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ENVI Tutorial: Introductionto ENVIIntroduction to ENVI 2Files Used in This Tutorial 2Getting Started with ENVI 3Starting ENVI 3Loading a Gray Scale Image 3ENVI File Formats 4Exploring the ENVI Header File 4ENVI Windows and Displays 5The ENVI Main Menu Bar 5The Display Group 5The Mouse Button Descriptions Dialog 9Basic ENVI Functions 10Displaying the Cursor Location and Value 10Displaying Image Profiles 10Performing Quick Constrast Stretching 10Displaying Interactive Scatter Plots 11Loading a Color Image 12Linking Two Displays 12Selecting Regions of Interest 13Annotating the Image 15Adding Grid Lines 16Saving and Outputting an Image 171ENVI Tutorial: Introduction to ENVIIntroduction to ENVIThis tutorial is designed to introduce you to the basic concepts of the ENVI software and some of its keyfeatures. It assumes that you are already familiar with general image-processing concepts. I n order torun this tutorial, you must have ENVI installed on your computer.Files Used in This TutorialENVI Resource DVD: D a t a \ c a n _ t mFile Descriptionc a n _ t m r . i m g Cañon City, CO, TM Datac a n _ t m r . h d r ENVI Header for Above2ENVI Tutorial: Introduction to ENVIGetting Started with ENVIBefore attempting to start the program, ensure that ENVI is properly installed as described in theInstallation Guide that shipped with your software.Starting ENVIWindows:Select S t a r t > P r o g r a m s > E N V I x . x > E N V I. (where x.x is the ...
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ENVI Tutorial: Introduction to ENVI
Introduction to ENVI Files Used in This Tutorial Getting Started with ENVI Starting ENVI Loading a Gray Scale Image ENVI File Formats Exploring the ENVI Header File ENVI Windows and Displays The ENVI Main Menu Bar The Display Group The Mouse Button Descriptions Dialog Basic ENVI Functions Displaying the Cursor Location and Value Displaying Image Profiles Performing Quick Constrast Stretching Displaying Interactive Scatter Plots Loading a Color Image Linking Two Displays Selecting Regions of Interest Annotating the Image Adding Grid Lines Saving and Outputting an Image
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ENVI Tutorial: Introduction to ENVI
Introduction to ENVI
This tutorial is designed to introduce you to the basic concepts of the ENVI software and some of its key features. It assumes that you are already familiar with general image-processing concepts. In order to run this tutorial, you must have ENVI installed on your computer.
Files Used in This Tutorial ENVI Resource DVD: Data\can tm _
File can tmr.img _ can tmr.hdr _
Description Cañon City, CO, TM Data ENVI Header for Above
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ENVI Tutorial: Introduction to ENVI
Getting Started with ENVI Before attempting to start the program, ensure that ENVI is properly installed as described in the Installation Guide that shipped with your software. Starting ENVI Windows: Select Start > Programs > ENVI x.x > ENVI . (where x.x is the ENVI version number) Unix: For ENVI, enter envi rt at the UNIX command line. For ENVI+IDL, enter envi at the UNIX _ command line. Macintosh: First, display an OSX, UNIX X-window. For ENVI, type envi rt at the UNIX command prompt. For _ ENVI +IDL, type envi at the UNIX command prompt. Loading a Gray Scale Image Open a multispectral Landsat Thematic Mapper (TM) data file representing Cañon City, Colorado, USA. Opening an Image File 1. From the ENVI main menu bar, select File > Open Image File . 2. Navigate to the Data\can tm directory, select the file can tmr.img from the list, and click _ _ Open . The Available Bands List ENVI provides access to both image files and to the individual spectral bands in these files. The Available Bands List is a special ENVI dialog containing a list of all the available image bands in all open files, as well as any associated map information. You can use the Available Bands List to load both color and gray scale images into a display. The File menu at the top of the Available Bands List dialog provides access to file opening and closing, file information, and canceling the Available Bands List. The Options menu provides a way to find the band closest to a specific wavelength, shows the currently displayed bands, allows toggling between full and shortened band names in the list, and provides the capability to fold all of the bands in a single open image into just the image name. Folding and unfolding the bands into single image names or lists of bands can also be accomplished by clicking on the + (plus) or – (minus) symbols to the left of the filename in the Available Bands List. Right-clicking in the Available Bands List displays a menu with access to different functions. The right-click menu selections will differ depending on what item is currently selected in the Available Bands List. 1. Select TM Band 4 . The band you have chosen is displayed in the field marked Selected Band. 2. Click the Gray Scale radio button then click Load Band to load the image into a new display. Band 4 will be loaded as a gray scale image.
