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Visualization Stages - Technical Notes:
I assume here that you have a number of exposures through different
filters for the same object. Each of these monochromatic datasets
needs to be converted to a black and white image.
Combining images increases the RGB value per pix and the higher the
RGB value the more white the pixel. So the trick is to ensure that
"whites" in an individual image are stretched so that they are more
grey. That is, your individual image should be dark. One has more
control using astronomical software so most imagemakers start with one
of these. I recommend that you use "kvis" from the
Karma package.
| Software | comments |
| IDL |
This costs money and you have to do your
own coding. |
| RGBSUN in IRAF | Requires
trial and error for the thresholds
and you can only combine 3 filters. |
| kvis |
Free in the Karma suite.
http://www.atnf.csiro.au/computing/software/karma/.
This let's you select thresholds in real time
via histograms.
As well as linear and log scaling, does
square root which is good for nebulae.
It has additional algorithms in its
pseudo-colour option (best is greyscale3).
Also it exports your scaled image to Portable Pixel Map
format which is accepted by all many packages.
|
In Karma, using kvis, you can reduce an image by loading a fits
file with the filter option turned on. Select the number of pixels
to "skip" (which is actually "add together"). Adjust your image and
then export this to a new ppm image. If you are using another
package for stretching the intensities, then a good format to save
the file as is tiff format, if it is available.
Back to Visualization Stages.
Next pump your output from the Stage 1 (above) into a manipulation
package such as GIMP or PhotoShop. Your work is done in "layers"
(rather than in colour channels) -- so open up the Layers Dialog Box in your manipulation
package. Create a layer for each Stage 1 black and white dataset
image. About "layers" in general:
- Layers allows one to assign any colour, not just a primary
colour, to an image.
(See an example at http://www.ras.ucalgary.ca/CGPS/press/shell/)
- In order to see all the images in your stack of layers,
"screen" is a good algorithm. This
mode is
set, using a menu on the Layer Dialog Box, for each individual layer.
A layer with this mode is like a transparency or positive slide,
allowing light from the image below it to shine through it.
- Each layer is individually adjusted so a noisey image, say from
one filter's dataset, can be suppressed without affecting the
other images. To suppress noise you can apply a gaussian filter
to the pixels, smoothing them out.
Using GIMP
- Click your right mouse button in the image window in order to
access the task menus.
Inserting images into layers
- In Gimp open up an image.
- Then open up a new image with the
background set to black by selecting the foreground or
background
as
appropriate. Ensure the Layers, Channels, Paths dialog
box is open.
- On your original image, right clicking to get the menu options,
find Edit --> Copy visible. On the
new image, Edit --> Paste. This will put the B&W image into a layer.
- In the Layers dialog box, click on the words "floating section" and
give the layer a name. This will also change layer into something you
can edit.
- Set the mode algorithm, in the Layers, Channels, Paths dialog box.
IMPORTANT: in order to see each of the layers, not just the
top one, you need to select a relevant mode. "screen" is a good
algorithm for combining images; it is set on each of the layers (or
you won't see the one underneath).
- Repeat for other images.
- Save this layered image as an ".xcf" format file.
Assigning Colours
Then for each layer you assign a colour. This you do using the
Levels tool.
- Click on the name of a layer in the
Layers, Channels, Paths dialog box
so that it is blue which
means active.
- Go get the levels tool.
[Image --> Colors --> Levels tool]
- Change the top
menu in the levels tool from value
to a colour and adjust the
OUTPUT levels to get your
requested colour. (For example,
if you want your layer to be green, then change the menu to
Red and drag the right slider in the output levels to zero...
repeat this for Blue... your layer should now be Green).
- Repeat for each layer.
- Save this as a .xcf format file.
- Adjust values and colours until you are happy with the results.
For example, if one filter image is particularly noisy (textured), this
can be reduced by applying to that layer a gaussian filter with the
width of the noise (e.g. a few pixels).
- Save changes as a .xcf format file.
Some advise: Make copies of your B&W layers and work on those so you
don't have to insert images again. Sometimes turn off the other layers (by
clicking on the eye icon) to check your colours.
This is very iterative. Enjoy!
Other options: Some imagemakers work on one RGB image (i.e. 3
filters) and then layer in other filters at this stage. For an example, at
http://heritage.stsci.edu/public/apr1/h301filt.html is an RGB
image from 3 filters with other filters layered on top.
Back to Visualization Stages.
After you are satisfied in general with your colour selection,
and have saved it as an .xcf file, then you flatten the image,
using the Layers dialog box options,
into a single tiff file with a different name in tif format.
Even better, open a new image (with a black background),
Edit --> Copy Visible the display of your .xcf file
and then Edit --> Paste into the new image. Set mode to screen and
flatten the new image and save as a single tiff file. To flatten you
use the submenus under Layers. Layers --> Merge Visible and
then Layers --> Flatten.
Back to Visualization Stages.
Use the image manipulation tool options (like
levels) for final colour and contrast adjustments. Use the clone tool to
remove chip seams and cosmic rays. Chose your orientation.
Save this file as a tiff (no compression) or, in a pinch, a
100% quality jpeg.
Back to Visualization Stages.
