[Paraview] graylevel displayed in color & empty box

[Berk Geveci]

> why is it that my graylevel image (loaded with the raw file reader) is
> displayed in color? For example, the background should be black but is
> displayed in bright green.

ParaView was not designed as an image processing tool. Because of
this, it does not know that you are loading a greyscale image. It
treats this data as a uniform grid which has scalar values at each
point. Because of this: 1. you will notice that interacting with
large 2D images is slow, because ParaView will generate a polygon for
each cell (pixel) and render them in full 3D, 2. the colors will not
match that of the image. Essentially, ParaView uses a lookup table 
to map scalar values (in this case, each pixel value) to colors.
The default lookup table maps the lowest value to red and highest
value to blue. You can change to a grayscale lookup table using
Edit Color Map in the Display page. See the documentation for datails. 

> When I load a 2D-image-stack (instead of just on 2D-image) an empty box
> appears in the display window but nothing else.

Indeed. Since ParaView does not support volume rendering, it has
no default way of displaying volumes. You will have to either
1. extract cut planes, 2. extract sub-grids, 3. extract iso-surfaces
or 4. extract external surfaces to look at your data. The head
screenshots on the ParaView web page were creating using the
cut and iso-surface tools.

-Berk