[R-sig-Geo] image, SpatialGridDataFrame, transparency

[Patrick Giraudoux]

Great !

Thanks !

Patrick

Roger Bivand a écrit :
> On Thu, 5 Aug 2010, Patrick Giraudoux wrote:
>
>> Hi,
>>
>> I was wondering if it is possible to manage color transparency with 
>> image.SpatialGridDataFrame ? Eg to export png that could be imported 
>> into Google Earth in the way indicated in Bivand et al. pp 97-98 
>> (functions kmlOverlay and GE_SpatialGrid of maptools). If yes how ?
>
> NAs are set transparent by default (on output devices supporting 
> transparency). Other values may be given an alpha of less than 1 - 
> here a single value for all:
>
> library(sp)
> data(meuse.grid)
> coordinates(meuse.grid) = c("x", "y")
> gridded(meuse.grid) = TRUE
> image(meuse.grid["dist"], col=heat.colors(12),
>  main = "Distance to river Meuse")
> image(meuse.grid["dist"], col=heat.colors(12, alpha=0.5),
>  main = "Distance to river Meuse")
>
> By setting the col= vector manually, it is possible to assign a 
> different alpha value for each element, but it may be difficult to 
> obtain a satisfactory visual impression consistently across output 
> devices. See col2rgb() and rgb(). See also the colorspace package for 
> a sophisticated treatment, and the use of hcl palettes, and the hsv() 
> function. All of these can help in constructing a colour vector to 
> pass to image.SpatialGridDataFrame().
>
> Hope this helps,
>
> Roger
>
>>
>> Patrick
>>
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>>
>