BandMathinENVI.docVIP

Band Math ENVIs Band Math function accesses data spatially by mapping user-defined variables to bands or files. You can use ENVIs Band Math dialog to define the bands or files used as input, to call a user Band Math function, and to write the result to a file or memory. See  HYPERLINK :7730/help/topic/com.rsi.envi.doc.working/BandMathMain.html Band Math (ENVI Users Guide) for complete details about writing proper Band Math expressions. Writing Band Math User Functions Since ENVI + IDL gives you access to IDL functionality, you can use the power of built-in IDL features, IDL user functions, or your own routines to perform custom Band Math operations. The only requirement for these functions is that they accept one or more image arrays as input and that they output a single-band, 2D array with the same dimensions as the input bands. Band Math user functions are simple to write and execute as Band Math expressions. For example, to execute a function called BM_RATIO with two input bands, enter the following in the Enter an expression field of the Band Math dialog: bm_ratio(b1, b2) The function declaration for this routine is as follows: FUNCTION bm_ratio, b1, b2 The processing that takes place within the Band Math user function has the same constraints as Band Math expressions. Input data are tiled; therefore, functions like min() and max() are invalid since they return only the minimum or maximum of the current tile and not the minimum or maximum of the input band. The function accepts the input bands, processes the data, and returns the result. Functions have the following model: FUNCTION bm_func, b1, [b2,..., bn, parameters and keywords] ???processing steps ???RETURN, result END To be compatible with tiled processing, custom Band Math functions should avoid processing that requires the entire band in memory at one time. Compiling Band Math User Functions Once the user function is complete, you should place the resulting .pro or .sav file in the sav