Re: [R] Transparent backgrounds in png files

From: Patrick Connolly <>
Date: Fri 30 Jul 2004 - 10:24:20 EST

On Thu, 29-Jul-2004 at 08:38AM +0100, Prof Brian Ripley wrote:

|> The bitmap() device does not support transparency. The png() device does.

Unfortunately, though png() does a fine job at a transparent background, it's rather lumpy even on a screen.

|> On Thu, 29 Jul 2004, Patrick Connolly wrote:

|> > Mine is the reverse (and I'm using standard graphics, not Lattice).
|> > I'm trying to get a transparent background but it always comes out
|> > white. Setting bg = "transparent", I've tried using a bitmap device
|> > to create a png file. I've also tried creating a postscript file and
|> > converting it to a PNG file using the Gimp. I've always used a
|> > resolution of 300 dpi in bitmaps since the default is far too low.
|> Really? You want PNG files of 2000+ pixels in each dimension?

Well, 300 dpi is somewhat excessive for onscreen, but not for printing (more below). For a screen at 1600 by 1200 resolution, a bitmap of over 1000 pixels in either direction is not excessive. Using a screen rated at .25mm dot pitch, 75dpi is rather a lot less than sufficient. According to my calculations, .25mm dot pitch corresponds to over 100 dpi, and a .27mm screen is over 90 dpi, so I don't get this 72 business. Perhaps there's something I need to know.

Evidently, there's something others know that I don't since png() generated files always turn out lumpy for me. It's worse than the unsatisfactory result of using PowerPoint's turning colours to transparent method I mentioned. People who are used to looking at TV screens might not think it's low resolution, so perhaps I'm too fussy.

Maybe I should be more fussy about getting an exact ratio between the number of pixels in the plotting device and the size of the image in PowerPoint. I'm somewhat confused by the fact that PP scales to fit to the slide PNG files that I produce using the Gimp, but not ones made using the png() method directly. What is the essential difference?

|> -- and you should not really be using bitmapped files for other
|> uses.)

Unfortunate as it may be, many people wish to put graphics in Word files and don't like being unable to see their graphics on their screen even if they have a postscript printer that could print them perfectly. That's where I use 300 dpi PNGs which print at least as well as WMFs I've seen.

There was a recent discussion on this list about graphics using OSX which covers most of the same thinking. Nothing in that discussion indicated to me a better way to get graphic files from Linux to Word. If there are any, I'd like to know about them.


Patrick Connolly
Mt Albert
New Zealand 
Ph: +64-9 815 4200 x 7188
I have the world`s largest collection of seashells. I keep it on all
the beaches of the world ... Perhaps you`ve seen it.  ---Steven Wright 

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Received on Fri Jul 30 10:28:36 2004

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