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Several of the plot output modes
write the plot in some graphics format or other.
When selecting an output format it is important to understand the
distinction between bitmapped and vector formats;
basically bitmapped formats represent the image as a grid of finite-sized
pixels while vector formats notionally draw smooth lines.
Bitmapped formats are fine for a computer screen, but for high quality
paper printouts you will want a vector format.
You can convert from vector to bitmapped but not (usefully) in the
other direction. There are a couple of subtleties to this distinction
specific to STILTS graphical output as discussed below.
The following formats are the available values for the
ofmt
parameter of the various plot commands:
-
png
- PNG format.
This is a flexible bitmapped format providing transparency
and an unlimited number of colours with good lossless compression.
It is widely supported by non-ancient browsers and other image viewers,
and is generally recommended for bitmapped output.
-
gif
- GIF format.
This is a bitmapped format providing transparency and lossless
compression.
The number of colours is limited to 255 however, so if you are using
auxiliary axes (colour variation to represent higher dimensionality)
or other plot features which use a wide range of colours you may see
image degradation.
It has long been widely supported by browsers and other image viewers.
-
jpeg
- JPEG format.
This is a bitmapped format with lossy compression
intended primarily for photographs.
Transparency is not supported, and although there is no limit on the
maximum number of colours, its lossiness means that plots generated
using it generally look a bit smudged.
-
pdf
- Portable Document Format.
This is the format which can be read by Adobe's Acrobat Reader.
It is a widely portable vector format, and is suitable for printing
at high resolution, either standalone or imported into some other
presentation format.
However, there are a couple of caveats when it comes to using it with
STILTS plots.
- If used to plot a very large number of points, the output PDF file
can get quite large, though it's much better than for
eps
output (see below).
- For certain colour shading options
(auto,
density,
and in some circumstances transparency),
the body of the plot will
be drawn as a bitmap rather than vector graphics.
This is sometimes a blessing in disguise since with very large numbers
of points a vector PDF file could get unmanageably large in any case.
In this case the interior of the plot will be pixellated.
The axes and annotations outside of the
plot will still be drawn in vector format however.
-
svg
- Scalable Vector Graphics.
This is an XML-based vector graphics format developed for display
in web pages, and
defined by the W3C.
This exporter can generate
OutOfMemoryErrors if asked to generate a large output file.
-
eps
- Encapsulated Postscript.
This is a vector format which is suitable for printing at high resolution
either standalone or imported into some other presentation format
(you may need to convert it via PDF depending on the intended destination).
However, there are a couple of caveats when it comes to using it with
STILTS plots.
- Unfortunately the postscript driver used by STILTS is not very
efficient and can result in large, sometimes very large, postscript
output files. This is likely to be a problem for plots with a large
number of non-transparent points.
For this reason
eps-gzip
or pdf
may be
a better choice.
- Postscript has no support for partial transparency, so if plots
are drawn with partially transparent points (common for very large
data sets) the only way they can be rendered is by drawing the body
of the plot as a bitmap rather than as vector graphics.
This is sometimes a blessing in disguise since with very large numbers
of points a vector postscript file would likely be unmanageably
large in any case.
So if there is any transparency in the plot, the interior of the
plot will be pixellated. The axes and annotations outside of the
plot will still be drawn in vector format however.
-
eps-gzip
- Just like the
eps
format above except that the output
is automatically compressed using the GZIP format as it is written.
Postscript compresses well (typically a factor of 5-10).
Next Previous Up Contents
Next: Colour Maps
Up: Plotting
Previous: auto
STILTS - Starlink Tables Infrastructure Library Tool Set
Starlink User Note256
STILTS web page:
http://www.starlink.ac.uk/stilts/
Author email:
m.b.taylor@bristol.ac.uk
Mailing list:
topcat-user@jiscmail.ac.uk