. This provides very
>>>>> flexible curve drawing (including curves very close to Bezier curves)
>>>>> based on the X-Splines implemented in xfig. The grid.curve() function
>>>>> provides a convenience layer that allows for at least certain
>>>>> parameterisations of arcs (you specify the arc end points and the
>>>>> angle).
>>>>> These functions are built on functionality within the core graphics
>>>>> engine, so exposing a similar interface (e.g., an xspline() function)
>>>>> within "traditional" graphics would be relatively straightforward.
>>>>> The core functionality draws the curves as line segments (but
>>>>> automatically figures out how many segments to use so that the curve
>>>>> looks smooth); it does NOT call curve-drawing primitives in the
>>>>> graphics device (like PostScript's curveto).
>>>>> In summary: there is some support for smooth curves, but we could still
>>>>> benefit from a specific arc() function with the standard
>>>>> centre-radius-angle parameterisation and we could also benefit from
>>>>> exposing the native strengths of different graphics devices (rather than
>>>>> the current lowest-common-denominator approach).
>>>>> Paul
>>>>>> Thank you very much.
>>>>>> Paulo Barata
>>>>>> (Rio de Janeiro - Brazil)
>>>>>> -----------------------------------------------------------------------
>>>>>> Prof Brian Ripley wrote:
>>>>>>> On Tue, 1 May 2007, Greg Snow wrote:
>>>>>>>> Here is an approach that clips the circle you like from symbols down
>>>>>>>> to
>>>>>>>> an arc (this will work as long as the arc is less than half a circle,
>>>>>>>> for arcs greater than half a circle, you could draw the whole circle
>>>>>>>> then use this to draw an arc of the bacground color over the section
>>>>>>>> you
>>>>>>>> don't want):
>>>>>>>> library(TeachingDemos)
>>>>>>>> plot(-5:5, -5:5, type='n')
>>>>>>>> clipplot( symbols(0,0,circles=2, add=TRUE), c(0,5), c(0,5) )
>>>>>>> I had considered this approach: clipping a circle to a rectangle isn't
>>>>>>> strictly an arc, as will be clear if the line width is large.
>>>>>>> Consider
>>>>>>> clipplot(symbols(0, 0 ,circles=2, add=TRUE, lwd=5), c(-1,5), c(-1,5))
>>>>>>> Note too that what happens with clipping is device-dependent. If R's
>>>>>>> internal clipping is used, the part-circle is converted to a polygon.
>>>>>> ______________________________________________
>>>>>> mailing list
>>>>>> PLEASE do read the posting guide
>>>>>> and provide commented, minimal, self-contained, reproducible code.

Brian D. Ripley,        
Professor of Applied Statistics,
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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