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SLIC Superpixels Compared to State-of-the-art
Superpixel Methods
Radhakrishna Achanta, Appu Shaji, Kevin Smith,
Aurelien Lucchi, Pascal Fua, and Sabine S
¨
usstrunk
Abstract—Computer vision applications have come to rely
increasingly on superpixels in recent years, but it is not always
clear what constitutes a good superpixel algorithm. In an effort to
understand the benefits and drawbacks of existing methods, we
empirically compare five state-of-the-art superpixel algorithms
for their ability to adhere to image boundaries, speed, memory
efficiency, and their impact on segmentation performance. We
then introduce a new superpixel algorithm, simple linear iterative
clustering (SLIC), which adapts a k-means clustering approach
to efficiently generate superpixels. Despite its simplicity, SLIC
adheres to boundaries as well as or better than previous methods.
At the same time, it is faster and more memory efficient, improves
segmentation performance, and is straightforward to extend to
supervoxel generation.
Index Terms—Superpixels, segmentation, clustering, k-means.
I. INTR ODUCTION
Superpixel algorithms group pixels into perceptually mean-
ingful atomic regions, which can be used to replace the rigid
structure of the pixel grid. They capture image redundancy,
provide a convenient primitive from which to compute image
features, and greatly reduce the complexity of subsequent
image processing tasks. They have become key building blocks
of many computer vision algorithms, such as top scoring multi-
class object segmentation entries to the PASCAL VOC Chal-
lenge [9], [29], [11], depth estimation [30], segmentation [16],
body model estimation [22], and object localization [9].
There are many approaches to generate superpixels, each
with its own advantages and drawbacks that may be better
suited to a particular application. For example, if adherence to
image boundaries is of paramount importance, the graph-based
method of [8] may be an ideal choice. However, if superpixels
are to be used to build a graph, a method that produces a more
regular lattice, such as [23], is probably a better choice. While
it is difficult to define what constitutes an ideal approach for all
applications, we believe the following properties are generally
desirable:
1) Superpixels should adhere well to image boundaries.
2) When used to reduce computational complexity as a pre-
processing step, superpixels should be fast to compute,
memory efficient, and simple to use.
3) When used for segmentation purposes, superpixels
should both increase the speed and improve the quality
of the results.
We therefore performed an empirical comparison of five
state-of-the-art superpixel methods [8], [23], [26], [25], [15],
evaluating their speed, ability to adhere to image boundaries,
All authors are with the School of Computer and Communication Sciences
(IC),
´
Ecole Polytechnique F
´
ed
´
erale de Lausanne (EPFL), Switzerland.
E-mail: firstname.lastname@epfl.ch
Fig. 1: Images segmented using SLIC into superpixels of size 64, 256,
and 1024 pixels (approximately).
and impact on segmentation performance. We also provide
a qualitative review of these, and other, superpixel methods.
Our conclusion is that no existing method is satisfactory in all
regards.
To address this, we propose a new superpixel algorithm:
simple linear iterative clustering (SLIC), which adapts k-
means clustering to generate superpixels in a manner similar
to [30]. While strikingly simple, SLIC is shown to yield state-
of-the-art adherence to image boundaries on the Berkeley
benchmark [20], and outperforms existing methods when used
for segmentation on the PASCAL [7] and MSRC [24] data
sets. Furthermore, it is faster and more memory efficient than
existing methods. In addition to these quantifiable benefits,
SLIC is easy to use, offers flexibility in the compactness and
number of the superpixels it generates, is straightforward to
extend to higher dimensions, and is freely available
1
.
II. EXISTING SUPERPIXEL METHODS
Algorithms for generating superpixels can be broadly cat-
egorized as either graph-based or gradient ascent methods.
Below, we review popular superpixel methods for each of
these categories, including some that were not originally de-
signed specifically to generate superpixels. Table I provides a
qualitative and quantitative summary of the reviewed methods,
including their relative performance.
A. Graph-based algorithms
Graph-based approaches to superpixel generation treat each
pixel as a node in a graph. Edge weights between two nodes
are proportional to the similarity between neighboring pixels.
Superpixels are created by minimizing a cost function defined
over the graph.
