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A.4.7 Density Map

Density map window

Density map window

The density map window plots a 2-dimensional density map of one or more pairs of table columns (or derived quantities); the colour of each pixel displayed is determined by the number of points in the data set which fall within its bounds. Another way to think of this is as a histogram on a 2-dimensional grid, rather than a 1-dimensional one as in the Histogram Window. If multiple datasets are being plotted, pixel colours can either be at a single brightness level derived from total number of points, or a combination of up to three independent (red, green, blue) colour channels from different datasets.

Density maps are suitable when you have a very large number of points to plot, since in this case it's important to be able to see not just whether there is a point at a given pixel, but how many points fall on that pixel. To a large extent, the transparency features of the other 2d and 3d plotting windows address this issue, but the density map gives you a bit more control. It can also export the result as a FITS image, which can then be processed or viewed using image-specific software such as GAIA or Aladin.

You can configure the axes, including zooming in and out, with the mouse or manually as described in Appendix A.4.1.2.

The Cut Percentile Levels panel below the plot controls how the number of counts in each pixel maps to a brightness. There are two sliders, one for the lower bound and one for the upper bound. They are labelled (logarithmically) with percentile values. If the upper one is set to 90, it means that any pixel above the 90th percentile of the pixels in the image in terms of count level will be shown with maximum brightness, and similarly for the lower one. These values apply independently to each colour channel if more than one is in use. Immediately below the sliders, the pixel values which correspond to minimum (black) and maximum brightness are displayed for each channel. If the image is not fairly completely covered, this control doesn't give you as much freedom as you might like - the user interface may be improved in future releases.

The following buttons are available on the toolbar:

Export as EPS
Pops up a dialogue which will write the current plot as an EPS file.
Export as GIF
Pops up a dialogue which will output the current plot to a GIF file. The output file is just the same as the plotted image that you see. Resize the plotting window before the export to control the size of the output GIF.
Export as FITS
Pops up a dialogue which will output the plotted map as a FITS array. If only one channel is visible (either one colour channel or monochrome mode) then the output FITS file will be a 2d array with dimensions the same as the displayed image. If there are multiple RGB channels then the output array will be 3d with the third dimension having an extent of 2 or 3, depending on the number of colour channels visible. In either case the FITS file will have a single (primary) HDU. Basic coordinate system information, as well as DATAMIN and DATAMAX cards, will be written to the header. The output array will have some integer type; its length (BITPIX) will depend on the maximum value of any of the pixels.
Rescale
Rescales the axes of the current plot so that it contains all the data points in the currently selected subsets. By default the plot will be scaled like this, but it it may have changed because of changes in the subset selection or from zooming in or out.
Configure Axes
Pops up a dialogue to allow manual configuration of axis ranges and labels - see Appendix A.4.1.2.
Log Intensity
Toggles between linear and logarithmic mapping for colour intensity as a function of number of counts.
Replot
Redraws the current plot. It is usually not necessary to use this button, since if you change any of the plot characteristics with the controls in this window the plot will be redrawn automatically. However if you have changed the data, e.g. by editing cells in the Data Window, the plot is not automatically redrawn (since this is potentially an expensive operation and you may not require it). Clicking this button redraws the plot taking account of any changes to the table data.
Colour
Toggles between one-channel (monochrome) and three-channel (RGB) images. When the display is monochrome, the brightness of every pixel depends on the number of counts that fall in its bounds summed from every plotted dataset, and the display runs from black (low cut) to white (high cut) via shades of grey. For RGB, each dataset is assigned one of the colours red, green or blue, and the brightness of each colour is individually determined by the number of counts from datasets assigned that colour.
Bigger Pixels
Increments the size of screen pixel corresponding to one density map bin.
Smaller Pixels
Decrements the size of screen pixel corresponding to one density map bin.
Draw Subset Region
Allows you to draw a region on the screen defining a new Row Subset. When you have finished drawing it, click this button again to indicate you're done. See Appendix A.4.1.3 for more details.
Subset From Visible
Defines a new Row Subset consisting of only the points which are currently visible on the plotting surface. See Appendix A.4.1.3 for more explanation.

The Export provides a number of ways to export the displayed image for external viewing or analysis. As well as options to export as GIF, JPEG, EPS and FITS, there is also the option to transmit the FITS image to one or all applications listening using the PLASTIC tool interoperability protocol which will receive images. In this way you can transmit the image directly to PLASTIC-aware image manipulation tools such as Aladin. See Section 8 for more information about PLASTIC.

How to set the colour channel corresponding to each dataset is explained in the following subsection.


Next Previous Up Contents
Next: Density Style Editor
Up: Graphics Windows
Previous: Spherical Plot

TOPCAT - Tool for OPerations on Catalogues And Tables
Starlink User Note 253
TOPCAT web page: http://www.starlink.ac.uk/topcat/
Author email: m.b.taylor@bristol.ac.uk