PixelwiseBandMathRaster Task

This task constructs an ENVIRaster from a source raster that has a mathematical expression applied on a pixel-by-pixel basis. If any of the bands referenced by the expression have a bad pixel state (due to masking or a data ignore value) and you set the DATA_IGNORE_VALUE parameter, then the corresponding output pixel will have a bad pixel state and will be transparent.

The virtual raster associated with this task is ENVIPixelwiseBandMathRaster.

Example

; Start the application

e = ENVI()

; Open an input file

File = Filepath('qb_boulder_msi', Subdir=['data'], $

Root_Dir=e.Root_Dir)

Raster = e.OpenRaster(File)

; Get the task from the catalog of ENVITasks

Task = ENVITask('PixelwiseBandMathRaster')

; Define inputs

Task.INPUT_RASTER = Raster

; Calculate enhanced NDVI

Task.EXPRESSION = 'float((b4 + b2) - 2*b1) / float((b4 + b2) + 2*b1)'

; Run the task

Task.Execute

; Get the collection of objects currently

; in the Data Manager

DataColl = e.Data

; Add the output to the Data Manager

DataColl.Add, Task.OUTPUT_RASTER

; Display the result

View = e.GetView()

Layer = View.CreateLayer(Task.OUTPUT_RASTER)

Syntax

Result = ENVITask('PixelwiseBandMathRaster')

Input parameters (Set, Get): DATA_IGNORE_VALUE, EXPRESSION, INPUT_RASTER, OUTPUT_RASTER_URI

Output parameters (Get only): OUTPUT_RASTER

Parameters marked as "Set" are those that you can set to specific values. You can also retrieve their current values any time. Parameters marked as "Get" are those whose values you can retrieve but not set.

Input Parameters

DATA_IGNORE_VALUE (optional)

Set this parameter to a numeric pixel value that ENVI should ignore when subsequently processing data or computing statistics from the output image.

This value should be different from any other pixel values in the raster, including expected values from the band math expression.
This value is stored as raster metadata. If you use the ENVIRaster::Export method to export the raster, you will not have to remember the value you used for DATA_IGNORE_VALUE.

EXPRESSION (required)

Enter a string indicating the mathematical expression.

Name the band variables as bx, where x is the order of the band in the raster. For example, b1 is the first band in the raster, b2 is the second band, etc.
You can specify any IDL function, including user-defined functions that can be found in !PATH.
The expression cannot change the dimensions of the raster. Functions such as TOTAL, MIN, and MEAN reduce an array to a scalar, so they are not allowed unless you use them like MIN([b1, b2, b3]) or b1 / MAX(b1).
The expression must evaluate to a scalar so that a single-band output raster is created.
When using division in an expression, be sure to add float in the denominator. See the above example.

INPUT_RASTER (required)

Specify an input raster to process.

OUTPUT_RASTER_URI (optional)

Specify a string with the fully qualified filename and path to export the associated OUTPUT_RASTER.

If you set this parameter to an asterisk symbol (*), the output raster will be virtual and not written to disk.
If you do not specify this parameter, or set it to an exclamation symbol (!), a temporary file will be created.
If you set it to the hash symbol (#), ENVI creates a file in the temporary directory, but this file will not be deleted when ENVI closes.