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.
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. >>>>>> >>>>>> >>>>> ______________________________________________ >>>>> R-help@stat.math.ethz.ch mailing list >>>>> https://stat.ethz.ch/mailman/listinfo/r-help >>>>> PLEASE do read the posting guide >>>>> http://www.R-project.org/posting-guide.html >>>>> and provide commented, minimal, self-contained, reproducible code. >> >> > ______________________________________________ R-help@stat.math.ethz.ch mailing list https://stat.ethz.ch/mailman/listinfo/r-help PLEASE do read the posting guide http://www.R-project.org/posting-guide.html and provide commented, minimal, self-contained, reproducible code.