NC05 – The Normalized cuts algorithm [23] recursively
partitions a graph of all pixels in the image using contour
and texture cues, globally minimizing a cost function defined
on the edges at the partition boundaries. It produces very
1
Cross-platform executables and source code for SLIC superpixels and
supervoxels can be found at http://ivrg.epfl.ch/research/superpixels
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TABLE I: Summary of existing superpixel algorithms. The ability of a superpixel method to adhere to boundaries found in the Berkeley data set [20]
is measured and ranked according to two standard metrics: under-segmentation error and boundar y recall (for ∼500 super pixels). We also report
the average time required to segment images using an Intel Dual Core 2.26 GHz processor with 2GB RAM, and the class-averaged segmentation
accuracy obtained on the MSRC data set using the method described in [11]. Bold entries indicate best performance in each category. Ability to
specify the amount of superpixels, control their compactness, and ability to generate supervoxels is also provided.
Graph-based Gradient-ascent-based
GS04 NC05 SL08 GCa10
b
GCb10
b
WS91 MS02 TP09
b
QS09 SLIC
[8] [23] [21] [26] [26] [28] [4] [15] [25]
Adherence to boundaries
Under-segmentation error (rank) 0.23 0.22 - 0.22 0.22 - - 0.24 0.20 0.19
Boundary recall (rank) 0.84 0.68 - 0.69 0.70 - - 0.61 0.79 0.82
Segmentation speed
320 × 240 image 1.08s
a
178.15s - 5.30s 4.12s - - 8.10s 4.66s 0.36s
2048 × 1536 image 90.95s
a
N/A
c
- 315s 235s - - 800s 181s 14.94s
Segmentation accuracy (using [11] on MSRC) 74.6% 75.9% - - 73.2% - - 62.0% 75.1% 76.9%
Control over amount of superpixels No Yes Yes Yes Yes No No Yes No Yes
Control over superpixel compactness No No No No
d
No
d
No No No No Yes
Supervoxel extension No No No Yes Yes Yes No No No Yes
a
Reported time includes parameter search.
b
Considers intensity only, ignores color.
c
NC05 failed to segment 2048 × 1536 images, producing “out of memory” errors.
d
Constant-intensity (GCa10) or compact (GCb10) superpixels can be selected.
regular, visually pleasing superpixels. However, the boundary
adherence of NC05 is relatively poor and it is the slowest
among the methods (particularly for large images), although
attempts to speed up the algorithm exist [5]. NC05 has a
complexity of O(N
3
2
) [15], where N is the number of pixels.
GS04 – Felzenszwalb and Huttenlocher [8] propose an alter-
native graph-based approach that has been applied to generate
superpixels. It performs an agglomerative clustering of pixels
as nodes on a graph, such that each superpixel is the minimum
spanning tree of the constituent pixels. GS04 adheres well to
image boundaries in practice, but produces superpixels with
very irregular sizes and shapes. It is O(N log N) complex and
fast in practice. However, it does not offer an explicit control
over the amount of superpixels or their compactness.
SL08 – Moore et al. propose a method to generate superpixels
that conform to a grid by finding optimal paths, or seams, that
split the image into smaller vertical or horizontal regions [21].
Optimal paths are found using a graph cuts method similar
to Seam Carving [1]. While the complexity of SL08 is
O(N
3
2
log N) according to the authors, this does not account
for the pre-computed boundary maps, which strongly influence
the quality and speed of the output.
GCa10 and GCb10 – In [26], Veksler et al. use a global
optimization approach similar to the texture synthesis work
of [14]. Superpixels are obtained by stitching together over-
lapping image patches such that each pixel belongs to only
one of the overlapping regions. They suggest two variants of
their method, one for generating compact superpixels (GCa10)
and one for constant-intensity superpixels (GCb10).
B. Gradient-ascent-based algorithms
Starting from a rough initial clustering of pixels, gradient
ascent methods iteratively refine the clusters until some con-
vergence criterion is met to form superpixels.
MS02 – In [4], mean shift, an iterative mode-seeking pro-
cedure for locating local maxima of a density function, is
applied to find modes in the color or intensity feature space
of an image. Pixels that converge to the same mode define the
superpixels. MS02 is an older approach, producing irregularly
shaped superpixels of non-uniform size. It is O(N
2
) complex,
making it relatively slow and does not offer direct control over
the amount, size, or compactness of superpixels.
