Hello everybody, I am new in the forum so let present myself. I am an Engineering student and I am learning the bases of remote sensing. I use Envi 4.7 and in my project I calculated the NDVI values variations of an area between two periods of time using the Band Math. Now the problem is: I need to keep fixed the values of NDVI for some pixels of the second image, while the other pixel have to change their NDVI value according to a proportion. I mean, suppose the pixels that I want to keep fixed in the second image are changed from the value of 0.4 (first image) to the value of 0.6 (second image). How can I keep fixed the value to 0.4 even in the second image? And how can I recalculated the values of other pixels in the second image according to this proportion (if I have not considered the change of +0.2 for some pixel of the image, even other pixels have to respect this proportion and decrease their value, don't they?) ? Could anyone help me, please? Sorry for my poor English and confused explanation. Thank you!