There are two programs, <khuei> and kview, that allow you to display different images on top of each other, while each image has a different colour coding. This can be used to display amplitude and phase together in one image, or compare different channels from a data cube by superimposing them.
These visualisation techniques require a 24-bit display.
<khuei> is quite user-friendly, allowing you to load two images of the same width and height. You can even load one image in FITS format and the other in Miriad or GIPSY format. A menu allows you to combine the two images in several ways.
Note that there is another way to superimpose images which involves drawing one image as a greyscale or false-colour image and the other is drawn over the top as contours. This can be done with <kview>, and does not require a 24-bit display.
You may also use the <send-contours> programme to draw contours over any image. This programme is a command-line tool, however, and is not normally used since the the GUI tools are much easier to use.
With the program <khuei> you can superimpose two images, where one image is taken to control the intensity and the other the colour. This can be used to display amplitude and phase together in one image, or line integral and a velocity field.
To display the images run
khuei
First load File1 and File2. You should now see the combined images. The sliders Start Hue and Stop Hue manipulate the colour of the image, the sliders Minimum Intensity and Saturation the intensity.
By default, File1 will control the intensity, and File2 the hue. You can swap this by choosing ``Phase, Amplitude'' from the RAW DATA FORM menu. If you select ``Real, Imaginary'' then a rectangular to polar transformation of the data is performed, where File1 is real image and File2 is the imaginary image. You can swap the sense of this by choosing ``Imaginary, Real''. After the rectangular to polar transformation, the amplitude value will control the intensity and the phase value will control the hue.
A screen snapshot is available here
Another possibility is to overlay images is that all images are coded as intensity but in a different colour. This you can use to overlay two or three different channels of a data cube and inspect subtle differences.
Unfortunately, kview cannot load separate image files and combine them into a single image like <khuei> can. This will change, but for now you have to manually convert the images to unsigned bytes and then combine the images outside of kview.
The program to be used here to convert Karma files to unsigned bytes is called convert. Type:
convert -lower 0 -upper 255 -scale range -type ubyte - <filename> "<units>"
The units you have to specify, has to be the value of the keyword BUNIT in the FITS-header of the original data cube. It is likely to be Jy/Beam or something like that. If the FITS-header did not have the keyword BUNIT, you have to specify Data Value. You can use more <fits-file> to have a look at the fits-header.
Also here note the space between the filename and the minus-sign.
The name of the output file is the name of the input file with convert_ prepended.
You now combine the unsigned byte images thus:
merge_planes <im 1> "Red Intensity" <im 2> "Green Intensity" <im 3> "Blue Intensity" <outfile>
You have to give three input images. If you want to overlay only two images, you can for example specify the same image for the green and the blue image and later in the program kview set the maximum blue intensity to zero. You will now have a 24 bit colour image file.
To display the images run
kview
Use the file selector to load your 24 bit colour image. Now choose the ``Direct Colour'' colourmap from the Colourmap menu. You can now manipulate the colourtransfer function for the three primary colours independently.