The default stretch applied to an image in ENVI is a 2% linear stretch taken from the scroll window. This setting is in the ENVI Preferences under Display. I believe this tech tip describes some of the behavior: http://www.ittvis.com/ser...echtip.asp?ttid=3869

ENVI's algorithm is more complex than what I show here, but you are on the right track when you wonder whether the histogram is involved. The image processing is some variation of IDL's HIST_EQUAL. The following code steps will not give you the exact result of ENVI, but it is close: ; My 'enviImg' image is an export to IDL of the 3 bands of ENVI example ; data 'cup95eff.init' which are displayed by default. help, enviImg ;ENVIIMG INT = Array[400, 350, 3] window, /FREE, XSIZE=400, YSIZE=350 tvscl, enviImg, TRUE=3 ; bytescale but no stretch produces low contrast stretchedImg = bytarr(400,350,3) ; A 2% linear sctretch of an RGB image done with HIST_EQUAL for i = 0, 2 do stretchedImg[*,*,i] = hist_equal(enviImg[*,*,i], PERCENT=2) WINDOW, /FREE, XSIZE=400, YSIZE=350 tv, stretchedImg, TRUE=3, /ORDER James Jones