[R-SIG-Mac] How to generate 24-bit PNG on Mac OSX

[Prof Brian Ripley]

On Thu, 31 Jul 2008, Fán Lóng wrote:

> Hi there,
>
> When I am using the png() function as the following to generate a PNG
> file on Mac OSX Tiger, I found that the bit depth is 32.  But I need
> 24 bit depth PNG. What should I do?
>
>> png(filename="/Users/usg/Desktop/new.png")
>> plot(1:20)
>> dev.off()
>
>
> To avoid using "cairo", and R-2.7.0 does not find a Xlib on my Mac, I
> also tried this one:
>> png(filename="/Users/usg/Desktop/new.png", type ="quartz" )
>> plot(1:20)
>> dev.off()
>
> It does not work, either.
>
> The manual says "png will use a palette if there are less than 256
> colours on the page, and record a 24-bit RGB file otherwise (or a
> 32-bit RGBA file if type = "cairo" and non-opaque colours are used)."
>
> I don't have to set the palette less than 256 colours, I just want a
> true color png file.

32-bit _is_ 'true color'. I think you need to turn anti-aliasing off if 
you want only opaque colours.

In general though, if you make unreasonable requirements you need to be 
prepared to do some work yourself -- there are many options to postprocess 
32-bit PNG or palette PNG or PDF to other formats.

-- 
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 272866 (PA)
Oxford OX1 3TG, UK                Fax:  +44 1865 272595