[R] Colour depth reduced to 12 by bitmap devices

[Prof Brian D Ripley]

On Thu, 29 Jun 2000, Clive Jenkins wrote:

> I am using R to create colour plots as bitmap files. I have created a
> palette of 256 colours that I want to use, but the resultant output
> files contain only 12 colours used in dither patterns. The result is the
> same whether I use bmp() or png().

What about bitmap()?  Have you tried that? 

Internally bmp/png/jpeg use take a copy of the screen, so what you get
depends on your graphics card driver. If you are seeing dithering in the
PNG file, do you see it on screen?  Is your screen set to a truecolour
mode?

bitmap() works via ghostscript.

> How can I get the true colours that I specified instead of dither
> patterns? (making the driver's job easier!)
> 
> I am using R 1.1.0 for Windows (rw1010). The attached files are:-
> 
>  src256.R     Test program to be run by source("src256.R")
>  col_256.R    Colour palette read by src256.R
>  col_256.dat  Test data representing a 16x16 square image using
>               all 256 colours
>  Rplot.png    The resulting bitmap.
> 
> If anyone can improve on my code in src256.R, then please do so!
> 
> A further problem is that I cannot reduce the point size of the tick
> labels (character strings in this case) using any of:-
> 
>  par(cex=0.2)
>  par(cex.axis=0.2)
>  axis(..., cex=0.2, ...)
>  axis(..., cex.axis=0.2, ...)
> 
> One final observation: in accordance with polymorphism of R, I would
> prefer the function image() to take a data frame parameter directly,
> e.g. image(a.df), so there would be no need to extract the row and
> column names and pass them separately. Unfortunately this cannot be done
> while retaining backward compatibility because the essential data item
> is the third parameter not the first. As a messy compromise, perhaps
> image() could be extended to accept image(,,a.df) where `a.df' is a data
> frame. 

I just don't see how this can work. image requires x, y and z, and 
x and y are intended to be one longer than the rows and columns of the
matrix z.  That the equal-length case works is a historical anomaly,
and only makes sense (to me) for reqularly spaced grids.

You are quite at liberty to write a `myimage' function to your own
specification.

-- 
Brian D. Ripley,                  ripley at stats.ox.ac.uk
Professor of Applied Statistics,  http://www.stats.ox.ac.uk/~ripley/
University of Oxford,             Tel:  +44 1865 272861 (self)
1 South Parks Road,                     +44 1865 272860 (secr)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595

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