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#### A.4.5.13 Density Form

Density form configuration panel

The Density form () plots a density map (2-d histogram) on the pixel grid of the plot surface, coarsened by a configurable factor. You can optionally use a weighting for the data values, and you can configure how the values are combined to produce the output bin values.

This form is somewhat experimental and may be modified in the future.

The shading is done using the shared colour map. This colour map is used by all currently visible Density, SkyDensity, Aux and Weighted layers. When at least one such layer is being plotted, the Aux Axis control is visible in the control panel, which allows you to configure the colour map, range, ramp display etc.

The configuration options are:

Weight
  The weight value applied to each plotted point. Fill this in with a column name or expression from the table just like for a positional coordinate. The exact way this quantity is used depends on the setting of the Combine control below. If it's left blank, the weighting is considered to be unity (all values are 1); this makes sense for some combination types (e.g. sum) but not others (e.g. mean).
Bin Size
  Determines the dimension of grid bins in pixels. Bins are square in pixel dimensions and this value gives the extent in pixels along each side. Currently, only integer values are allowed.
Combine
  Determines how the weight values associated with markers plotted covering a given bin are combined to produce the numeric value used for that bin's colour.

  The following options (some are more useful than others) are currently available:

  - `sum`: the sum of all weights
  - `mean`: the mean of all weights
  - `median`: the median of all weights; (note, this is expensive to calculate and may be slow)
  - `variance`: the sample variance of all weights
  - `min`: the minimum weight
  - `max`: the maximum weight
  - `count`: the number of points plotted (weight value is ignored, this is like Density mode)
  - `hit`: one if any point is plotted, blank otherwise (weight value is ignored, this is like Flat mode)
Opaque Limit
  Determines transparency of the points. By default, they are fully opaque, but if you slide the slider to the right, they will become progressively more transparent.

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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@bristol.ac.uk