You can define the color composition for the visualization of image layers for display in the project view. In addition, you can choose from different equalizing options.
You can use different visualizations without changing the image data. Thus, the color of an image area informs the viewer about the particular image layer, but not about the real coloring of the image area.
Note
Changing the image layer mixing only changes the visual display of the image but not the underlying image data as such—it has no impact on the process of image
analysis. Therefore, using this option enables you to better visualize the image according to your preferences without actually changing it.
When creating a new project, the first three image layers are displayed in red, green and blue.
1. To change the layer mixing, open the Edit Image Layer Mixing dialog box:
• Choose View > Image Layer Mixing on the main menu bar.
• Double-click in the right pane of the View Settings window.
Figure 69: Edit Image Layer Mixing dialog box. Changing the layer mixing and equalizing options affects the display of the image only.
Show Next Image Layer
Edit Image Layer Mixing
2. Define the display color of each image layer. For each Image Layer _ you can set the weighting of the Red, Green and Blue color channel. Your choices can be displayed together as additive colors in the project view.
3. Choose among different layer mixes from several Presets P. The One layer gray preset displays a layer in grayscale mode with the Red, Green, and Blue together.
The Three layer mix displays layer 1 in the Red channel, layer 2 in Green and layer 3 in Blue. Choose Six layer mix to display additional layers.
Figure 70: Layer Mixing presets (from left to right): One layer gray, Three layer mix, Six layer mix.
4. Change these settings to your preferred options with the Shift button a or by clicking in the respective R, G, B cell. One layer can be displayed in more than one color, and more than one layer can be displayed in the same color.
5. Individual weights can be assigned to each layer. Clear the No layer weights check box b and click the color in which you want the layer to be displayed. Left-clicking increases the layer's color weight while right-clicking decreases it.
The Auto update check box b refreshes the view with each change of the layer mixing settings. Clear this check box to show the new settings after clicking OK.
With the Auto Update check box cleared, the Preview button becomes active.
6. Compare the available image Equalization methods c and choose one that gives you the best visualization of the objects of your interest.
Equalization settings are stored in the workspace and applied to all projects within the workspace. In the Options dialog box you can define a default equalization setting.
7. Click the Parameter button d to changing the equalizing parameters, if available.
6.3.5.1 Edit Image Layer Mixing for Thumbnails
Change the way thumbnails display in the Heat Map window and in the Thumbnail Views of the workspace.
To edit image layer mixing for thumbnails:
1. Right click on the Heat Map window or go to View > Thumbnail Settings to open the Thumbnail Settings dialog box.
Figure 71: Thumbnail Settings dialog box.
2. Choose among different layer mixes in the Layer Mixing drop-down list.
The One layer gray preset displays a layer in grayscale mode with the Red, Green,
About Image Equalization on page 99
Options on page 456
View Thumbnails in the Heat Map Window on page 383
and Blue together. The Three layer mix displays layer 1 in the Red channel, layer 2 in Green and layer 3 in Blue. Choose Six layer mix to display additional layers.
3. Open the Equalizing drop-down box and select a method that gives you the best display of the objects in the thumbnails.
4. If you select an equalization method you can also click the Parameter button to changing the equalizing parameters.
6.3.5.2 About Image Equalization
Image equalization is performed after all image layers are mixed into a raw RGB (Red, Green, Blue) image. If, as is usual, only one image layer is assigned to each color, this approach is the same as applying equalization to the individual raw layer gray value images. On the other hand, if more than one image layer is assigned to one screen color (Red, Green or Blue), image equalization leads to higher quality results if it is performed after all image layers are mixed into a raw RGB image.
There are different modes for image equalization available:
None
No (None) equalization allows you to see the image data as it is what can be helpful at the beginning of rule set development when looking for an approach.
The output from the image layer mixing is displayed without further modification.
Linear Equalization
Linear equalization with 1.00% is the default for new scenes. Commonly it displays images with a higher contrast as without image equalization.
Standard Deviation Equalization
With its default parameter of 3.0, Standard deviation renders a similar display as Linear equalization. Use a parameters around 1.0 for an exclusion of dark and bright outliers.
Gamma Correction Equalization
Gamma correction equalization is used to improve the contrast of dark or bright areas by spreading the corresponding gray values.
Histogram Equalization
Histogram equalization is well suited for LANDSAT images but can lead to substantial over-stretching on many normal images. It can be helpful in cases where you want to display dark areas with more contrast.
Manual Image Layer Equalization
Manual image layer equalization allows you to control equalization in detail. For each image layer individually, you can set the equalization method specifying the mapping function. In addition, you can define the input range by setting minimum and maximum values.
About Image Equalization on page 99
For details see the Reference Book
Manual Image Layer Equalization on page 100
Examples
Compare the following displays of the same scene:
Figure 72: Left: Three layer mix (red, green, blue) with Gamma correction (0.50). Right: One layer mix with Linear equalizing (1.00%)
Figure 73: Left: Three layer mix (red, green, blue) without equalizing. Right: Six layer mix with Histogram equalization.
(Image data courtesy of Ministry of Environmental Affairs of Sachsen-Anhalt, Germany.)
6.3.5.3 Manual Image Layer Equalization
Manual image layer equalization allows you to control equalization of image layer display in detail. You can select the method of equalization and limit it to a range of gray values that is important for the display of the objects in which you are interested.
1. Go to the Equalizing drop-down list box of the Edit Image Layer Mixing dialog box.
2. Select Manual and click the Parameter button. The Image Layer Equalization dialog box opens.
Edit the Image Layer Mixing on page 97
Figure 74: Image Layer Equalization dialog box.
3. Select one image Layer ` for equalization. Only those image layers that you chose (with a dot or layer weight value) in the Edit Image Layer Mixing dialog box are listed.
4. Select an Equalizing a method from the drop-down list box:
• Linear: Use to improve the contrast evenly over the selected range of gray values.
• Linear - inverse: Same as Linear but with inverted gray values. This can be used, for example, in radiology to display a negative image positively, in other words, changing black to white and vice versa.
• Gamma correction (negative): Use to improve the contrast in brighter areas.
• Gamma correction (positive): Use to improve the contrast in darker areas.
• Gamma correction (negative) - inverse: Same as Gamma correction (negative) but with inverted gray values. This can be used, for example, in radiology for displaying a negative image positively, in other words, changing black to white and vice versa.
• Gamma correction (positive) - inverse: Same as Gamma correction (positive) but with inverted gray values. This can be used, for example, in radiology for displaying a negative image positively, in other words, changing black to white and vice versa.
5. The Histogram b displays the equalization method as function curve. You can modify minimum and maximum values in the histogram. Commonly you select a range of gray values that is important for the display of the objects of your interest.
This allows you to limit the display on a defined range of gray values that is used for the selected equalization method a. Likewise, you may exclude the display of disturbing gray values.
• Drag the orange marker.
• Enter a value in the text box.
• Click in the Histogram display and use arrow keys on your keyboard:
and to move the minimum value and to move the maximum value
6. The effect of the settings is updated as a preview in the project view. For Preview c, you can switch between the display of all image layers or the selected one only.
7. Confirm with OK and go back to the Edit Image Layer Mixing dialog box.
Check Image Layer Histograms on page 102