There is no concept of 'no data' in ENVI.  Every pixel value is valid.  Sometimes users will assign 'NaN' or 'Inf' to background values but they are not always handled correctly by other programs.  You can set the value to -9999 which may be recognized as 'no data' by  something like ArcGIS.  Background values usually only become relevant during processing and in this case, it is best to use a mask with your input data file to exclude background values.

Hello  A quick question related to this post [QUOTE]markgibb2 wrote In ENVI 4.2 - when subsetting an image via an ROI, am I able to assign the pixels outside the ROI to no data? Thanks. [/QUOTE] I am doing an image subsetting via and ROI, and I don't quite understand what is the funtion of the Mask pixels oustide the ROI option. I thought this subsetting was a sort of cut function. Can someone shed somelight about this??   S  

All images are stored as grids in ENVI with a certain number of samples and lines.  ROIs can be defined areas of any shape because they are created with polygons or other means.  So lets say I have a diagonal, irregular shaped ROI that crosses my image area and I want to use to subset an image.  The output still must a rectangular image but it will be the smallest bounding box you can make around the ROI.  So, you could have areas outside of the ROI included in the output.  The masking allows you to set these areas to a value you specify, such as 0 or -9999.  ENVI doesn't really have a concept of 'No data' in an image, like you can have in ArcGIS.  But if you saved that image from Arc to an image format such as TIFF, it would also be rectangular with 'background' set to some value.