Assigning Colors to an Image
You can enhance images by assigning colors to represent distinct areas of an
image, based on relative brightness. These colors will represent the relative
intensity values of the image itself, not the underlying data. For example, if
you use the Image Processing dialog to change the relative brightness or contrast of an image, the
colorization will respond to those changes. If you wish the colors to directly
(linearly) represent the underlying data, you should apply the colors to an image
that has no image enhancements, and uses default processing values.
Colorizing an image also prepares it to be used as an upper layer of a composite image.
To assign colors to an image, select the image and click Color Assignments from the Image menu. This will bring up the Image Color Assignments dialog. Each of the
dialog controls is described in more detail below.
Color Assignments List
Since you can add more than one color assignment to an image, a list of them
is kept here. If there are no colors currently assigned to an image, you will
have one default assignment already provided. Click the Add, Remove, and Remove All buttons to add, remove, or remove all of the assignments, respectively. The
rest of the controls in the Current Selection group reflect parameters of the color assignment currently selected in this
list.
Comment
Type a comment in this field (optional). The comment will appear next to the
index of the color assignment in the Color Assignments List box and can make it easier to track multiple colors.
Contrast Range
The From and To fields of the Contrast Range represent the lower and upper limits (in percent
of the full-scale contrast) within which this color will be assigned (0%
representing the darkest, 100% representing the lightest). For example, if the From and To fields are 60% to 100% respectively, the selected color(s) will be assigned
to the brightest 40% of the image.
One Color, Two Color
The One Color and Two Color radio buttons select whether this color assignment is represented by a single
color, or by a blend of two colors. When Two Color is selected, the result is
a blend of the two assigned colors, starting with one color at the lowest
contrast, and the other color at the highest contrast.
Color Swatches
There are three Color Swatches. You can click each of them to bring up the standard Windows Color Selection
dialog and change the color. The leftmost swatch (under the One Color radio control) represents the color used if one color is selected. The
rightmost two swatches represent the two colors that will be blended if Two Color is selected. The first (left) of these will be assigned to the lowest
contrast percentage (the From field), the second (right) of these will be assigned to the highest contrast
(the To field).
Some Tips on Colorizing
Colorizing an image is most useful when creating composites, since every layer but the base layer must be a colorized image. You will
often want to assign one unique color to each element you will composite. The
range of data will normally reflect the area of the element where counts are
strongest. Often, you will map this color to a large percentage of the data range
(for example, 10% – 100%), ignoring the lowest counts, which are more typically background or
noise.