# Questions about dirorder
**URL:** https://community.mrtrix.org/t/questions-about-dirorder/4706
**Category:** Uncategorized
**Tags:** preprocessing
**Created:** [March 29, 2021, 1:01am UTC](https://community.mrtrix.org/t/questions-about-dirorder/4706 "2021-03-29T01:01:33Z")
**Posts on this page:** 5
**Page:** 1
### Author:  [@ruifate](https://community.mrtrix.org/u/ruifate)
#### Post date: [March 29, 2021, 1:01am UTC](https://community.mrtrix.org/t/questions-about-dirorder/4706/1 "2021-03-29T01:01:33Z")
Hi MRTrix-ers,
Hope you guys are doing well.
I would like to use dirorder to downsampling angular resolution from a multi-shell high-angular-resolution dwi. However when I used
```auto
dirorder bvecs reorder_bvecs
```
this command returned [ERROR] unexpected number of columns for directions file “bvecs”. But my bvecs can be successfully recognized by “dirstat”. So I need you help. Thank you very much.
Best regards,
Rui
---
### Author:  [@jdtournier](https://community.mrtrix.org/u/jdtournier)
#### Post date: [March 30, 2021, 9:40am UTC](https://community.mrtrix.org/t/questions-about-dirorder/4706/2 "2021-03-30T09:40:31Z")
Apologies, `dirorder` is indeed not very well documented. It expects as input either a 2-column matrix of [az el] pairs, or a 3-column matrix of [x y z] triplets (one row per direction). The latter is effectively the transpose of the bvecs¹.
Also, just in case: note that this implies that the directions you’re providing correspond to a _single shell_. You can’t provide all the directions for a multi-shell scheme and expect the output to be sensible, it won’t know which direction belongs to which shell…
* * *
¹ bearing in mind that the bvecs are defined with respect to the image axes, not the scanner / world frame as would typically be expected for MRtrix applications – though this shouldn’t make any difference here.
---
### Author:  [@dieloz](https://community.mrtrix.org/u/dieloz)
#### Post date: [March 21, 2022, 10:58am UTC](https://community.mrtrix.org/t/questions-about-dirorder/4706/3 "2022-03-21T10:58:56Z")
Hi Jacques-Donald,
As Rui, I’m insterested to downsample a DWI dataset from 64 to 32 directions. I was able to run the dirorder on bvecs file that I previously transposed using python. Two questions:
- Do you know which mrtrix function that exports ONLY the 3 Cartesian coordinates (mrinfo -grad exports the [x y z bvals]) column-wise?
- the bvect dirorder output is exactly the same as the input. Does it mean that if I select the first 32 directions I can correctly and homogeneously dowsample the DWI data?
In anticipation, thanks for your atention and time
Diego
---
### Author:  [@jdtournier](https://community.mrtrix.org/u/jdtournier)
#### Post date: [March 21, 2022, 1:03pm UTC](https://community.mrtrix.org/t/questions-about-dirorder/4706/4 "2022-03-21T13:03:33Z")
> [@dieloz](#):
>
> - Do you know which mrtrix function that exports ONLY the 3 Cartesian coordinates (mrinfo -grad exports the [x y z bvals]) column-wise?
No, we don’t provide that functionality, but it’s easy enough with a bit of old-school Unix, for example:
```auto
awk '{ print $1, $2, $3 }' < dw_scheme.txt > dirs.txt
```
Where `dw_scheme.txt` is a text file in the format produced by `mrinfo -dwgrad`, and `dirs.txt` is the output file with only the direction vectors in it.
> [@dieloz](#):
>
> - the bvect dirorder output is exactly the same as the input. Does it mean that if I select the first 32 directions I can correctly and homogeneously dowsample the DWI data?
Feeding the output of that command as its input produces the same output again, so if that’s the case, then yes, I think you can safely assume that you can pick the first N directions and that they would be relatively evenly distributed (within the limits of what can be done when simply picking directions from an existing set).
---
### Author:  [@dieloz](https://community.mrtrix.org/u/dieloz)
#### Post date: [March 24, 2022, 8:16am UTC](https://community.mrtrix.org/t/questions-about-dirorder/4706/5 "2022-03-24T08:16:58Z")
Dear Jacques-Donald,
Thank you very much for your prompt reply and sorry for the delay in my response. I can cofirm that I correclty picked up the b0 plus the first 30 directions doing this:
`mrconvert dwi.mif -coord 3 0:30 dwi30.mif`
All the best
Diego