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A.4.2.4 Plot Export Window

Plot export window

Plot export window

The Plot Export Window can be reached with the Export plot to file () toolbar button in any of the plot windows.

You can select a file in the usual way, and save the plot in one of the following graphics formats:

png
PNG bitmap. The background is opaque.
png-transp
PNG bitmap with a transparent background. Background pixels that fall outside the plot surface itself (for instance outside the axes for a Plane plot or outside the celestial sphere for a Sky plot) are transparent.
gif
GIF bitmap; note the number of colours is limited to 256.
jpeg
JPEG bitmap; note that this is a lossy format, better suited to photographs than plots, and colours will be blurred.
pdf
Portable Document Format; in most cases this vector format gives pretty good output, in particular text will be rendered properly.
eps
Encapsulated PostScript; PostScript cannot handle transparency, which means that in some cases the output will come out wrong. PostScript files can also be very large if there are many data points.
eps-gzip
Just like eps, but the output is gzipped before output.

There are two additional controls on the right hand side of this window:

File Format
Selects the output file format as above. The default setting is (auto), which guesses what format you want to use from the filename, and which usually does the right thing.
Force Bitmap
This option only has an effect for vector graphics formats (PDF and PostScript). If selected, it draws the data contents of the plot as a pixel map and embeds that into the output file rather than plotting each point in the output. This may make the output less beautiful (round markers will no longer be perfectly round), but it may result in a much smaller file if there are very many data points. Plot annotations such as axis labels will not be affected - they are still drawn as vector text. Note that in some cases (e.g. use of the auto, density or weighted shading modes) this kind of pixellisation will happen in any case.

Exporting to the pixel-based formats (GIF, JPEG, PNG) is fairly straightforward: each pixel on the screen appears as one pixel in the output file. PNG is generally recommended. GIF works well in most cases, but if there are more than 255 colours some of the colour resolution will be lost. JPEG can preserve a wide range of colours, but does not support transparency and is lossy, so close inspection of image features will reveal blurring.

When exporting to Portable Document Format (PDF) or Encapsulated PostScript (EPS), which are vector graphics formats, there are a few things to consider:

Positional Quantisation
Some of the shading modes (Density, Weighted, Auto) are inherently pixellated, and others (Flat, Aux) are not. In the former case you will see pixel boundaries for the plotted points rather than nice rounded edges at high magnifications (though text and axes will always be plotted nicely). In both cases, at present the positional resolution is the same as it would be on the screen, so if you have a 400-pixel high plot for instance, there are only 400 possible Y coordinates at which a marker can be plotted, which in general is not obvious by looking at the output plot. In future versions the positional resolution of non-pixellated modes may be improved. In either case, increasing the size of the plot on the screen by resizing the window before performing an export to PDF or EPS will reduce the effect of the positional quantisation. Note it will also have the effect of making the text labels proportionally smaller to the graphics, so you may want to increase the font size too.
Transparency
For technical reasons transparent markers cannot easily be rendered when a plot is exported to PostScript. In some cases the plot is done using a bitmap in the PostScript output to permit transparency and in some cases the points are just plotted opaque. PDF does a bit better, but the compositing of transparent shapes is sometimes a bit different on the screen and rendered to a PDF. It's a good idea to check the output of screen exports by looking at the produced file - if it doesn't look like it should do, setting the Force Bitmap option will probably make sure it does, though this will also pixellate the plotted symbols. There is more discussion of this point in the subsections for the various shading modes.
File Size
In some cases (2D and 3D scatter plots with many thousands of points or more) output EPS files can get extremely large; the size scales with the number of points drawn, currently with a factor of a few hundred bytes per point. In some cases you can work round this by plotting some points as transparent so that the plot is rendered as a bitmap (see the discussion of transparency above) which scales as the number of pixels rather than the number of points. The Gzipped EPS format helps somewhat (though can be slow); PDF output is better still. Even PDF files may be unmanageably large for very many points however.


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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
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