R：使用colorRampPalette的透明色

Question

I have the following code that draws arrows with a gradient:

csa <- function(x1,y1,x2,y2,first.col,second.col,length=0.15, ...) {
  cols <- colorRampPalette( c(first.col,second.col))(250)
  x <- approx(c(0,1),c(x1,x2), xout=seq(0,1,length.out=251))$y
  y <- approx(c(0,1),c(y1,y2), xout=seq(0,1,length.out=251))$y

  arrows(x[250],y[250],x[251],y[251], col=cols[250],length=length, ...)
  segments(x[-251],y[-251],x[-1],y[-1],col=cols, ...)

}

color.scale.arrow <- Vectorize(csa, c('x1','y1','x2','y2') )

# Create sample data
x <- c(1,3,5,3,2,1,6,2)
y <- c(2,5,3,7,2,1,5,6)

x1 <- c(1,3,5,3)
y1 <- c(2,5,3,7)
x2 <- c(2,1,6,2)
y2 <- c(2,1,5,6)

# Plot sample data
plot(x,y, main='')
color.scale.arrow(x1,y1,x2,y2,'#5F9EA0','#CD3333',lwd=2)

Which produced this Figure:

I want to make these lines transparent, but simply adding 50 to the colour code (ie proportion of transparency = 50%) does not work unfortunately:

# Plot sample data (transparent?)
plot(x,y, main='')
color.scale.arrow(x1,y1,x2,y2,'#5F9EA050','#CD333350',lwd=2)

Any idea why this doesn't work, and how to make these lines transparent?

Answer 1

I would add the transparency after you generate the gradient, not before. You'll need to do this within your csa function, so consider adding an alpha argument to both csa and color.scale.arrow . Assuming you have done so, you can add the alpha transparency to the color gradient after generating it:

cols <- colorRampPalette( c(first.col,second.col))(250)
cols <- paste0(cols, alpha)

Also worth noting the alpha transparency is also hexadecimal: so "50" is not 50%, "7f" is.