Show a basic storage type to handle a simple RGB raster graphics image,
and some primitive associated functions.

If possible provide a function to allocate an uninitialised image,
given its width and height, and provide 3 additional functions:

   one to fill an image with a plain RGB color,
   one to set a given pixel with a color,
   one to get the color of a pixel.


(If there are specificities about the storage or the allocation, explain those.)

These functions are used as a base for the articles in the category raster graphics operations, 
and a basic output function to check the results
is available in the article write ppm file.




#  See the class definitions and constructors with, e.g. 
getClass("pixmapIndexed", package=pixmap)
pixmapIndexed
 
# Image with all one colour
plot(p1 <- pixmapIndexed(matrix(0, nrow=3, ncol=4), col="red"))
 
# Image with one pixel specified
cols <- rep("blue", 12); cols[7] <- "red"
plot(p2 <- pixmapIndexed(matrix(1:12, nrow=3, ncol=4), col=cols))
 
# Retrieve colour of a pixel
getcol <- function(pm, i, j)
{
   pmcol <- pm@col
   dim(pmcol) <- dim(pm@index)
   pmcol[i,j]   
}
getcol(p2, 3, 4)  #red