# 3dedgedog¶

## Overview¶

```
This program calculates edges in an image using the Difference of Gaussians
(DOG) method by Wilson and Giese (1977) and later combined with work by
Marr and Hildreth (1980) to provide a computationally efficient
approximation to their Lagrangian of Gaussian (LOG) method for calculating
edges in an image. This is a fascinating set of papers to read. But you
don't have to take *my* word for it!...
Generating edges in this way has some interesting properties, such as
numerical efficiency and edges that are closed loops/surfaces. The edges
can be tuned to focus on structures of a particular size, too, which can be
particularly useful in some applications.
written by: PA Taylor and DR Glen (SSCC, NIMH, NIH)
```

### Description¶

```
The primary papers for learning more about the DOG and LOG methods are:
Wilson HR, Giese SC (1977). Threshold visibility of frequency
gradient patterns. Vision Res. 17(10):1177-90.
doi: 10.1016/0042-6989(77)90152-3. PMID: 595381.
Marr D, Hildreth E (1980). Theory of edge detection. Proc R Soc
Lond B Biol Sci. 207(1167):187-217.
doi: 10.1098/rspb.1980.0020. PMID: 6102765.
Thanks to C. Rorden for pointing these papers out and discussing them.
The current code here extends/tweaks the MH1980 algorithm a bit. It runs
in 3D by default (a straightforward extension), it also employs the
Euclidean Distance Transform (EDT) to pick out the actual edges from the
DOG step---see 3dDepthMap for more information about the EDT.
The DOG-based edges require specifying a couple parameters, the main
one being interpretable as a minimal 'scale size' for structures. In this
code, this is the 'sigma_rad' (or 'sigma_nvox', if you want to specify it
in terms of the number of voxels along a given axis), which is the 'inner
Gaussian' sigma value, if you are following MH1980. The default for this
sigma_rad parameter is set based on the expected average thickness of adult
human GM, but it is easily alterable at the command line for any other
values.
```

## Command usage and option list¶

```
3dedgedog [options] -prefix PREF -input DSET
where:
-input DSET :(req) input dataset
-prefix PREF :(req) output prefix name
-mask MASK :mask dataset. NB: this mask is only applied *after*
the EDT has been calculated. Therefore, the boundaries
of this mask have no affect on the calculated distance
values, except for potentially zeroing some out at the
end. Mask only gets made from [0]th vol.
-automask :alternative to '-mask ..', for automatic internal
calculation of a mask in the usual AFNI way. Again, this
mask is only applied after all calcs (so using this does
not speed up the calc or affect distance values).
** Special note: you can also write '-automask+X', where
X is some integer; this will dilate the initial automask
X number of times (as in 3dAllineate); must have X>0.
-sigma_rad RRR :radius for 'inner' Gaussian, in units of mm; RRR must
by greater than zero (def: 1.40). Default is chosen to
capture useful features in typical adult, human GM,
which has typical thickness of 2-2.5mm. So, if you are
analyzing some other kind of data, you might want to
adapt this value appropriately.
-sigma_nvox NNN :define radius for 'inner' Gaussian by providing a
multiplicative factor for voxel edge length, which will
be applied in each direction; NNN can be any float
greater than zero. This is an alternative to the
'-sigma_rad ..' opt (def: use '-sigma_rad' and its
default value).
-ratio_sigma RS :the ratio of inner and outer Gaussian sigma values.
That is, RS defines the size of the outer Gaussian,
by scaling up the inner value. RS can be any float
greater than 1 (def: 1.40). See 'Notes' for more about
this parameter.
-output_intermed :use this option flag if you would like to output some
intermediate dataset(s):
+ DOG (difference of Gaussian)
+ EDT2 (Euclidean Distance Transform, dist**2 vals),
[0]th vol only
+ BLURS (inner- and outer-Gaussian blurred dsets),
[0]th vol only
(def: not output). Output names will be user-entered
prefix with a representative suffix appended.
-edge_bnd_NN EBN :specify the 'nearest neighbor' (NN) value for the
connectedness of the drawn boundaries. EBN must be
one of the following integer values:
1 -> for face only
2 -> for face+edge
3 -> for face+edge+node
(def: 1).
-edge_bnd_side EBS :specify which boundary layer around the zero-layer
to use in the algorithm. EBS must be one of the
following keywords:
"NEG" -> for negative (inner) boundary
"POS" -> for positive (outer) boundary
"BOTH" -> for both (inner+outer) boundary
"BOTH_SIGN" -> for both (inner+outer) boundary,
with pos/neg sides keeping sign
(def: "NEG").
-edge_bnd_scale :by default, this program outputs a mask of edges, so
edge locations have value=1, and everything else is 0.
Using this option means the edges will have values
scaled to have a relative magnitude between 0 and 100
(NB: the output dset will still be datum=short)
depending on the gradient value at the edge.
When using this opt, likely setting the colorbar scale
to 25 will provide nice images (in N=1 cases tested,
at least!).
-only2D SLI :instead of estimating edges in full 3D volume, calculate
edges just in 2D, per plane. Provide the slice plane
you want to run along as the single argument SLI:
"axi" -> for axial slice
"cor" -> for coronal slice
"sag" -> for sagittal slice
```

## Notes¶

### The value of sigma_rad¶

```
(... which sounds like the title of a great story, no? Anyways...)
This parameter represents the ratio of the width of the two Gaussians that
are blurred in the first stage of the DOG estimation. In the limit that
sigma_rad approaches 1, the DOG -> LOG. So, we want to keep the value of
this parameter in the general vicinity of 1 (and it can't be less than 1,
because the ratio is of the outer-to-the-inner Gaussian). MH1980 suggested
that sigma_rad=1.6 was optimal 'on engineering grounds' of bandwidth
sensitivity of filters. This is *very approximate* reasoning, but provides
another reference datum for selection.
Because the DOG approximation used here is for visual purposes of MRI
datasets, often even more specifically for alignment purposes, we have
chosen a default value that seemed visually appropriate to real data.
Values of sigma_rad close to one show much noisier, scattered images---that
is, they pick up *lots* of contrast differences, probably too many for most
visualization purposes. Edge images smoothen as sigma_rad increases, but
as it gets larger, it can also blend together edges of features---such as
gyri of the brain with dura. So, long story short, the default value here
tries to pick a reasonable middle ground.
```

## Examples¶

```
1) Basic case:
3dedgedog \
-input anat+orig.HEAD \
-prefix anat_EDGE.nii.gz
2) Same as above, but output both edges from the DOG+EDT steps, keeping
the sign of each side:
3dedgedog \
-edge_bnd_side BOTH_SIGN \
-input anat+orig.HEAD \
-prefix anat_EDGE_BOTHS.nii.gz
3) Output both sides of edges, and scale the edge values (by DOG value):
3dedgedog \
-edge_bnd_side BOTH_SIGN \
-edge_bnd_scale \
-input anat+orig.HEAD \
-prefix anat_EDGE_BOTHS_SCALE.nii.gz
4) Increase scale size of edged shapes to 2.7mm:
3dedgedog \
-sigma_rad 2.7 \
-edge_bnd_scale \
-input anat+orig.HEAD \
-prefix anat_EDGE_BOTHS_SCALE.nii.gz
5) Apply automasking, with a bit of mask dilation so outer boundary is
included:
3dedgedog \
-automask+2 \
-input anat+orig.HEAD \
-prefix anat_EDGE_AMASK.nii.gz
```