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#### A.4.6.5 Density Mode

Example Density shading mode plot

Density mode selection

The Density shading mode () uses a configurable colour map to indicate how many points are plotted over each other. Specifically, it colours each pixel according to how many times that pixel has has been covered by one of the shapes plotted by the layer in question. To put it another way, it generates a false-colour density map with pixel granularity using a smoothing kernel of the form of the shapes plotted by the layer. The upshot is that you can see the plot density of points or other shapes plotted.

This is like Auto mode, but with more user-configurable options. The options are:

Density Shader
  The colour map for displaying density values. There are two types, relative and absolute. Relative maps have names marked by a star ("*"), and alter the basic dataset colour, for instance by darkening or lightening it, while absolute maps (the rest) ignore the basic dataset colour altogether. For a single-dataset plot, the absolute maps are best, but for multiple subsets it may be less confusing to use a relative one.
Shader Clip
  Select a sub-range of the full colour map above. If the Default checkbox is checked, then all or most of the colour ramp from the Shader control is used. If you want to configure the range of colours from the map yourself, uncheck the Default checkbox, and slide the handles in from the end of the slider to choose exactly the range you want.

  The default range is clipped at one end for colour maps that fade to white, so that all the plotted colours will be distinguisable against a white background. If you don't want that, you can uncheck Default and leave the handles at the extreme ends of the slider.

Shader Flip
  Whether the density scale should map forwards or backwards into the colour map.
Shader Quantise
  Allows the colour map to be quantised. By default, the colour map is effectively continuous. If you slide the slider to the right, or enter a value in the text field, the map will be split into a decreasing number of discrete colours. This can be used to generate a contour-like effect, and may make it easier to trace the boundaries of regions of interest by eye.
Scaling
  Determines the function used to map the range of density values onto the colour map. Options are linear, logarithmic, square and square root.
Density Subrange
  Adjusts the density range over which the colour map is applied. By default the colour map is scaled using limits found from the data density in the plot (the most dense few pixels are ignored), but you can restrict the range using this slider.

Although these options give you quite some control over how densities are mapped to colours, this mode does not display the colour mapping in a way that shows you quantitatively which colours correspond to which numeric density values. If you want that kind of visual feedback, you should use the Weighted shading mode, which can be configured to display point densities (as well as other quantities), and also causes a colour ramp to be displayed under control of the Aux Axis control.

Exporting: When exported to vector formats, the output is automatically forced to a bitmap for Density-mode layers. In the case of PostScript, this completely obscures any previous layers.

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TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk
Mailing list: topcat-user@jiscmail.ac.uk