QS08 – Quick shift [25] also uses a mode-seeking segmen-
tation scheme. It initializes the segmentation using a medoid
shift procedure. It then moves each point in the feature space to
the nearest neighbor that increases the Parzen density estimate.
While it has relatively good boundary adherence, QS08 is quite
slow, with an O(dN
2
) complexity (d is a small constant [25]).
QS08 does not allow for explicit control over the size or
number of superpixels. Previous works have used QS08 for
object localization [9] and motion segmentation [2].
WS91 – The watershed approach [28] performs a gradient
ascent starting from local minima to produce watersheds, lines
that separate catchment basins. The resulting superpixels are
often highly irregular in size and shape, and do not exhibit
good boundary adherence. The approach of [28] is relatively
fast (O(N log N) complexity), but does not offer control over
the amount of superpixels or their compactness.
TP09 – The Turbopixel method progressively dilates a set
of seed locations using level-set based geometric flow [15].
The geometric flow relies on local image gradients, aiming
to regularly distribute superpixels on the image plane. Unlike
WS91, TP09 superpixels are constrained to have uniform size,
compactness, and boundary adherence. TP09 relies on algo-
rithms of varying complexity, but in practice, as the authors
claim, has approximately O (N ) behaviour [15]. However, it is
among the slowest algorithms examined and exhibits relatively
poor boundary adherence.
III. SLIC SUPERPIXELS
We propose a new method for generating superpixels
which is faster than existing methods, more memory efficient,
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exhibits state-of-the-art boundary adherence, and improves
the performance of segmentation algorithms. Simple linear
iterative clustering (SLIC) is an adaptation of k-means for
superpixel generation, with two important distinctions:
1) The number of distance calculations in the optimization
is dramatically reduced by limiting the search space to a
region proportional to the superpixel size. This reduces
the complexity to be linear in the number of pixels N
– and independent of the number of superpixels k.
2) A weighted distance measure combines color and spatial
proximity, while simultaneously providing control over
the size and compactness of the superpixels.
SLIC is similar to the approach used as a preprocessing step
for depth estimation described in [30], which was not fully
explored in the context of superpixel generation.
A. Algorithm
SLIC is simple to use and understand. By default, the
only parameter of the algorithm is k, the desired number
of approximately equally-sized superpixels.
2
For color images
in the CIELAB color space, the clustering procedure begins
with an initialization step where k initial cluster centers
C
i
= [l
i
a
i
b
i
x
i
y
i
]
T
are sampled on a regular grid spaced
S pixels apart. To produce roughly equally sized superpixels,
the grid interval is S =
p
N/k. The centers are moved to
seed locations corresponding to the lowest gradient position
in a 3 × 3 neighborhood. This is done to avoid centering a
superpixel on an edge, and to reduce the chance of seeding a
superpixel with a noisy pixel.
Next, in the assignment step, each pixel i is associated with
the nearest cluster center whose search region overlaps its
location, as depicted in Fig. 2. This is the key to speeding up
our algorithm because limiting the size of the search region
significantly reduces the number of distance calculations, and
results in a significant speed advantage over conventional k-
means clustering where each pixel must be compared with all
cluster centers. This is only possible through the introduction
of a distance measure D, which determines the nearest cluster
center for each pixel, as discussed in Section III-B. Since
the expected spatial extent of a superpixel is a region of
approximate size S × S, the search for similar pixels is done
in a region 2S × 2S around the superpixel center.
Once each pixel has been associated to the nearest cluster
center, an update step adjusts the cluster centers to be the mean
[l a b x y]
T
vector of all the pixels belonging to the cluster.
The L
2
norm is used to compute a residual error E between
the new cluster center locations and previous cluster center
locations. The assignment and update steps can be repeated
iteratively until the error converges, but we have found that
10 iterations suffices for most images, and report all results
in this paper using this criteria. Finally, a post-processing
step enforces connectivity by re-assigning disjoint pixels to
nearby superpixels. The entire algorithm is summarized in
Algorithm 1.
2
Optionally, the compactness of the superpixels can be controlled by
adjusting m, which is discussed in Section III-B.
(a) standard k-means searches (b) SLIC searches
the entire image a limited region
Fig. 2: Reducing the superpixel search regions. The complexity of SLIC
is linear in the number of pixels in the image O(N ), while the conven-
tional k -means algorithm is O(kN I) where I is the number of iterations.
