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Last Post 05 Mar 2018 10:59 AM by  MariM
Maintain background pixels through raster stretch
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Solar Smith



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23 Feb 2018 10:56 AM

    I'm trying to write automated processing routines to stretch and reformat my binary file data.
    I have floating point raster data in which the background pixels are zero and all valid values are greater than zero.
    I need to stretch the data to byte values and keep the background pixels distinct from the data pixels.
    I'm trying to use ENVILogStretchRaster to stretch the data. Unfortunately, pixel values near the MIN value are being mapped to zero which then look like background values. I'd like to be able to specify the valid output range for valid data but I don't see the ability to do that in the ...StretchRaster documentation.

    Lacking an alternative, I'm resorting to using 'WHERE' to first identify the non-background pixels before the stretch, then another 'WHERE' to change all of the non-backgournd stretched values that are zero to '1'. However, this requires a couple of extra 'GetData' processes which are very inefficient with large datasets. Is there a faster, more efficient way to stretch the data and preserve background pixels as background?

    MariM



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    Posts:2396
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    26 Feb 2018 11:35 AM
    Have you tried one of the Stretch options after applying a mask to the 0 values using the Data ignore value?

    http://www.harrisgeospatial.com/docs/Masks.html

    Solar Smith



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    Posts:5
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    26 Feb 2018 10:35 PM
    Yes, I did try setting the Data_Ignore_Value but I still saw no zero for both the output background pixels and the lowest data pixels.

    Solar Smith



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    26 Feb 2018 10:51 PM
    It's possible I didn't understand how to properly create and apply a mask using the 'Data_Ignore_Value' with the ENVI??StretchRaster routine. It might be helpful if I saw a script sample of how to correctly do that with say 'ENVILogStretchRaster'.
    Thanks

    MariM



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    Posts:2396
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    05 Mar 2018 10:59 AM
    I could stretch the data using the following code. This includes setting the data ignore value, calculating the statistical min/max and then stretching the remaining values to bytes:

    pro stretchFloats
    compile_opt idl2
    e=envi()

    file='c:\temp\Quac_scaled.dat'

    raster=e.openraster(file)
    ;start task
    task1 = ENVITask('EditRasterMetadata')
    task1.input_raster=raster
    task1.data_ignore_value = 0.0
    task1.output_raster_uri = e.GetTemporaryFilename()
    task1.execute
    DataColl = e.Data
    DataColl.Add, Task1.Output_Raster

    Task2=ENVITask('RasterStatistics')
    task2.input_raster=task1.output_raster
    Task2.Execute

    Print, Task2.MAX
    Print, Task2.MIN

    Task3=ENVITask('LogStretchRaster')
    task3.input_raster=task1.output_raster
    Task3.MIN = task2.min
    Task3.MAX = task2.max
    Task3.OUTPUT_RASTER_URI = e.GetTemporaryFilename()
    task3.Execute

    DataColl = e.Data
    DataColl.Add, Task3.Output_Raster
    view = e.GetView()
    layer = view.CreateLayer(task3.output_raster)

    end
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