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Pyramids Background

Pyramids Background

ENVI builds pyramids for each image while loading the image into the display. Pyramids are copies of the data file at various reduced resolutions. They are used to speed image display by reducing the resampling required when displaying large portions of an image at low resolution.

The Process Manager in the Status bar shows the progress of building pyramids when you open a data file. ENVI only builds pyramids for a large data file the first time you open it. When you open the image thereafter, it displays more quickly because it accesses the previously built pyramid. By default, a pyramid file is created in the same directory as the image file. If this directory is unavailable (due to write permission), ENVI attempts to create the pyramid file in directory set in the Auxiliary File Directory preference. If this is not available, ENVI uses the IDL temporary directory.

Some data formats build internal pyramids and do not require ENVI to build pyramid files. These include File/Enterprise geodatabases, JPIP, OGC WMS/WCS, JPEG2000, MrSID, and NITF 2.1 C3/C8.

ENVI may perform an additional step of downsampling imagery while a pyramid file is being created. Downsampling is the process of retrieving lower resolution data on demand. It is not available to Windows users who access an image file over a network and for which there are no pyramids available.

If a pyramid file does not exist and downsampling is not available, some display tools will not be available. These include 2D Scatter Plot, Histogram Stretch, and Contrast. If a pyramid file or internal pyramid does not exist, regardless of downsampling, the Overview and Stretch on View Extent tools will not be available.

Files on network drives may be slower to build pyramids due to network issues.

To benefit from the high-performance display in ENVI, ensure that the Optimize Display Performance and Copy Pyramids Locally preference settings are set to True.

Note: If your image has .ovr pyramid files, ENVI can use those.


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