Graham G - 4 months ago 24

R Question

Is there a straight forward way to plot a non-convex surface in R?

I have used something like the following for convex surfaces and it works fine:

`xyz <- cbind(y,x,z)`

tbr <- t(surf.tri(xyz, delaunayn(xyz)))

rgl.triangles(xyz[tbr,1], xyz[tbr,2], xyz[tbr,3])

However, for non-convex surfaces, concave areas become filled. I think this is a problem with the function

`delaunayn()`

Any suggestions appreciated.

P.S.

I have data as an ascii file but it has 3 columns and is 225 lines long. What is the best way of providing this?

Data available at: http://pastebin.com/R2p4Cf7d

The top of the plot should be concave! This is an image created using

`persp3d()`

Answer

The following solution is not perfect, but works if you have a copy of Matlab.

```
library(rgl)
library(geometry)
# Read in data - also at: http://pastebin.com/R2p4Cf7d
simDat <- read.csv("testDat.csv")
#
x <- simDat[,1];y<-simDat[,2];z<-simDat[,3]
xyz <- cbind(simDat[,1],simDat[,2],simDat[,3])
#
triNodes <- delaunayn(xyz)
tbr1 <- t(surf.tri(xyz, triNodes ))
# Plot data from R generated triangles
open3d()
rgl.triangles(xyz[tbr1,1], xyz[tbr1,2], xyz[tbr1,3])
#
# Import data generated by Matlab function delaunay()
# - also at: http://pastebin.com/vQV2Zaii
nodeDat_ML <- read.csv("testDatNodes.csv")
triNodes2 <- cbind(nodeDat_ML[,1], nodeDat_ML[,2], nodeDat_ML[,3],1)
#
tbr2 <- t(surf.tri(xyz, triNodes2))
# Plot data from Matlab generated triangles
open3d()
rgl.triangles(xyz[tbr2,1], xyz[tbr2,2], xyz[tbr2,3])
```

The code produces the following two surfaces. The left plot is based on triangulation generated by the R function delaunayn(), and the right plot is based on triangulation generated from the Matlab function delaunay().

The Matlab generated data is available at: http://pastebin.com/vQV2Zaii

and the associated Matblab code is:

```
fname = 'testDat.csv';
tt = table2array(readtable(fname)); % get data
x = tt(:,1);y = tt(:,2);z = tt(:,3);
tri = delaunay(x,y); % the triangulation data
trisurf(tri,x(:,1),y(:,1),z(:,1)); % surface plot
```

The result is not entirely perfect as the final plot (right) has a spurious triangle.

I hope the above will be useful to anyone with a similar problem.