This image of Rhea against Saturn's globe was created from three raw
images downloaded from Cassini's
raw images website. Credit:
NASA / JPL / SSI / Emily Lakdawalla
One very common operation with spacecraft images is to make a color image
from individual frames captured through different filters. To make approximate
true color images, an image of the same target needs to be captured through
red, green, and blue filters at about the same time. False color images
can be made from any combination of three filters.
If you'd like to try this at home, you will need:
A copy of Adobe Photoshop Elements. The screen caps in this tutorial
are from Photoshop Elements 5.0, but earlier versions may also work.
(Note: making RGB images is possible with many different
image processing packages; this tutorial presently covers only Photoshop.)
Three images captured through different color filters (for the example
on this page, download these three images: N00084955_red, N00084956_grn, N00084957_bl1)
Consider these three images taken from the Cassini raw images website. They
were taken by Cassini on June 15, 2007 and show the moon Rhea against
Saturn's globe. From left to right, they were taken through red, green,
and blue filters. These images are courtesy of NASA, JPL, and the Space
Science Institute.
This image was taken from 1,197,277 kilometers away, using the BL1 and CL2
filters.
Image Credits All raw images posted on space mission websites
are in the public domain. However, you should always give credit to the
appropriate institutions for taking the original images. Each mission's
website will offer guidance on how images should be credited.
(For clarity, from here forward these images will be shown cropped more tightly
around Rhea.)
This tutorial will show you how to convert three grayscale
images into one color image. However,
if you go straight to that step, you will find that because Cassini is
not a fixed platform but instead a moving orbiter, Rhea shifts from frame
to frame. If
you do not align the frames before merging them, you will produce an image
that looks like this, where each sharp edge is fringed with color:
If images captured through different filters are not aligned, color fringes
will appear around sharp edges in the image. Credit: NASA / JPL / SSI / Emily
Lakdawalla
Aligning the Frames
Begin by opening the three different-filter images of Rhea.
Paste the three frames into layers on one image.
Go to File > New > Photomerge™ Panorama
...
Select "Normal", "Snap
to Image" and "Keep as Layers". Click OK
Tutorial: RGB images: Photomerge
Make sure to select "Normal", "Snap to Image" and "Keep as Layers."
Credit: NASA / JPL / SSI / Eric Hartwell
Photoshop Elements creates
a new RGB image with each of the original images in a separate layer.
Conveniently, it converts each image to RGB mode and gives each layer
the name of the original file.
Now would be a good time to save (Ctrl-S or File > Save).
Tutorial: RGB images: layers
The three images have now been stacked as layers in a single Photoshop document. In this example, the Blue filter image is Layer 1, the Red filter image is Layer 2, and the Green filter image is Layer 3.
Credit: NASA / JPL / SSI / Eric Hartwell
Increase the size of the image
Go to Image > Image Size... and make the image 200% or
300% its original size. This step will permit you to make finer
adjustments to the alignment of the frames.
Align the Green-filter image to the Blue-filter image
In the Layers palette, turn off the top layer (Layer 2) by clicking
the eye icon next to it.
Make the middle layer (Layer 1) active by clicking the text "Layer
1."
Set the blending mode for this layer to Difference by using the drop-down
menu in the Layers palette. This blending mode subtracts one of
the layers from the other. Since the two layers are very similar
the image will now appear mostly black, because the two layers will nearly
cancel each other out. However, where they are poorly aligned,
there will be white fringes.
Tutorial: RGB images: fringes
With one layer set to the Difference blending mode, that layer's pixel values are subtracted from the ones below it. The result is an image that is nearly black (because the two images are largely similar), but there are bright fringes because the two layers are not perfectly aligned.
Credit: NASA / JPL / SSI / Eric Hartwell
Using the Move tool (), move Layer 1 around until the
white fringes disappear (or are minimized). If the necessary
adjustments are small, it may be easiest to use the arrow keys on your
keyboard to nudge the layer back and forth.
Set the blending mode to Screen.
Align the Red-filter image to the Blue-filter image
In the Layers palette, turn off Layer 1 and turn on Layer 2.
Repeat steps 3b through 3e for Layer 2.
Crop the image so that every layer reaches the edge of the canvas
Because you moved layers around, there will be "blank" or "transparent" areas
at the extreme edges of some layers. These transparent areas
can cause problems later when you paste the layers into a new document. You
want to crop these transparent areas away from the image. You
can do this either using the Crop tool () or by shrinking
the size of the canvas (Image > Canvas Size…)
Now would be a good time to save.
Merging the aligned frames into an RGB image
Make the background layer (the Blue-filter image) the active layer
by clicking on its name in the Layers palette.
Adjust the layer's levels so that its pixels have only blue information,
and no red or green information.
Go to the Levels dialog (Ctrl-L or Enhance > Adjust Lighing > Levels...)
Select the Red channel from the dropdown menu and adjust the maximum
output level to 0.
Do the same for the Green channel.
Click OK.
The blue channel now contains the blue-filtered Cassini image. You
may notice that the combined image looks blue.
Tutorial: RGB images: final
The image now has gets its red, green, and blue channels from the separate images.
Credit: NASA / JPL / SSI / Eric Hartwell
To convert the image from a three-layered file to a single RGB image,
select all, copy the layers merged (Shift-Ctrl-C or Edit
> Copy Merged),
create a new document, and paste.
Finally, you may wish to play around with the image's brightness, contrast,
hue, and saturation using the various controls available to you in the
Image menu.
Now that the layers have been properly aligned, the resulting color image of
Rhea has no annoying color fringes. Credit: NASA / JPL / SSI / Emily Lakdawalla
Please
with
any comments or questions on this tutorial.
), move Layer 1 around until the
white fringes disappear (or are minimized). If the necessary
adjustments are small, it may be easiest to use the arrow keys on your
keyboard to nudge the layer back and forth.
) or by shrinking
the size of the canvas (Image > Canvas Size…)
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