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To limit the raster processing extent using a spatial mask, you can follow these general steps:

1. Create a spatial mask: You can create a spatial mask using a vector file, such as a shapefile, or by creating a new raster with the same extent, resolution, and coordinate reference system as the original raster and assigning a value of 1 to the pixels that you want to include in the mask and 0 to the pixels that you want to exclude.

2. Load the mask and the original raster: Once you have created the mask, you can load it into your GIS software along with the original raster that you want to limit the processing extent.

3. Apply the mask: You can apply the mask to the original raster using the "Extract by Mask" tool, which is available in most GIS software. This tool will clip the original raster to the extent of the mask and assign NoData values to the pixels outside the mask.

4. Perform raster processing: With the masked raster, you can perform any raster processing operation, such as calculating statistics, applying a function, or generating a new raster layer.

Here's an example using the arcpy Python package in ArcGIS: 

import arcpy

# Set the input and output paths
input_raster = "C:/data/myraster.tif"
mask = "C:/data/mymask.shp"
output_raster = "C:/data/myoutput.tif"

# Execute Extract by Mask
arcpy.gp.ExtractByMask_sa(input_raster, mask, output_raster)

This code will create a new raster, "myoutput.tif", that includes only the pixels in "myraster.tif" that intersect with the features in "mymask.shp".