[R] to draw a smooth arc

[Prof Brian Ripley]

On Tue, 1 May 2007, Jim Lemon wrote:

> Paulo Barata wrote:
>> Dear R-list members,
>>
>> I would like to draw a smooth arc. I can draw an arc
>> parametrically, but this produces an arc too coarse,
>> even allowing for different increments in sequence t
>> in the example below. Function "symbols" (graphics) does
>> produce a smooth circle, but it cannot produce an arc.
>>
>> Please see the following example, drawing complete circles:
>>
>> plot(-5:5,-5:5,type='n')
>> ## draws circle with function symbols (package graphics)
>> ## - inner circle is very smooth:
>> symbols(0,0,circles=2,add=TRUE)
>> ## draws circle parametrically - outer circle is too coarse:
>> pi <- 4*atan(1)
>> t <- seq(0,2*pi,0.02)
>> lines(4*cos(t),4*sin(t))
>>
>> Package "plotrix" has a function "draw.arc", but arcs produced
>> with this function are also either too coarse or too polygonal,
>> depending on the number of polygons used to approximate the arc.
>>
>> Is there a way to harness the characteristics of function
>> "symbols" (graphics) to draw a smooth arc, not just a complete
>> circle?
>>
> Hi Paulo,
> I may be misunderstanding you, but have you tried to increase the number
> of segments in the arc using the "n" argument?
>
> draw.arc(1,1,1,n=100)

Put it another way, drawing arcs is not a primitive in the R graphics 
system but drawing circles is.  So there is no low-level way to draw an 
arc of a circle except via line segments.  (Quite a few graphics devices 
draw circles via line segments, but not all and vector-graphics systems 
like postscript can often do better.)


-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595