Overviews

An overview, or pyramid, is a reduced-resolution raster version of an image. CATALYST Professional modules, such as , use overviews to open images much faster than full-resolution ones, making it more efficient for you to work with large files. You can create overview levels in powers of two or powers of three.

You can also create overviews for file formats that do not provide native support of overviews.

You can create overviews for data, such as orthorectified images, that you create with OrthoEngine. While creating overviews from large files requires some processing time, it increases the speed at which your images display and for all of the operations that use image correlation.

You can specify whether to create overviews on input files, output files, or both. You can also specify how you want to create the overviews: never, always, or when prompted.

On input files

You can select from the following options to create overviews on input files:

Never create overviews: no overviews are created
Always generate: overviews for each large raster that does not have an existing overview are created automatically
Prompt: displays the Create Overviews window each time you view or add a large raster

Store overviews externally

Select this check box to write overviews for input files to an external .pox file. By storing overviews externally, the input data remains unchanged.

To create overviews internally for input files with formats for which GDB supports creation of native overviews, clear the Store overviews externally check box. GDB supports creating native (internal) overviews for PCIDSK (.pix) , TIFF (.tif) and Imagine (.img) formats. With this check box clear, and data is added for which GDB does not create native overviews, a .pox file is created to store the overviews.

On output files

You can select from the following options to create overviews on output files:

Never create overviews: no overviews are created
Always generate: overviews for each large raster that does not have an existing overview are created automatically

Downsampling method

With both input and output files, you can select the downsampling method to use, which specifies the resampling technique to use to create overviews.

The available options are as follows:

Nearest neighbor: Each pixel in the resampled raster acquires the same value as its nearest neighbor in the original raster. Although this method computes quickly, it can introduce small errors in the resampled raster, which may cause it to have a "jagged" appearance.
Block average: Calculates the average value of a group of image pixels and uses that value for the screen pixel. This method often provides a more meaningful representation of the image in the viewer. This method is well suited to continuous-tone images, such as radar imagery.
Block mode: Finds the most common value in a block of the image pixels and assigns that value to the output pixels. This method is well suited to thematic data, where any mathematical operations on pixel values (class numbers) have no meaning.

Decimation levels

You can also select the level of decimation to use:

Powers of 2: creates overview levels at 2, 4, 8, 16, 32, 64, and so on, depending on your image size. It uses more disk space; however, the display is improved significantly.
Powers of 3: creates overview levels at 3, 9, 27, 81, and so on, depending on your image size. It uses less disk space; however, it may slow the image display, as it requires computing an intermediate level (dynamically) when you zoom out by a factor of two.