GEOG 5110/6110
Department of Geography,
Lab 2 –
Laboratory Objectives:
I. Density Slicing
Use density slicing to select data
ranges and colors for highlighting areas in your gray scale image. The
data ranges used in the density slice coloring can be from the displayed image
or from another image of the same size.
Use Interactive Density Slicing
Editing
To change the range values or color of
a density slice:
Density Slice Options
Use the Options menu to add new
ranges, set default ranges, and set the number of default ranges to be used.
Adding Ranges
To add new ranges to the list in the
Density Slice dialog:
1.
Select Options > Add New
Ranges
2.
When the
3.
Choose the starting color from
the Colors menu. The colors of multiple ranges will follow the lost of
graphics colors.
4.
Click OK to return to the
Density Slice dialog. The determined ranges are listed in the dialog and
can be edited as desired.
5.
Click Apply to apply the density
slicing colors to the image.
Changing Bands
1.
In the Density Slice dialog, select Options
> Change Density Slice Band. The Density Slice Band Choice
dialog appears.
2.
Change the band used for the density
slice data ranges by clicking on the band name.
The new density
slice coloring is automatically applied to the new band selected.
Closing the Density Slice Dialog
1.
To close the Density Slice dialog and
reset the original color table, select File > Cancel.
II. Contrast Stretching
Contrast stretching is a form of radiometric
enhancement. Radiometric enhancement
deals with the individual values of the pixels in the image. Whereas spatial enhancement (to be discussed in a future
lab) takes into account the values of neighboring pixels.
Contrast stretching enhancements are not applied to the
data files (i.e. the data files are not physically changed).
Contrast stretching is applied to adjust the color or gray scale range of a
selected image so it fills the computer display’s dynamic range and improves
the contrast in the image. Available stretching options allows you to do
this using both linear and non-linear methods.
Interactive Contrast Stretching
Interactive Contrast Stretching allows you to stretch
your image data by interacting with histograms. Interactive Contrast
Stretching plots a histogram and allows you to interactively control the contrast
of the displayed image. Many different types of stretches can be
applied. By default, a linear 2% stretch is applied to the data when it
is first displayed.
1. To access ENVI’s
interactive contrast stretching functions, from the Display menu bar, select Enhance
> Interactive Stretching. An Input and an Output histogram
appear in the Interactive Contrast Stretching dialog. They show the
current input data and applied stretch respectively. Initially, the input
and output histograms reflect the default stretch applied to the data when the
image was displayed. Two vertical dotted lines mark the current minimum and
maximum values of the stretch. For color images, the color of the
histogram matches the color of the selected band (the red band is displayed by
default). At the bottom of the Interactive Histogram window, the stretch type
and the histogram source are listed.
a. To view the histogram for the green or
blue bands (if your image is displayed in RGB), click the G or B toggle button.
Applying Different Stretch Types
Use the Stretch_Type menu in
the Interactive Contrast Stretching Dialog to select from a list of all
available types of interactive stretches. In this lab we will experiment
with the following four contrast stretches:
1. Applying Linear Contrast Stretches
A 2% linear contrast stretching is the
default interactive stretch. A linear stretch sets the minimum and
maximum input value to 0 and 255 respectively and all other values in between
are linearly aligned to intermediate output values.
a. From within the Interactive Contrast
Stretching dialog, select Stretch_Type > Linear.
b. To determine the minimum and maximum
input value move the minimum and maximum vertical bars (dotted white lines) on
the input histogram to the desired location using the left mouse button or
enter the minimum and maximum input values by entering the desired DN values or
a percentage of the data (e.g. 5% for minimum and 95% for maximum) in the
Stretch text boxes.
c. When clicking Input Histogram, Numbers
below the plot display the current data value, the number of pixels and the
percentage of pixels that have that value, and the cumulative percentage of
pixels with values less than or equal to the current value.
d. Click Apply to apply the stretch
to the displayed data.
