# SciVisFall2008/Assignment 3/FAQ

You can post your questions to assignment 3 here:

## Exercise 1

### Can you explain what is meant by "effect of the different scanning modality" in Problem 1e?

The question is aimed at exploring the relationship between the quantity that a scanner is measuring and the type of visualization that can be produced. As explained in the assignment description, a CT scanner measures tissue density while an MRI measures the effect of water in the volume. This has important consequences when visualizing the final data even if the volumes are essentially the same (in this case, both are human heads).

## Exercise 2

## Exercise 3

### Could you explain how do I compute discrete differences em 3D? I looked at the "Gradient Generation" python source from Assignment 2 but I did not really understand it and I'm not sure how to apply these discrete differences to a 3D space.

The gradient is defined as:

grad{f(x_0, ... , x_n)} = (df/dx_0, ... df/dx_n)

In assignment 2, only two dimensions were used. You have to include the z component. The magnitude of this would be the standard vector length calculation.

A discrete differencing approach to find the gradient is explained in:

http://en.wikipedia.org/wiki/Finite_difference