ENVI Tutorial
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E N V I T u t o r i a l : I n t e r a c t i v eD i s p l a y F u n c t i o n sI n t e r a c t i v e D i s p l a y F u n c t i o n s 2F i l e s U s e d i n t h i s T u t o r i a l 2O p e n i n g a P a n c h r o m a t i c ( S P O T ) I m a g e F i l e 3P e r f o r m i n g I n t e r a c t i v e C o n t r a s t S t r e t c h i n g 4L i n e a r S t r e t c h i n g O p t i o n s 4G a u s s i a n S t r e t c h i n g O p t i o n s 5E q u a l i z a t i o n S t r e t c h i n g O p t i o n s 5A p p l y i n g E N V I C o l o r T a b l e s 7U s i n g t h e P i x e l L o c a t o r 8D i s p l a y i n g t h e G e o r e f e r e n c e d C u r s o r L o c a t i o n 9A p p l y i n g I n t e r a c t i v e F i l t e r s 1 0C o m p a r i n g I m a g e s U s i n g D y n a m i c O v e r l a y s 1 1R e v i e w i n g G e o S p o t M a p I n f o r m a t i o n 1 2O p e n i n g a n d O v e r l a y i n g D X F V e c t o r F i l e s 1 3C o m p o s i n g a B a s i c M a p 1 5A d d i n g a G r i d t o Y o u r I m a g e 1 5A n n o t a t i n g t h e I m a g e w i t h a M a p K e y 1 5S a v i n g a n d R e s t o r i n g A n n o t a t i o n 1 6S a v i n g a n d O u t p u t t i n g a n I m a g e ( B u r n - I n ) 1 61E N V I T u t o r i a l : I n t e r a ct i ve D i sp l a y F u n ct i o n sI n t e r a c t i v e D i s p l a y F u n c t i o n sT h i s t u t o r i a l p r o v i d e s a n i n t r o d u c t i o n t o u s i n g E N V I w i t h P a n c h r o m a t i c ( S P O T ) d a t a , i ...
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ENVI Tutorial: Interactive Display Functions
Interactive Display Functions Files Used in this Tutorial Opening a Panchromatic (SPOT) Image File Performing Interactive Contrast Stretching Linear Stretching Options Gaussian Stretching Options Equalization Stretching Options Applying ENVI Color Tables Using the Pixel Locator Displaying the Georeferenced Cursor Location Applying Interactive Filters Comparing Images Using Dynamic Overlays Reviewing GeoSpot Map Information Opening and Overlaying DXF Vector Files Composing a Basic Map Adding a Grid to Your Image Annotating the Image with a Map Key Saving and Restoring Annotation Saving and Outputting an Image (Burn-In)
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ENVI Tutorial: Interactive Display Functions
Interactive Display Functions This tutorial provides an introduction to using ENVI with Panchromatic (SPOT) data, including display, contrast enhancement, basic information about ENVI and some suggestions for your initial investigations of the software. It is designed to introduce first-time ENVI users to the basic concepts of the package and to explore some of its key features. It assumes that you are already familiar with general image-processing concepts. It is recommended that new users begin with either the Quick Start to ENVI tutorial or the Introduction to ENVI tutorial before beginning this tutorial. Files Used in this Tutorial ENVI Resource DVD: Data\enfidavi File Description enfidavi.bil SPOT Panchromatic Data, Enfidaville, Tunisia enfidavi.hdr ENVI Header for Above enfidavi.dsc GeoSpot Volume Descriptor File enfidavi.rep GeoSpot report file (REP/B: GEOSPOT Structure) enfidavi.rsc GeoSpot Raster Source Description File dxf.txt DXF coding descriptor file alti.dxf Spot height DXF file energy.dxf Oil or Gas Pipeline DXF file hydro.dxf Hydrology DXF file industry.dxf Industrial Areas DXF file physio.dxf Physiographic areas DXF file popu.dxf Urban Features (Population Centers) DXF file transpor.dxf Transportation Networks DXF file copyright.txt Data Copyright Notice Panchromatic Data and Vector Overlays This dataset is a SPOT Panchromatic image and corresponding DXF files of Enfidaville, Tunisia, courtesy of ITT Visual Information Solutions International France. These data are Copyright CNES-Spot Image and IGN France.