*To re-display the original stretch
select Options > Rest Stretch
*For images displayed using RGB, the
above can be done for each channel by selecting the appropriate RGB radio
button.
2. Applying Piecewise Linear Contrast
Stretches
A piecewise linear contrast stretch can
be interactively defined using the mouse to position points in the input
histogram. Linear segments connect the points to provide linear
stretching between them.
a. Select Stretch_Type
> Piecewise Linear. A transfer function is plotted on the input
histogram.
b. Click the middle mouse button (scroll dial) anywhere in the Input
Histogram plot to add a node (break point) to the transfer function. Line
segments are plotted to connect the endpoints to the plotted node symbol.
i. To move the position of a point, click
the left mouse button on the symbol and drag it to a new position.
ii. To delete points, click on the symbol
with the right mouse button.
iii. To enter input and output values
manually, select Options > Edit Piecewise Linear.
c. The resulting histogram is plotted in
the output display. The output histogram shows the distribution of the data
with the new stretch applied.
d. Click Apply to apply the stretch
to the displayed data.
3. Applying Gaussian Stretches
The default Gaussian stretch is
centered at a mean DN of 127 with the data values 3 standard deviation set to 0
and 255.
a. Select Stretch_Type
> Gaussian
b. Enter the minimum and maximum values
for the stretch. The output histogram shows the selected Gaussian
function as a red curve. The stretched data distribution is shown superimposed
in white on the red Gaussian function.
i. To enter the desired number of standard
deviations manually, select Options > Set Gaussian Stdv... .
c. Click Apply to apply the stretch
to the displayed data.
i. If Options > AutoApply is selected, the stretch will
automatically be applied. I would suggest turning this option off.
4. Applying Histogram Equalization
Contrast Stretches
To automatically scale the data to
equalize the number of DNs in each histogram bin:
a. Select Stretch_Type
> Equalization. The input histogram shows the unmodified data
distribution. The output histogram shows the equalization function as a red
curve, and the stretched data distribution is shown superimposed in white.
b. Click Apply to apply the stretch
to the displayed data.
i. If Options > AutoApply is selected, the stretch will
automatically be applied. I would suggest turning this option off.
III. Image
Sharpening
Image sharpening can be explained as a
combination of a high-spectral but low-spatial resolution image with a
low-spectral but high-spatial resolution one to produce a high-spectral and
high-spatial resolution image. We are going to use one of the image
sharpening techniques to see the effects of high spectral and spatial
resolution image on identifying geographic features in the visually appealing
sharpened image.
Navigate to the Data folder under geog5110
on 'Syntax L M Server (geogshare)' drive. Find 4
files named, ETM8subset,
ETM8subset.hdr, ETM123subset, and
ETM123subset.hdr and copy them into your N:\geog5110\Lab2 folder.
Load the ETM123subset
as a true color composite image and the ETM8subset
as a gray-scale image.
When you want to open the multiple images
(ETM8subset and ETM123subset), choose the images by clicking left mouse button
on the file names with CTRL key pressed in the Enter Data Filenames. Click Open.
You can see Map Info in the Available Bands List. Click on the +
of Map Info, observe information carefully. Load ETM123subset as a
true color composite image into New Display and ETM8subset as a gray
scale image into New Display. Click on the + of Map
Info, look at information carefully. Compare the two images.
Now we are ready to do the image sharpening.
The Color Normalized
(Brovey) sharpening will be used as an example
sharpening technique. This technique uses a mathematical combination of the
color image and high resolution data. Each band in the color image is
multiplied by a ratio of the high resolution data divided by the sum of the
color bands (see text p.168). The output sharpened RGB images will
have the pixel size of the input high-resolution data. Please see the text for
details.
Importing Images into MS Word
A portion of the assignment will ask
you to import display or zoom windows images into
Microsoft Word.
Here are instructions for doing so....