How to make RGB image from three/four greyscale images?

[Enno Middelberg]

Hi,

I am trying to make an RGB image from 3 and 4 greyscale images,
respectively. The easiest case is working fine: I open three greyscale
images of equal size and use image -> mode -> compose to make the RGB
image. So far everything is fine.

However, I am using this for a scientific purpose and there are two
things I would like to do:

1 - can I change the color of the channels? IE, two of the images I
want to put in are rather close in "color", so transforming them into
colors that have rather equally-spaced frequencies or wavelengths
falsifies the result. What I mean is that two out of these three
images are "red" and "yellow", whereas the third is "blue". When I
compose the RGB like I described above, then the "yellow" image
becomes "green".

2 - can I use four colors? I don't know much about colors in modern
image processing systems, but the options that I have in GIMP to
compose an image don't seem to satisfy my needs (when composing an
image out of greyscale images, I can choose RGB, RGBA, HSV, CMY, CMYK
- but none of these seems to be right).

I have the idea to turn each of the greyscale images into a
monochromatic color image. Can't I tune the color of each to a certain
"wavelength" and then overlay them? How do I do that?

I apologize if this seems trivial to you, but I am only an occasional
user of the GIMP and I am not very familiar with it, as you noticed.

Kind regards and many thanks,

Enno

[Marc 'BlackJack' Rintsch]

In <slrnb1gboh....@pc069.MPIfR-Bonn.MPG.de>, Enno Middelberg
wrote:

> I have the idea to turn each of the greyscale images into a
> monochromatic color image. Can't I tune the color of each to a certain
> "wavelength" and then overlay them? How do I do that?

I think the most flexible solution is the following:

1. make a layer for each of your images
2. add a layer mask to each layer and place your grayscale image there
3. now you can fill the individual layers with any color you want
4. adjust the opacity slider in the layers dialog of every layer

Ciao,
Marc 'BlackJack' Rintsch

[Rodrigo Cunha]

What you are asking for, combining N different grayscale images, with
their corresponding arbitrary frequency response curves, into a standart
RGB color image is a complex task. You will also most certainly lose
information in the process.

There are some freeware scientific packages, mostly in the astronomy
area, to manipulate such multichannel data (in FITS format, probably)
and generate color images, both false color (to enhance some details)
and true color.

Search for FITS in this page:

http://sal.kachinatech.com/D/1/index.shtml

also have a look at this one:

http://fits.gsfc.nasa.gov/fits_viewer.html

[Enno Middelberg]

Hi,

>There are some freeware scientific packages, mostly in the astronomy
>area

Ooops... :-) Maybe some other people already solved the same
problem. I need this for combining radio observations.

>Search for FITS in this page:
>
>http://sal.kachinatech.com/D/1/index.shtml
>
>also have a look at this one:
>
>http://fits.gsfc.nasa.gov/fits_viewer.html

I'll have a look, thanks!

Enno

[Rodrigo Cunha]

Enno Middelberg wrote:
> Ooops... :-) Maybe some other people already solved the same
> problem. I need this for combining radio observations.

Yes, probably already solved. Since you are observing out of visible
spectrum all your images are going to be "false color", obviously.

But comming from a radio-telescope your data is probably intrinsecally
multi-spectral, that is, you don't have many grayscale images taken
throught a narrow-spectrum filter, you have spectral curves taken from a
radio receiver for each pixel, isn't that true? I think what you want is
a function that takes each pixel's spectral curve and assignes a RGB
value to it.

Now from a non-astronomic enviroment comes this nice and very flexible
program capable displaying many kinds of data:

http://www.research.ibm.com/dx/

Perhaps you can costumize it to better suit you :-)

--
Rodrigo