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.