If you do a search in IDL Online Help's Search tab page for the word "gradient", I think you will see that there are many built-in functions in IDL designed to measure gradient in grayscale images. There are also functions that expect the user/programmer to supply the analytic gradient. Reading through these options makes me think that there is no simple built-in routine designed to output the data you are looking for in a single line or two of code. Generally speaking, IDL should be able to take a 3D volume of density magnitude values and calculate for each point the directional vector toward which the greatest density increase points. Is that what you are looking for? Unfortunately, I cannot think of what single IDL function or procedure would provide all the vectors in a single call. James Jones