|ENVI User's Guide: Getting Started with ENVI|
This section describes standard ENVI file opening procedures and options and ENVI functions common in many ENVI windows and dialogs.
Before you apply any of ENVI's functions to a specific data set, you must first select the file containing the data. To ensure consistency, nearly every ENVI image processing function uses a standard input file selection dialog. In the input selection dialog, you can select an input file or a single band, perform a spatial or spectral subset, and in some cases, mask the input data.
The title bar of the input file selection dialog changes to reflect the current function and/or input file type. For example, the title bars in the following two figures specify whether the input selection is by band or by file (respectively). (This was accomplished using the ENVI_SELECT procedure with the TITLE keyword.)
If the Select By toggle in the dialog is set to File, you can select a subset by clicking the Spatial Subset or Spectral Subset buttons and using the standard subsetting procedures (see Selecting a Spatial Subset or Selecting a Spectral Subset).
To automatically start the selected function without selecting any subsetting, double-click on the file or band name.
The Open drop-down button contains selections enabling you to open a new image file, a previously opened file, a spectral library, an ROI file, or an EVF file.
Follow these steps to open a new file for processing.
To open a previously opened file, click the Open drop-down button, and select Previous File from the menu options to reveal the last 20 files used in previous ENVI sessions. Selecting a file from the list opens that file.
Follow these steps to open a spectral library for processing.
Follow these steps to open an ROI file to use when subsetting the selected image.
Follow these steps to open an EVF file to use when subsetting the selected image.
To exit the Input Selection dialog without loading any files, click Cancel.
For detailed information about how to apply the selected function to only a portion of the entire file or band, see Selecting a Spatial Subset or Selecting a Spectral Subset.
Use spatial subsetting to limit application of a function to a spatial subset of the image. Spatial subsets can be selected using the following methods: entering samples and line values, selecting interactively from the image, entering map coordinates, using the same spatial subset that was previously used on another file, using the image shown in the meta scroll window (see Scroll Window Meta Zoom), or by using the bounding box around a region of interest.
For subsetting by spectral bands instead of by spatial regions, see Selecting a Spectral Subset.
To select by starting and ending values of the subset, enter the starting and ending values of the samples and/or lines into the appropriate text boxes or enter the desired number of lines or pixels in the boxes labeled NS or NL respectively.
The size of the original data set and the size of the currently-selected subset appear below the text boxes.
To select the spatial subset interactively from the image:
The starting and ending sample and line coordinates appear in the text boxes labeled Samples and Lines.
For georeferenced images, you may select the subset by map coordinates or latitude/longitude coordinates:
The default values displayed are the upper-left and lower-right coordinates of the full image.
To change the map projection of the coordinates, click the corresponding Change Proj button and follow the instructions in Selecting Map Projection Types.
There are two options for subsetting a file based on the subset area of a previously subset image: use the first option to subset your image based on a subset of an image that was originally the same size as the one you are subsetting; use the second to subset your image based on an image of the same or smaller size that was previously subset by map or pixel values.
To use the same spatial subset previously entered for a file with the same spatial size, click Previous in the File Selection dialog or the Select Spatial Subset dialog.
The input file is subset based on the subset parameters of the file you select.
To make previously saved ROIs available for selection in the Subset Image by ROI/EVF Extent dialog, click Open in the Select Spatial Subset dialog, select ROI File or EVF File, and choose the desired file in the subsequent file selection dialog.
To subset a file using the bounding box (i.e., the area encompassing an ROI or group of ROIs) follow these steps:
To subset a file based on the image currently shown in the meta zoomed scroll window for the selected file, click Scroll in the Select Spatial Subset dialog. The starting and ending values are set to include only the Scroll window subset.
This option is only available if the Scroll window displaying the selected file has been meta zoomed (for further information, see Scroll Window Meta Zoom). If the file is displayed in multiple meta zoomed Scroll windows, the subset of the first window is used.
