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8.6 Export Formats

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.
  1. 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).
  2. 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.
  1. 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.
  2. 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).


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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