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ENVI Tutorial: Interactive Display Functions
Opening a Panchromatic (SPOT) Image File
Before attempting to start the program, ensure that ENVI is properly installed as described in the Installation Guide that shipped with your software. 1. From the ENVI main menu bar, select File > Open Image File . 2. Navigate to the Data\enfidavi directory, select the file enfidavi.bil from the list, and click Open . The Available Bands List appears on your screen. This is a SPOT Panchromatic image of Enfidaville, Tunisia, courtesy of ITT Visual Information Solutions France. The data in this file is copyrighted as Copyright CNES-Spot Image and IGN France. Select Band 1 . Ensure the Gray Scale radio button is selected, then click Load Band to load the image into a new display group. Examine the image in the display group.
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ENVI Tutorial: Interactive Display Functions
Performing Interactive Contrast Stretching 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. From the Display group menu bar, select Enhance > Interactive Stretching . An Interactive Stretching dialog for the displayed band appears. This dialog allows you to change the contrast stretch of the displayed image. Two histogram plots display the color or gray scale range of the input image (left) and the output image after contrast stretching (right). Initially, the input and output histograms reflect the default stretch applied to the data when the image was displayed.
2. Input histogram information can be extracted from the Image window, from the Scroll window (subsampled), from the Zoom window, from the entire image band (all pixels of the data), or from a user defined region-of-interest (ROI). Explore the options available in the Histogram_Source menu on the Interactive Stretching dialog menu bar. Linear Stretching Options When images are loaded into the display group, a 2% linear contrast stretch is applied by default. This can be modified by selecting File > Preferences from the Display group menu bar, then clicking the Display Defaults tab. 1. From the Interactive Stretching dialog menu bar, select Stretch_Type > Linear . Two vertical dotted lines appear in the input histogram plot: these bars can be repositioned to control the minimum and maximum value used in the contrast stretch. 2. In the Input Histogram window, there are two dotted vertical bars on either side of the histogram. These bars can be repositioned to control the minimum and maximum value used in the contrast stretch. Position the mouse cursor over the left bar then click and hold the left mouse button as you drag the bar from side to side (see figure below). As the dotted vertical bar is moved across
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ENVI Tutorial: Interactive Display Functions
the plot, numbers appear on the status bar of the Interactive Stretching dialog. Whenever the left mouse button is clicked over the histogram plot, the status bar displays the current data value, the number of pixels and percentage of pixels that have that value, and the cumulative percentage of pixels with values less than or equal to the current value.
3. You can choose to have stretching or histogram changes applied to your images automatically, by selecting Options > Auto Apply from the Interactive Stretching dialog menu bar. To have changes applied to your image only when you click the Apply button, toggle the Auto Apply option off. For this exercise, toggle the Auto Apply option on. 4. Position the left dotted vertical bar in the Input Histogram window so that a cumulative percentage of pixels equaling approximately 5% is selected. Now move the right bar so that the cumulative percentage is approximately 95% of the pixels. 5. You can also position the bars by entering a minimum and maximum value (numbers or percentages) in the Stretch fields provided in the dialog. Enter 4% in the left text field and 96% in the right text field and press the Enter key. The % values are converted to digital numbers and the left and right bars in the display are updated with the data values at 4% and 96%, respectively. Gaussian Stretching Options The default Gaussian stretch is centered at a mean of 127 with the data values +/- 3 standard deviations set to 0 and 255. 1. From the Interactive Stretching dialog menu bar, select Stretch_Type > Gaussian . 2. From the Interactive Stretching dialog menu bar, select Options > Set Gaussian Stdv to set the standard deviation. The Set Gaussian Stdv dialog appears allowing you to adjust the standard deviation value and see the effect when the new setting is applied to the display group. 3. Set the Gaussian Stdvs to 4.00 and click OK . 4. You can choose to have the stretch automatically applied to the image display group by ensuring Options > Auto Apply is selected from the Interactive Stretching dialog menu bar.s Equalization Stretching Options You can use the Equalization option in the Interactive Stretching dialog to automatically scale the data to equalize the number of digital numbers in each histogram bin.
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ENVI Tutorial: Interactive Display Functions
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From the Interactive Stretching dialog menu bar, select Stretch_Type > Equalization .