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ENVI Tutorial: Introduction to ENVI
ENVI File Formats ENVI uses a generalized raster data format consisting of a simple flat-binary file and a small associated ASCII (text) header file. This file format permits ENVI to use nearly any image file, including those that contain their own embedded header information. ENVI also supports a variety of data types: byte, integer, unsigned integer, long integer, unsigned long integer, floating-point, double-precision floating-point, complex, double-precision complex, 64-bit integer, and unsigned 64-bit integer. Generalized raster data are stored as a binary stream of bytes in either Band Sequential (BSQ), Band Interleaved by Pixel (BIP), or Band Interleaved by Line (BIL) format. l BSQ is the simplest format, with each line of data followed immediately by the next line of the same spectral band. BSQ format is optimal for spatial (x,y) access to any part of a single spectral band. l BIP format provides optimal spectral processing performance. Images stored in BIP format have the first pixel for all bands in sequential order, followed by the second pixel for all bands, followed by the third pixel for all bands, etc., interleaved up to the number of pixels. This format provides optimum performance for spectral (z) access of the image data. l BIL format provides a compromise in performance between spatial and spectral processing and is the recommended file format for most ENVI processing tasks. Images stored in BIL format have the first line of the first band followed by the first line of the second band, followed by the first line of the third band, interleaved up to the number of bands. Subsequent lines for each band are interleaved in similar fashion. Exploring the ENVI Header File The separate text header file provides information to ENVI about the dimensions of the image, any embedded header that may be present, the data format, and other pertinent information. The header file is normally created (sometimes with your input) the first time a particular data file is read by ENVI. It can also be created outside of ENVI using a text editor. 1. From the ENVI main menu bar, select File > Edit ENVI Header . 2. Click once on can tmr.img to view the ENVI header file for this image. _ 3. Click the Cancel button to close the header file. 4. Right-click on an image name in the Available Bands List and select Edit Header as an alternate way to view the ENVI header file. Close the header file.
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ENVI Tutorial: Introduction to ENVI
ENVI Windows and Displays The ENVI Main Menu Bar In ENVI, activities are initiated by using the menus in the ENVI main menu bar, which may be oriented horizontally as shown below, or vertically (as set via the option on the File > Preferences > Miscellaneous tab).
The Display Group As you work with ENVI, a group of windows will appear on your screen allowing you to manipulate and analyze your image. This group of windows is collectively referred to as the “display group” and the default setup consists of an Image window, a Scroll window, and a Zoom window.
You can choose which combinations of windows appear on the screen by right-clicking on any image window to display the right-click menu and selecting a style from the Display Window Style submenu. Use the ENVI main menu bar File > Preferences > Display Defaults tab to change the default settings for which windows you wish to display and where you wish to position them. You may have many display groups open on the screen at any time. A wide variety of other types of ENVI windows may also be displayed, such as scatter plots, spectral profiles, spectral plots, and vector windows. All windows can be resized by grabbing and dragging a window corner with the left mouse button.
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ENVI Tutorial: Introduction to ENVI
1. Resize the Image window to be as large as possible (until the Scroll window disappears). 2. Now, make the Image window smaller than the full extent of the image data (the Scroll window will reappear). 3. Resize the Zoom window and notice how the outlining box changes in the Image window. The Image Window The Image window shows a portion of the image at full resolution.
