Combining Channels into Images

We can create RGB images by loading three different grayscale images into the R, G and B channels of the new RGB image using Edit - Load Mask/Channel . The most frequent application is creating a false color RGB image from multi-channel satellite sensor data. See also the Masks topic for more information on using images as masks.

Combining Per-Channel Grayscale Images into One RGB Image

1. Create an image of the desired size. This can be done by simply copying one of the channel images and renaming it.

2. Use Edit - Load Mask/Channel to load the red channel with the desired grayscale image.

3. Use Edit - Load Mask/Channel to load the green channel with the desired grayscale image.

4. Use Edit - Load Mask/Channel to load the blue channel with the desired grayscale image.

Example

The classic example of such work is creating false color RGB images using infrared bands from the Landsat satellite's thematic mapper sensor.

Using its Enhanced Thematic Mapper instrument the Landsat 7 satellite delivers images in seven standard Landsat bands plus a higher resolution panchromatic band from green color to near infrared. All the standard seven bands provide 30 meter resolution except the thermal infrared band, which delivers 150 meter resolution. The panchromatic band achieves 15 meter resolution.

Band	Wavelength (mm)	Description	Resolution (meters)	Comments
1	.45 to .52	Blue	30	Discriminates between soil and vegetation. Map coastal water areas.
2	.52 to .60	Green	30	Shows healthy vegetation.
3	.63 to .69	Red	30	Discriminates between plant species. Useful in geology.
4	.76 to .90	Reflective infrared	30	Crop identification. Shows vegetation well.
5	1.55 to 1.75	Mid-infrared	30	Shows water content of plants well. Detects drought.
6	10.4 to 12.5	Thermal infrared	150	Low resolution band. Shows thermal pollution and geothermal sites.
7	2.08 to 2.35	Mid-infrared	30	Geologic uses. See changes in soil types.
PAN	0.50 to 0.90	Green to infrared	15	Panchromatic (gray scale). Highest resolution. Co-registered with other bands.

We will take a sample Landsat 7 image from bands 4, 5 and 7, the most useful infrared bands.

$images\img_landsat_band4.gif$

Band 4 we will use as the green channel since it is most sensitive to the mass of vegetation at a particular location.

$images\img_landsat_band5.gif$

Band 5 we will use as the red channel since it is brighter and has better contrast than the band 7 image. This should provide a good distinction from the green channel.

$images\img_landsat_band7.gif$

Band 7 we will use as our blue channel. Since this is the darkest image, the resulting composite image should be mostly greens, reds and some yellows.

$images\img_landsat_false.gif$

The resulting RGB image is a typical "false color" rendition of colors invisible to human eyes (from the infrared part of the spectrum in this case) translated into visible colors. We created this image by loading the above bands into the R, G and B channels using Edit - Load Mask/Channel. The image brings out areas of large plant biomass and also shows regions with contrasting water content of plants.

$images\img_landsat_band7inv.gif$

If desired, we can alter the various grayscale images before combining them into the false color image. This image takes band 7 and Inverts it using an inversion center that's below the default so that the image does not become too bright.

$images\img_landsat_false_inv7.gif$

If we load this new, inverted band 7 grayscale image into the blue channel we get an entirely different look to the image.

Using the Mode Option

The Mode option to combine a mask by adding it, subtracting it or otherwise combining it in a sophisticated way with an existing channel provides a powerful means to combine multi-spectral images into powerful and highly informative false color images.

$images\img_landsat_modecombo_01.gif$

Using a straight, unprocessed combination of the Red, Green and Blue bands from the Landsat 7 Enhanced Thematic Mapper instrument (bands 3, 2 and 1) as the Red, Green and Blue channels in an image results in the muddy and uninformative image above.

$images\img_landsat_modecombo_02.gif$

We can enhance the image by using a Band 4 image as a mask and using Add mode to add its intensity values to the Green channel of the image. Band 4 is a reflective infrared band that is highly responsive to vegetation. By using it to "kick" the Green channel up we enhance the green element normally associated with vegetation.

$images\img_landsat_modecombo_03.gif$

We can further enhance the image by using a Band 7 image as a mask and Add mode to add it to the Red channel of the image. Band 7 is a lower mid-infrared band that is responsive to soil types. This lets us see not only vegetation but also soils.