How to generate 24-bit PNG on Mac OSX

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.

Any help is appreciated.

Thanks in advance.
Long