I think you can do this in hexadecimal. To see this, compare the HEX output
from rgb(1,0,0) and rgb(1,0,0,0.5):
#FF0000
#FF000080
They differ only by the addition of the alpha digits hex 80 which are
(almost) half way between 00 and FF. Making a non-transparent scale
transparent is thus just a question of appending the appropriate digits:
paste(myScale,"80",sep="")
which worked well for my code. As you say, the in built scales support alpha
by default ending all their codes in FF.
James Sturman wrote>
> Hello,
>
> I can't seem to add transparency to any color ramp I create using
> colorRampPalette(). Can someone tell me if it is possible to create
> transparent colour ramps?
>
> I am attempting to map shaded relief under elevation data. I know I can
> use the terrain.colors() with the alpha option, but I would prefer to
> create my own colour ramp using more vibrant colours.
>
> My most recent attempt at doing this involved generating multiple rgb
> colours with transparency (alpha = 0.5) and then combining these colours
> using colorRampPalette(). I am able to use the resulting colour ramp, but
> it has no transparency.
>
> # Make terrain colour ramp
> rgb7 <- rgb(red=0, green=255, blue=77, alpha = 0.5, maxColorValue
> 255)
> rgb8 <- rgb(red=26, green=255, blue=0, alpha = 0.5, maxColorValue
> 255)
> rgb9 <- rgb(red=128, green=255, blue=0, alpha = 0.5, maxColorValue
> 255)
> rgb10 <- rgb(red=230, green=255, blue=0, alpha = 0.5,
maxColorValue
> = 255)
> rgb11 <- rgb(red=255, green=255, blue=0, alpha = 0.5,
maxColorValue
> = 255)
> rgb12 <- rgb(red=255, green=229, blue=60, alpha = 0.5,
maxColorValue
> = 255)
> rgb13 <- rgb(red=255, green=219, blue=119, alpha = 0.5,
> maxColorValue = 255)
> rgb14 <- rgb(red=255, green=224, blue=178, alpha = 0.5,
> maxColorValue = 255)
> rgb15 <- rgb(red=255, green=255, blue=255, alpha = 0.5,
> maxColorValue = 255)
> terraincolours <- colorRampPalette(c(rgb7, rgb8, rgb9, rgb10,
rgb11,
> rgb12, rgb13, rgb14, rgb15))
>
>
> # Plot map
> image(terrain, col = shadecolours(512), axes = TRUE) # plots the
> shaded relief
> #image(dem, col = terrain.colors(512, alpha = 0.5), add = TRUE) #
> this option works, but I don't like the colours in the terrain.colors
> pallet.
> image(dem, col = terraincolours(512), add = TRUE) # draws the
> elevation data with no transparancy and it won't accept an alpha value
> Thank you,
>
> James
>
> Please consider the environment before printing this email.
>
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Atmospheric
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