The Display group menu bar provides access to many ENVI features related to the images in the display group. By default, it appears at the top of the Image window. If you have chosen to display only the Scroll and Zoom windows or just the Zoom window, the menu bar will appear at the top of the Zoom window. The Zoom box (the colored box in the Image window) indicates the region that is displayed in the Zoom window. 1. Place the mouse cursor in the Zoom box in the Image window, hold down the left mouse button, and move the mouse. The Zoom window is updated automatically when the mouse button is released. 2. Place the cursor anywhere in the Image window (outside of the Zoom box) and click the left mouse button to move the magnified area instantly. If you click, hold, and drag the left mouse button in this fashion, the Zoom window is updated as you drag. 3. Click once in the Zoom box in the Image window and use the arrow keys on your keyboard to move the box. To move several pixels at a time, hold down the Shift key while using the arrow keys. You can choose to have scroll bars displayed in the Image window. These scroll bars provide a way to move through the Scroll window, allowing you to select which portion of the image appears in the Image window. 1. Right-click in the Image window and select Toggle > Display Scroll Bars .
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ENVI Tutorial: Introduction to ENVI
2. To have scroll bars appear in the Image window by default, use the ENVI main menu bar to select the File > Preferences > Display Defaults tab. Set the Image Window Scroll Bars toggle to Yes . The Scroll Window The Scroll window displays the entire image at reduced resolution (subsampled). The subsampling factor is listed in parentheses in the window Title Bar at the top of the image. The highlighted Image box (red by default) indicates the area shown at full resolution in the Image window.
1. Place the mouse cursor inside the Image box, hold down the left mouse button, drag to the desired location, and release to reposition the portion of the image shown in the Image window. The Image window is updated automatically when the mouse button is released. 2. Click anywhere within the Scroll window using the left mouse button to instantly move the selected Image window area. If you click, hold, and drag the left mouse button in this fashion, the Image window will be updated as you drag (the speed depends on your computer resources). 3. Click in the Image box and press the arrow keys on your keyboard. To move the image in larger increments, hold down the Shift key while using the arrow keys. The Zoom Window The Zoom window shows a portion of the image magnified the number of times indicated by the number shown in parentheses in the Title Bar of the window. The zoom area is indicated by a highlighted box (the Zoom box) in the Image window.
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ENVI Tutorial: Introduction to ENVI
There are three Zoom controls (red by default) in the lower left corner of the Zoom window. These control the zoom factor and the crosshair cursor in both the Zoom and Image windows. 1. Place the mouse cursor in the Zoom window and click the left mouse button to reposition the magnified area (displayed in the Zoom box in the Image window) by centering the zoomed area on the selected pixel. 2. Click and hold the left mouse button in the Zoom window while dragging. This causes the Zoom window to pan within the Image window. 3. Click the left mouse button on the graphic in the lower left corner of the Zoom window to zoom out by a factor of 1. 4. Using a three button mouse, click the middle mouse button on the graphic to zoom out by a factor of 2. 5. Click the right mouse button on the graphic to return the Zoom window to the default zoom factor. 6. Click the left mouse button on the graphic to zoom in by a factor of 1. 7. Click the middle mouse button on the graphic to zoom in by a factor of 2. 8. Click the right mouse button on the graphic to return the Zoom window to the default zoom factor. 9. Click the left mouse button on the graphic to toggle the crosshair cursor in the Zoom window on or off. 10. Click the middle mouse button on the graphic to toggle the crosshair cursor in the Image window on or off. 11. Click the right mouse button on the graphic to toggle the Zoom box in the Image window on or off. 12. The Zoom window can also have optional scroll bars, which provide an alternate method for moving through the Zoom window. Right-click in the Zoom window and select Toggle > Zoom Scroll Bars . To have scroll bars appear on the Zoom window by default, use the ENVI main
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ENVI Tutorial: Introduction to ENVI
menu bar to select the File > Preferences > Display Defaults tab. Set the Zoom window Scroll Bars toggle to Yes .