Use spectral subsetting to limit application of a function to selected bands of an image. For subsetting by spatial regions instead of spectral bands, see Selecting a Spatial Subset.
For images with a Bad Bands list, click Apply BBL to apply the Bad Bands List (see Editing Ancillary Header Information). ENVI automatically deselects individual bands that are marked Bad. Bad bands do not appear in the output image.
To select the same spectral subset previously used for a file with the same number of spectral bands, click the Open drop-down button and select Previous File to reveal the last 20 files and subsets used in previous ENVI sessions. Selecting a file from the list opens the file.
When selecting a range of bands to subset, the dialog default initially shows all bands selected. To select a specific range of bands instead of the default:
Spectral subsetting from an ASCII file is useful if you have defined several special-use spectral subsets for a file with a large number of bands. To spectrally subset your data using an ASCII file:
The format of the ASCII file must have the same number of lines in the file as the number of bands in the image.
For instance, a 5-band AVHRR file could use an ASCII file with the following data:
0 1 1 0 0 1 0 1 1 1 0 0 1 1 0
Selecting column 1 deselects bands 1, 2 and 3, and selects bands 4 and 5.
Selecting column 2 selects bands 1, 3 and 5, and deselects bands 2 and 4.
Selecting column 3 selects bands 1, 2 and 3, and deselects bands 4 and 5.
Regardless of the number of rows in the ASCII file, only the top 5 bands are shown in the Input ASCII File dialog.
When you select a file to process you can apply a spatial mask. When a mask is used, ENVI does not apply the selected function to the masked portion of the image. A mask must be previously defined using Basic Tools Masking Build Mask. You can build a spatial mask from data ranges, Regions of Interest, and other types of input. See Masking for more details about building masks.
Only certain ENVI functions allow spatial masking before processing. These functions include statistics, classification, un-mixing, matched filtering, continuum removal, and spectral feature fitting.
To apply a previously built spatial mask to your image:
When using ENVI, you often have to select from a list of items. Some lists in ENVI allow you to select more than one item. The following describes the different ways to select multiple items in a list.
To select a group of files that are listed consecutively, select the first file in the group, press and hold the Shift key, and click the last file in the group. Or, click and drag the cursor over the group of items with the left mouse button. The items are selected when they are highlighted.
To select multiple files that are not listed consecutively, press and hold the Ctrl key on your keyboard and click on each desired file.
If the dialog you are working in displays a Select All Items button, click the button to select all of the items in the list.
You cannot select multiple items from the Available Bands List or the Available Vectors List.
To de-select a single item, hold the Ctrl key down and click on the item.
If the dialog you're working in displays a Clear All Items or Clear button, click it to de-select all of the items in the list.
You are given the option to save your images and data to File or Memory. Saving an image to memory saves it in your computer's system memory and allows the image to be displayed from the Available Bands List or the Available Vectors List. When you close ENVI, the image is deleted. Saving an image to a file saves the image to your hard drive. When you close ENVI, the item is not deleted. For more information, see File vs. Memory.
Use the Queue button to save the parameters that you've set for a particular function (without executing the function) so that you can execute the function later. To execute the function later, use the ENVI Queue Manager (see Using the ENVI Queue Manager).
The dialog closes, the parameters are saved, and the function can be executed later from the ENVI Queue Manager.
If you exit ENVI without executing the queued functions, a warning appears.
Use the Compress check box to compress your output. ENVI applies a lossless GZIP format compression to the output file. When a compressed file is opened in ENVI, ENVI reads the file and un-compresses it on-the-fly.
Be aware that ENVI cannot read compressed files that are larger than two gigabytes. If you compress a large file and it remains larger than two gigabytes when compressed, you will not be able to read it with ENVI.
To output a file in compressed format: In any dialog where the Compress check box appears, select the Compress check box before clicking OK.
Compressed files are slower to output and input than un-compressed files; the Compress check box is only available in functions that output files sequentially.
ENVI Online Help (August 12, 2005)