This is achieved by limiting the search space of each cluster center in the
assignment step. (a) In the conventional k-means algorithm, distances
are computed from each cluster center to every pixel in the image.
(b) SLIC only computes distances from each cluster center to pixels
within a 2S × 2S region. Note that the expected superpixel size is only
S × S, indicated by the smaller square. This approach not only reduces
distance computations but also makes SLIC’s complexity independent
of the number of superpixels.
Algorithm 1 SLIC superpixel segmentation
/∗ Initialization ∗/
Initialize cluster centers C
k
= [l
k
, a
k
, b
k
, x
k
, y
k
]
T
by
sampling pixels at regular grid steps S.
Move cluster centers to the lowest gradient position in a
3 × 3 neighborhood.
Set label l(i) = −1 for each pixel i.
Set distance d(i) = ∞ for each pixel i.
repeat
/∗ Assignment ∗/
for each cluster center C
k
do
for each pixel i in a 2S × 2S region around C
k
do
Compute the distance D between C
k
and i.
if D < d(i) then
set d(i) = D
set l(i) = k
end if
end for
end for
/∗ Update ∗/
Compute new cluster centers.
Compute residual error E.
until E ≤ threshold
B. Distance measure
SLIC superpixels correspond to clusters in the labxy color–
image plane space. This presents a problem in defining the
distance measure D, which may not be immediately obvious.
D computes the distance between a pixel i and cluster center
C
k
in Algorithm 1. A pixel’s color is represented in the
CIELAB color space [l a b]
T
, whose range of possible values
is known. The pixel’s position position [x y]
T
, on the other
hand, may take a range of values that varies according to the
size of the image.
Simply defining D to be the five-dimenensional Euclidean
distance in labxy space will cause inconsistencies in clustering
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behavior for different superpixel sizes. For large superpixels,
spatial distances outweigh color proximity, giving more rela-
tive importance to spatial proximity than color. This produces
compact superpixels that do not adhere well to image bound-
aries. For smaller superpixels, the converse is true.
To combine the two distances into a single measure, it is
necessary to normalize color proximity and spatial proximity
by their respective maximum distances within a cluster, N
s
and N
c
. Doing so, D
0
is written
d
c
=
p
(l
j
− l
i
)
2
+ (a
j
− a
i
)
2
+ (b
j
− b
i
)
2
d
s
=
p
(x
j
− x
i
)
2
+ (y
j
− y
i
)
2
D
0
=
r
d
c
N
c
2
+
d
s
N
s
2
. (1)
The maximum spatial distance expected within a given cluster
should correspond to the sampling interval, N
S
= S =
p
(N /K). Determining the maximum color distance N
c
is
not so straightforward, as color distances can vary significantly
from cluster to cluster and image to image. This problem can
be avoided by fixing N
c
to a constant m so that Eq. 1 becomes
D
0
=
q
d
c
m
2
+
d
s
S
2
, (2)
which simplifies to the distance measure we use in practice
D =
s
d
c
2
+
d
s
S
2
m
2
. (3)
By defining D in this manner, m also allows us to weigh the
relative importance between color similarity and spatial prox-
imity. When m is large, spatial proximity is more important
and the resulting superpixels are more compact (i.e. they have
a lower area to perimeter ratio). When m is small, the resulting
superpixels adhere more tightly to image boundaries, but have
less regular size and shape. When using the CIELAB color
space, m can be in the range [1, 40].
Equation 3 can be adapted for grayscale images by setting
d
c
=
p
(l
j
− l
i
)
2
. It can also be extended to handle 3D
supervoxels, as depicted in Figure 3, by including the depth
dimension to the spatial proximity term of Eq. 3
d
s
=
q
(x
i
− x
j
)
2
+ (y
i
− y
j
) + (z
i
− z
j
)
2
. (4)
C. Post-processing
Like some other superpixel algorithms [8], SLIC does not
explicitly enforce connectivity. At the end of the clustering
procedure, some “orphaned” pixels that do not belong to the
same connected component as their cluster center may remain.
To correct for this, such pixels are assigned the label of the
nearest cluster center using a connected components algorithm.
