‘image_key’ and ‘image_key_control’

NXtomo application is defining the format to be used to handle NXtomo.

In order to know the frame type (dark field, flat field, projection…) we use the image_key dataset. It gives an int value for each frame:

• projection = 0

• flat field = 1

• dark field = 2

• invalid = 3

But on our case we want to distinguish some specific projection which are the alignment or return projections. So projections done again at a specific angle for alignment, check if the sample has moved… As this was not part of the initial application we created the ‘image_key_control’ dataset which can contain a -1 value for those projections.

Now the format allows to provide negative values to image_key. But until now we have keep the two datasets.

Note

to interprete an NXtomo we first try to access image_key_control else we fall back on image_key. This process ensure compatibility with any NXtomo

Hint

to edit values of image_key there is several options. The first one is that you create your own python script and edit the file. The second one is to use the nxtomo library. Here is a tutorial presenting the edition of an NXtomo with this library. There is also a tomwer widget to achieve it. See the video