The Mouse Button Descriptions Dialog ENVI has many interactive functions, and the mouse button combinations and actions are different for each one. The Mouse Button Descriptions dialog is available to assist you in understanding what the mouse buttons do in each graphics window. Start the Mouse Button Descriptions dialog by performing either of these methods: l From the Display group menu bar, select Window > Mouse Button Descriptions . l From the ENVI main menu bar, select Window > Mouse Button Descriptions and place your cursor over the Image window. When the Mouse Button Descriptions dialog is open, the mouse button assignments for each ENVI display or graphics window are shown in the dialog when the cursor is poised over a display group window. In the dialog, MB1 is the left mouse button, MB2 is the middle mouse button, and MB3 is the right mouse button.
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ENVI Tutorial: Introduction to ENVI
Basic ENVI Functions This section of the tutorial takes you on a step-by-step tour of ENVI’s basic functions. Displaying the Cursor Location and Value You can choose to display the location of your mouse cursor, screen value (RGB color), and the actual data value of the pixel underneath the crosshair cursor using the Cursor Location/Value dialog. When several display groups are open, this dialog specifies which display group’s location and value are being reported. 1. To display the cursor location and value, select Window > Cursor Location/Value from either the ENVI main menu bar or the Display group menu bar. You can also right-click in the Image window and select Cursor Location/Value . 2. Move the mouse cursor over the Image, Scroll, or Zoom windows to display location and value information in the Cursor Location/Value dialog. 3. Double-click in the Image window to hide the Cursor Location/Value dialog. Double-click again to show the Cursor Location/Value dialog. 4. From the Cursor Location/Value menu bar, select File > Cancel to close the dialog. Displaying Image Profiles X (horizontal), Y (vertical), and Z (spectral) profile plots can be selected and displayed interactively. These profiles show the data values across an image line (X), column (Y), or spectral bands (Z). To display these profiles, perform the following steps. 1. From the Display group menu bar, select Tools > Profiles > X Profile to display a window plotting data values versus sample number for a selected line in the image. 2. From the Display group menu bar, select Tools > Profiles > Y Profile to display a plot of data value versus line number. 3. From the Display group menu bar, select Tools > Profiles > Z Profile to display a spectral plot. You can also open a Z profile from the right-click menu in any Image window. 4. From the Display group menu bar, select Window > Mouse Button Descriptions to view the descriptions of the mouse button actions in the Profile displays. 5. A red crosshair extends to the top and bottom and to the sides of the Image window. The red lines indicate the line or sample locations for the vertical or horizontal profiles. Move the crosshair around the image (just as you move the Zoom box) to see the three image profile plots update to display data on the new location. 6. Close the profile plots by selecting File > Cancel from the menu bar within each window. Performing Quick Constrast Stretching You can perform quick contrast stretches using default parameters and data from either the Image window, the Zoom window, or the Scroll window.
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ENVI Tutorial: Introduction to ENVI
1. From the Display group menu bar, select Enhance and apply various contrast stretches (Linear, Linear 0-255, Linear 2%, Gaussian, Equalization, and Square Root). 2. Compare the effects of the various stretches on the display group. Displaying Interactive Scatter Plots You can plot the data values of two selected image bands versus each other in a scatter plot to graphically display the overlapping values. 1. From the Display group menu bar, select Tools > 2D Scatter Plots . The Scatter Plot Band Choice dialog appears, allowing you to choose the two image bands to compare. 2. Select one band for the x axis and another band for the y axis and click OK . It may take a few seconds for ENVI to extract and tabulate the data values. 3. Once the scatter plot has appeared, position the mouse cursor anywhere in the Image window and drag with the left mouse button pressed. Pixel values contained in a ten-pixel by ten-pixel box surrounding the crosshair will be highlighted in red on the scatter plot.
1. From the Display group menu bar, select Window > Mouse Button Descriptions to display the functions of the different mouse button actions when applied in the Scatter Plot display. 2. Click and drag the cursor around in the Image window to observe the dancing pixels effect. You can also use the scatter plot to highlight specific data values in the Image window. 1. Place the mouse cursor in the Scatter Plot window and click and drag with the middle mouse button. Pixel values contained in a ten-pixel by ten-pixel box surrounding the crosshair will be highlighted in red on the scatter plot Pixels with the values contained in the box are highlighted in the Image window and appear to dance. 2. From the Scatter Plot menu bar, select File > Cancel to close the Scatter Plot window.
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