D. Complexity
By localizing the search in the clustering procedure, SLIC
avoids performing thousands of redundant distance calcula-
tions. In practice, a pixel falls in the neighborhood of less than
eight cluster centers, meaning that SLIC is O(N) complex.
In contrast, the trivial upper bound for the classical k-means
algorithm is O(k
N
) [17], and the practical time complexity
t =0 t =20 t =40 t =60 t =80 t =100 t =120 t =140
Fig. 3: SLIC supervoxels computed for a video sequence. (top) frames
from a short video sequence of a flag waving. (bottom left) A volume
containing the video. The last frame appears at the top of the volume.
(bottom right) A supervoxel segmentation of the video. Supervoxels with
orange cluster centers are removed for display purposes.
is O(N kI) [6], where I is the number of iterations required
for convergence. While schemes to reduce the complexity of
k-means have been proposed using prime number length sam-
pling [27], random sampling [13], local cluster swapping [12],
and by setting lower and upper bounds [6], these methods
are very general in nature. SLIC is specifically tailored to
the problem of superpixel clustering. Finally, unlike most
superpixel methods and the aforementioned approaches to
speed up k-means, the complexity of SLIC is linear in the
number of pixels, irrespective of k.
IV. COMPARISON WITH STATE-OF-THE-ART
We performed a quantitative comparison of SLIC and five
state-of-the-art superpixel methods using publicly available
source code. These algorithms include GS04
3
, NC05
4
, TP09
5
,
QS09
6
, and two versions of the algorithm proposed in [26],
GCa10 and GCb10
7
. Examples of superpixel segmentations
produced by each method appear in Fig. 7.
A. Adherence to boundaries
Arguably, the most important property of a superpixel
method is its ability to adhere to image boundaries. Boundary
recall and under-segmentation error are standard measures for
boundary adherence [15], [26]. In Fig. 4(a) and (b), SLIC,
GS04, NC05, TP09, QS09, and GC10, are compared using
these measures on the Berkeley database [20]. In addition, a
baseline performance obtained by segmenting the image into
uniform squares is denoted as “Squares”. The Berkeley data set
contains three-hundred 321 × 481 images, and approximately
10 human-annotated ground truth segmentations correspond-
ing to each image.
Boundary recall measures what fraction of the ground truth
edges fall within at least two pixels of a superpixel boundary.
The boundary recall of each method is plotted in Fig. 4(a)
3
http://people.cs.uchicago.edu/
∼
pff/segment/
4
http://www.cs.sfu.ca/
∼
mori/research/superpixels/
5
http://www.cs.toronto.edu/
∼
babalex/turbopixels supplementary.tar.gz
6
http://www.vlfeat.org/download.html
7
http://www.csd.uwo.ca/faculty/olga/Code/superpixels1pt1.zip
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(a) Boundary Recall
500 1000 1500 2000
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Number of superpixels
Boundary recall
GS04
NC05
TP09
QS09
GCa10
GCb10
Squares
SLIC
GSLIC
ASLIC
(b) Under-segmentation Error
500 1000 1500 2000
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
Number of superpixels
Under−segementation error
GS04
NC05
TP09
QS09
GCa10
GCb10
Squares
SLIC
GSLIC
ASLIC
(c) Segmentation Speed
Fig. 4: Boundary adherence and segmentation speed. (a) Boundary
recall measures the fraction of the ground truth edges that fall within
at least two pixels of a superpixel boundary. While GS04 demonstrates
the best boundary recall, reducing m from the default value increases
the boundary recall of SLIC over that of GS04. (b) Under-segmentation
error measures the amount of superpixel “leak” for a given ground truth
region. SLIC outperforms the other methods, showing the lowest under-
segmentation error for most of the useful operating regime. (c) Time
required to generate superpixels for images of increasing size. SLIC is
the fastest superpixel method, followed closely by GS04, and then a
significant gap. NC05 is not plotted due to its particularly slow speed.
for increasing numbers of superpixels. A high boundary recall
indicates that very few true edges were missed. Superpixels
generated by SLIC and GS04 demonstrated the best boundary
recall performance. If we reduce SLIC’s compactness m from
its default value of 10, SLIC shows superior performance to
GS04.