You can choose to have the stretch automatically applied to the image display group by ensuring Options > Auto Apply is selected from the Interactive Stretching dialog menu bar.
From the Interactive Stretching dialog.
Stretching dialog menu bar,
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click File
> Cancel to close
the
Interactive
ENVI Tutorial: Interactive Display Functions
Applying ENVI Color Tables You can use Color Mapping to apply color tables to images, create interactive density sliced images, control the RGB image planes and to change classification color mapping. 1. From the Display group menu bar, select Tools > Color Mapping > ENVI Color Tables . The ENVI Color Tables dialog appears.
2. Explore the color options in the Color Table list and observe the color-coded image. 3. Use the Stretch Bottom and Stretch Top sliders to apply quick stretches to the displayed image. 4. From the Color Tables dialog menu bar, select Options > Reset Color Table to return to the original stretch and gray scale color table. 5. From the Color Tables dialog menu bar, select File > Cancel to close the Color Tables dialog.
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ENVI Tutorial: Interactive Display Functions
Using the Pixel Locator The Pixel Locator allows exact positioning of the cursor. You can manually enter a sample and line location to position the cursor in the center of the Zoom window. If an image contains georeferenced data, you can optionally locate pixels using map coordinates. If the image contains an associated DEM, elevation information displays. The Pixel Locator pertains to the display group from which it was opened. You can open a Pixel Locator for each display group shown on your screen. 1. From the Display group menu bar, select Tools > Pixel Locator to open the Pixel Locator dialog.
2. Place the cursor in any of the three windows of the display group and click the left mouse button. Notice that the Pixel Locator provides the pixel location for the selected pixel. 3. Click the toggle button next to the projection field to toggle between true map coordinates and latitude/longitude geographic coordinates. You can also choose to change the selected projection by clicking the Change Proj button. 4. From the Pixel Locator dialog menu bar, select File > Cancel to close the Pixel Locator dialog.
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ENVI Tutorial: Interactive Display Functions
Displaying the Georeferenced Cursor Location You can use ENVI’s Cursor Location/Value to view image values and geographic location. The Cursor Location/Value option displays the sample (horizontal, X) and line (vertical, Y) coordinates of the cursor along with the data value of the pixel at the cursor location. The position is continuously updated as you move the cursor around the image. 1. From the Display group menu bar, select Tools > Cursor Location/Value . The Cursor Location/Value dialog box appears displaying the location of the cursor in the Image, Scroll, or Zoom windows. The dialog also displays the screen value (color) and the actual data value of the pixel underneath the crosshair cursor.
2. From the Cursor Location/Value dialog menu bar, select File > Cancel to close the Cursor Location/Value dialog.
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ENVI Tutorial: Interactive Display Functions
Applying Interactive Filters
ENVI gives you the ability to apply several different pre-defined or user-defined filters to a display. The following exercise shows you how to apply a pre-defined filter to the image in the Image window. 1. From the Display group menu bar, select Enhance > Filter and choose a desired filter type to apply to the displayed image. 2. Experiment with the different sharpening, smoothing, and median filters on the displayed image. 3. From the Available Bands List, click the Display #1 drop-down button and select New Display to create a second display group. 4. Select Band 1 and click Load Band to load the image into the second display. 5. From the Display group menu bar in image #2, select Enhance > Filter and select a filter different from that applied to Image #1.
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ENVI Tutorial: Interactive Display Functions
Comparing Images Using Dynamic Overlays
Use image linking and dynamic overlays to overlay portions of multiple images simultaneously (or flicker images) and to perform identical operations on multiple images from within a single Image window. 1. From the Display group menu bar, select Tools > Link > Link Displays . You can also right-click in the image and select Link Displays . 2. Click OK in the Link Displays dialog to establish the link. 3. Click the left mouse button in one of the Image windows to see the image displays overlaid on one another. 4. To create a smaller overlay area, position the mouse cursor anywhere in either Image window (or Zoom window) and hold down and drag with the middle mouse button. Upon button release, the smaller overlay area is set and a small portion of the linked image will be superimposed on the current Image window. 5. Click the left mouse button in the Image window and drag the small overlay window around the image to see the overlay effects. 6. Resize the overlay area at by clicking and dragging the middle mouse button until the overlay area is the desired size.
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