AFNI Message Board

Dear AFNI users-

We are very pleased to announce that the new AFNI Message Board framework is up! Please join us at:

https://discuss.afni.nimh.nih.gov

Existing user accounts have been migrated, so returning users can login by requesting a password reset. New users can create accounts, as well, through a standard account creation process. Please note that these setup emails might initially go to spam folders (esp. for NIH users!), so please check those locations in the beginning.

The current Message Board discussion threads have been migrated to the new framework. The current Message Board will remain visible, but read-only, for a little while.

Sincerely, AFNI HQ

History of AFNI updates  

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March 04, 2009 09:14AM
The importance of the warning depends on the data in each volume
you care about. The voxel values that contribute to a high misfit
average are those very close to zero, which are often of the least
interest.

But if you have a single huge value in a volume, it can throw off
the numerical resolution of the rest of the volume. Keep in mind
that signed shorts have only 15 bits (for the unsigned part).
There are only 32,767 possible numbers (call it 30000), plus a
scalar.

So if the largest absolute value in your dataset were 300, then the
scalar would be 1/100 (so 300.0 would be stored as 30000 times the
scalar). The smallest non-zero value one could represent like this
is 0.01 (1 times the scalar). So numbers less than 0.01 would be
truncated to zero, and those affect the misfit the most.

Now suppose there is one really large value, like 60000. So the
scalar would be 1/2, meaning one could not tell the difference
between 7 and 8. Anything less than 2 would affect the misfit most
significantly.

- rick

Subject Author Posted

Undoing contrasts?

Aditya March 01, 2009 08:31PM

Re: Undoing contrasts?

rick reynolds March 02, 2009 04:13PM

Re: Undoing contrasts?

Aditya March 03, 2009 12:00AM

Re: Undoing contrasts?

rick reynolds March 03, 2009 09:36AM

Re: Undoing contrasts?

Aditya March 03, 2009 01:15PM

Re: Undoing contrasts?

rick reynolds March 03, 2009 03:34PM

Re: Undoing contrasts?

Aditya March 04, 2009 12:53AM

Re: Undoing contrasts?

rick reynolds March 04, 2009 09:14AM

Re: Undoing contrasts?

Aditya March 05, 2009 05:01PM

Re: Undoing contrasts?

Aditya March 03, 2009 12:10AM

Re: Undoing contrasts?

Aditya March 03, 2009 12:14AM