Under-segmentation error, shown in Fig. 4(b), is another
measure of boundary adherence. Given a region from the
ground truth segmentation g
i
and the set of superpixels re-
quired to cover it, s
j
|s
j
T
g
i
, it measures how many pixels
from s
j
“leak” across the boundary of g
i
. If |.| is the size of a
segment in pixels, M is the number of ground truth segments,
and B is a minimum number of pixels in s
j
overlapping g
i
,
under-segmentation error is expressed as
U =
1
N
M
X
i=1
X
s
j
|s
j
T
g
i
>B
|s
j
|
− N
. (5)
B is set to 5% of |s
j
| in our experiments to account for
ambiguities in the ground truth. Superpixels that do not tightly
fit the ground truth result in a high value of U .
B. Computational and memory efficiency
Superpixels are often used to replace the pixel-grid to
help speed up other algorithms. Thus, it is important that
superpixels can be generated efficiently in the first place.
In Fig. 4(c), we compare the time required for the various
superpixel methods to segment images of increasing size on
an Intel Dual Core 2.26 GHz processor with 2GB RAM.
SLIC, with its O(N) complexity, is the fastest superpixel
method, and its advantage increases with the size of the image.
While GS04 is competitive with O(N ) log N complexity, the
remaining methods show a significant gap in processing speed.
It is also important that a superpixel algorithm is memory
efficient in order to handle large images. SLIC is the most
memory efficient method, requiring only N floats to store
the distance from each pixel to its nearest cluster center.
Other methods have comparatively high memory requirements:
GS04 and GC10 require 5N floats to store edge weights and
thresholds for 4-connectivity (or 9N for 8-connectivity).
C. Segmentation performance
Superpixels are commonly used as a pre-processing step in
segmentation algorithms. A good superpixel algorithm should
improve the performance of the segmentation algorithm that
uses it. We compared the segmentation resulting from SLIC,
GS04, NC05, TP09, QS09, and GC10 on the MSRC data
set [24]. These results were obtained using the method of [11],
which uses superpixels to compute color, texture, geometry,
and location features. It then trains classifiers for the 21 object
classes and learns a CRF model. The results appearing in Table
I show that SLIC superpixels yield the best performance. SLIC
also reduces the the computational time by a factor of over
500 over NC05, the method used in [11]. Example images
segmented using SLIC are shown in Fig. 5.
We also tested on the PASCAL VOC 2010 data set [7] using
the approach of [10]. As shown in Table II, SLIC provided a
boost in segmentation accuracy over QS09 and reduced the
time spent generating superpixels by an order of magnitude.
D. Discussion
In addition to the properties discussed above, other consider-
ations should factor into the quality of a superpixel algorithm.
One such consideration is the ease of use. Superpixel methods
with many difficult-to-tune parameters can result in lost time
![Fig. 5: Multi-class segmentation. (top) Images from the MSRC data set. (middle) Ground truth annotations. (bottom) Results obtained using SLIC superpixels in place of NC05, following the method proposed in [11].](/figures/fig-5-multi-class-segmentation-top-images-from-the-msrc-data-3e9k4vdg.webp)
![TABLE I: Summary of existing superpixel algorithms. The ability of a superpixel method to adhere to boundaries found in the Berkeley data set [20] is measured and ranked according to two standard metrics: under-segmentation error and boundary recall (for ∼500 superpixels). We also report the average time required to segment images using an Intel Dual Core 2.26 GHz processor with 2GB RAM, and the class-averaged segmentation accuracy obtained on the MSRC data set using the method described in [11]. Bold entries indicate best performance in each category. Ability to specify the amount of superpixels, control their compactness, and ability to generate supervoxels is also provided.](/figures/table-i-summary-of-existing-superpixel-algorithms-the-litgcln7.webp){kind=tracking}
![Fig. 6: SLIC applied to segment mitochondria from 2D and 3D EM images of neural tissue. (a) SLIC superpixels from an EM slice. (b) The segmentation result from the method of [18]. (c) SLIC supervoxels on a 1024 × 1024 × 600 volume. (d) Mitochondria extracted using the method described in [19]. (e) Segmentation performance comparing SLIC supervoxels vs. cubes of similar size for the volume in (c).](/figures/fig-6-slic-applied-to-segment-mitochondria-from-2d-and-3d-em-1471k9kw.webp)
