R plot with color gradient

Question

Below is a 3D curve using the R function persp3d and specifying the colors.

library(rgl)

y = seq(-5,25,by=0.1)
x = seq(5,20,by=0.2)

NAs <- rep(NA, length(x)*length(y))
z <- matrix(NAs, length(x), byrow = T)
for(i in seq(1,length(x))) {
    for(j in seq(1,length(y))) {
        val = x[i] * y[j]
        z[i,j] = val
        if(z[i,j] < 0.02) {
            z[i,j] = NA
        }

    }
}

col <- rainbow(length(x))[rank(x)]

open3d()
persp3d(x,y,z,color=col,xlim=c(5,20),ylim=c(5,10),axes=T,box=F,xlab="X Axis",ylab="Y Axis",zlab="Z Axis")

And it produces this image:

In the current version, for an x value of 15, the color is blue regardless of the z value. But I'd like it so that high z values are dark blue whereas low z values are light blue, if that makes sense. How can I do something like this, so that color not only distinguishes x values but also z values?

Answer 1

You will have to play around with colors in HSV format rather than RGB format for this. It's easier that way I think.

See my sample code below.

library(rgl)

y = seq(-5,25,by=0.1)
x = seq(5,20,by=0.2)

NAs <- rep(NA, length(x)*length(y))
z <- matrix(NAs, length(x), byrow = T)
for(i in seq(1,length(x))) {
  for(j in seq(1,length(y))) {
    val = x[i] * y[j]
    z[i,j] = val
    if(z[i,j] < 0.02) {
      z[i,j] = NA
    }

  }
}

Create unique color for each value of x.

col <- rainbow(length(x))[rank(x)]

Create grid of colors by repeating col length(y) times

col2 <- matrix(rep(col,length(y)), length(x))
for(k in 1:nrow(z)) {

  row <- z[k,]
  rowCol <- col2[k,]  
  rowRGB <- col2rgb(rowCol) #convert hex colors to RGB values
  rowHSV <- rgb2hsv(rowRGB) #convert RGB values to HSV values

  row[is.na(row)] <- 0
  v <- scale(row,center=min(row), scale=max(row)-min(row)) # scale z values to 0-1

  rowHSV['s',] <- v #update s or v values by our scaled values above
  # rowHSV['v',] <- v  # try changing either saturation or value i.e. either s or v

  newRowCol <- hsv(rowHSV['h',], rowHSV['s',], rowHSV['v', ]) #convert back to hex color codes
  col2[k,] <- newRowCol #Replace back in original color grid
}

open3d()
persp3d(x,y,z,color=col2,xlim=c(5,20),ylim=c(5,10),axes=T,box=F,xlab="X Axis",ylab="Y Axis",zlab="Z Axis")

This should give following. You can play around scaling of saturation or value of colors to get desired "lightness" or "darkness" of shades.