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Image Conversions for Astronomers:
Postscript vs. Powerpoint
by Mark Cornell
4/2/04
As you know, I have looked into the question of how to insert a line
drawing made by sm, originally stored as a Postscript file, into a
Powerpoint presentation, preserving as much resolution as possible. The
previous method using JPEGs as an intermediate format produced Powerpoint
images that were too fuzzy (because of the inherently lossy nature of JPEG
image compression). I believe that I have found an acceptable scheme.
Note that I made no attempt to also make the output file as small as
possible, but they do come out fairly small anyway.
The bottom line is to take the Postscript file, transfer it to a Mac, read
it into Adobe Illustrator, and then select File...Export. Save the file in
PNG format, specifying High (300 ppi) Resolution with a background color of
White,  making sure that "Anti-Alias" is selected, and that "Interlaced" is
not selected. The resulting PNG file can be directly inserted as a picture
into Powerpoint (and most other modern software), and produces a nice
result when scaled inside Powerpoint. It is not necessary to try to adjust
the size or resolution of the image further, before inserting into
Powerpoint. Rasterizing at 300dpi is necessary to preserve the original
Postscript file's native resolution. One could select "Interlaced" images
for output instead. This makes bigger images, but ones that can be
displayed more nicely in successive scans on a web page over a slow network
link. I don't know if Powerpoint would take any advantage of that.
Note that Illustrator is better for this purpose than Photoshop, because
Illustrator is vector-graphics-based, while Photoshop is
bitmap-graphics-based. Illustrator does not rasterize the image until you
execute the Export command, and then does so exactly as you specify, once.
Photoshop, on the other hand, rasterizes the image immediately upon input,
and it is easy to end up further interpolating the pixels, possibly
reducing resolution even more. If you must run Photoshop on a Postscript
image, be sure to rasterize at a high resolution (like 300dpi) when you
first input the image. Do not take the default resolution of 72dpi.
PNG format is the replacement for GIF, and has been around since 1995. PNG
is now supported in our latest patched xv (as of today), and in many other
programs such as Mozilla, Adobe Illustrator, Adobe Photoshop, Microsoft
Powerpoint, etc., so in principle one could use it instead of JPEG format.
However PNG's lossless compression often produces files that are bigger
than JPEG's inherently lossy compression, and JPEG remains the recommended
format for actual pictures.
To reiterate, PNGs are not a particularly good way to make image files
smaller, although they are smaller than GIFs. Their lossless compression
usually results in larger files than JPEGs. PNGs are a way to make high
quality intermediate files that can be inserted into other graphics
applications, like Powerpoint. If you already have a JPEG file, there is no
point in converting it to a PNG file, since you have already lost
resolution in the original JPEG compression.
Related topic:
If your original image is a JPEG file, and you want to make a SMALL (and
hence lower quality) Postscript file for insertion into a TeX document
destined for astro-ph, then do the following. Load the JPEG file into xv
on a Sun, and re-save it as a JPEG file, but with the quality set to 50%.
Then convert the compressed JPEG into a Postscript file with the command:
jpeg2ps -h whatever.jpg > whatever.eps
If your original image is a LARGE Postscript file, and you want to make a
SMALL Postscript file for insertion into a TeX document destined for
astro-ph, then try the following:
ps2ps input.ps output.ps
gzip output.ps
This Postscript-rewriting program can be quite amazing:
Original file:
-rw-r--r-- 1 cornell 11 708592 Mar 30 10:48 n1353phot.ps
Just gzipping that produces a result that is too big for astro-ph:
-rw-r--r-- 1 cornell 11 151042 Mar 30 10:48 n1353phot.ps.gz
Instead run ps2ps on it to make test.ps:
-rw-r--r-- 1 cornell 11 458026 Apr 2 15:10 test.ps
and run gzip to compress a lot:
-rw-r--r-- 1 cornell 11 38805 Apr 2 15:10 test.ps.gz
The result is just fine for astro-ph and I can't tell the difference
between it and the original when printed.
There are all-Mac and all-UNIX variants of the above steps.
Dr. Mark Cornell is the former head of McDonald Observatory Computing Services.
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