The MNF transform is used to determine the inherent dimensionality of image data, to segregate and equalize the noise in the data, and to reduce the computational requirements for subsequent processing.

  1. You can optionally enter a Number of output MNF bands value. However, in most cases, you should use the default (based on the number of input bands) to preserve the full data dimensionality. The Spectral Hourglass Wizard helps determine if all of the MNF bands are required later, depending on the inherent data dimensionality revealed in the MNF process.
  2. To use only a subset of a file to calculate noise statistics for the whole file, click Shift Difference Spatial Subset and enter subsetting parameters. Noise statistics are calculated from the data based on a shift difference method that uses local pixel variance to estimate noise. A virtual noise image is created by subtracting line-shifted and sample-shifted images from the original data and averaging and scaling these two results. You can select a subset of the data to speed the noise statistic calculations or select a uniform area to improve noise estimates.
  3. Click Next. The View MNF Results panel appears, and progress windows display the processing status. After the processing is complete, the View MNF Results panel and an MNF Eigenvalues plot window appear.
  4. The MNF Eigenvalues plot shows the eigenvalue (y-axis) for each MNF-transformed band (an eigenvalue number, shown in the x-axis). Larger eigenvalues indicate higher data variance in the transformed band and may help indicate data dimensionality. When the eigenvalues approach 1, only noise is left in the transformed band, as the noise floor has been scaled to unity in each output MNF band.

  5. Click Load MNF Result to ENVI Display to display an RGB image of the first three MNF bands. Use this image to quickly locate dominant spectral materials, which are displayed in bright, pure colors. Or, select Load Animation of MNF Bands to load the MNF result as a gray scale animation.
  6. After displaying and analyzing the MNF result, click Next in the Wizard. The Determine Data Dimensionality panel appears.