Semantic Segmentation

1. Citation: Cheng, X.; Lei, H. Semantic
Segmentation of Remote Sensing
Imagery Based on Multiscale
Deformable CNN and DenseCRF.
Remote Sens. 2023, 15, 1229.
https://doi.org/10.3390/rs15051229
Academic Editor: Silvia Liberata Ullo
Received: 18 January 2023
Revised: 21 February 2023
Accepted: 21 February 2023
Published: 23 February 2023
Copyright: © 2023 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
remote sensing
Article
Semantic Segmentation of Remote Sensing Imagery Based on
Multiscale Deformable CNN and DenseCRF
Xiang Cheng 1,2 and Hong Lei 1,*
1 Department of Space Microwave Remote Sensing System, Aerospace Information Research Institute,
Chinese Academy of Sciences, Beijing 100190, China
2 School of Electronic, Electrical and Communication Engineering, University of Chinese Academy of Sciences,
Beijing 100039, China
* Correspondence: hlei@mail.ie.ac.cn
Abstract: The semantic segmentation of remote sensing images is a significant research direction in
digital image processing. The complex background environment, irregular size and shape of objects,
and similar appearance of different categories of remote sensing images have brought great challenges
to remote sensing image segmentation tasks. Traditional convolutional-neural-network-based models
often ignore spatial information in the feature extraction stage and pay less attention to global context
information. However, spatial context information is important in complex remote sensing images,
which means that the segmentation effect of traditional models needs to be improved. In addition,
neural networks with a superior segmentation performance often suffer from the problem of high
computational resource consumption. To address the above issues, this paper proposes a combination
model of a modified multiscale deformable convolutional neural network (mmsDCNN) and dense
conditional random field (DenseCRF). Firstly, we designed a lightweight multiscale deformable
convolutional network (mmsDCNN) with a large receptive field to generate a preliminary prediction
probability map at each pixel. The output of the mmsDCNN model is a coarse segmentation result
map, which has the same size as the input image. In addition, the preliminary segmentation result
map contains rich multiscale features. Then, the multi-level DenseCRF model based on the superpixel
level and the pixel level is proposed, which can make full use of the context information of the
image at different levels and further optimize the rough segmentation result of mmsDCNN. To be
specific, we converted the pixel-level preliminary probability map into a superpixel-level predicted
probability map according to the simple linear iterative clustering (SILC) algorithm and defined the
potential function of the DenseCRF model based on this. Furthermore, we added the pixel-level
potential function constraint term to the superpixel-based Gaussian potential function to obtain a
combined Gaussian potential function, which enabled our model to consider the features of various
scales and prevent poor superpixel segmentation results from affecting the final result. To restore the
contour of the object more clearly, we utilized the Sketch token edge detection algorithm to extract
the edge contour features of the image and fused them into the potential function of the DenseCRF
model. Finally, extensive experiments on the Potsdam and Vaihingen datasets demonstrated that the
proposed model exhibited significant advantages compared to the current state-of-the-art models.
Keywords: semantic segmentation of remote sensing imagery; deep learning; convolutional neural
network (CNN); conditional random field (CRF)
1. Introduction
At present, image semantic segmentation (ISS) is one of the most-significant areas
of research in the field of digital image processing and computer vision. Compared with
traditional image segmentation, ISS adds semantic information to the target and foreground
of the image on this basis and can obtain the information that the image itself needs
to express according to the texture, color, and other high-level semantic features of the
Remote Sens. 2023, 15, 1229. https://doi.org/10.3390/rs15051229 https://www.mdpi.com/journal/remotesensing

2. Remote Sens. 2023, 15, 1229 2 of 22
image, which is more practical [1]. The difference between remote sensing image semantic
segmentation and ordinary image semantic segmentation lies in the different processing
objects. Specifically, the semantic segmentation of remote sensing images refers to the
analysis of the spectrum, color, shape, and spatial information of various ground objects
in remote sensing images, divides the feature space into independent subspaces, and
finally, assigns each pixel in the image predetermined semantic tags. Remote sensing
images contain richer ground object information and vary in size, color, and orientation,
which leads to the emergence of inter-class similarities and intra-class variability [2–4]. For
instance, “stadium”, “church”, and “baseballfield” may appear in “school”, and there may
be large differences in different “school” scenes, as shown in Figure 1. Moreover, remote
sensing images often have a complex background environment, irregular object shapes,
similar appearances in different categories, and other factors that are not conducive to
image segmentation. In particular, compared with natural images that only contain the
three channels of RGB, high-resolution multispectral remote sensing images contain more
channels, which contain richer ground features and a more complex spatial distribution.
On account of the above-mentioned factors, the task of the semantic segmentation of remote
sensing images is far more complicated than that of natural images. Furthermore, many
natural image semantic segmentation models do not perform satisfactorily on remote
sensing images. Recently, there has been increasing research on the semantic segmentation
of remote sensing images. However, due to the above problems, the semantic segmentation
of remote sensing images is still worth further study [5–8].
Figure 1. The figure shows the inter-class similarity and intra-class variability of remote sensing
images.
Currently, several new segmentation tasks have emerged, including instance seg-
mentation and panoptic segmentation [9,10]. Different from basic semantic segmentation,
instance segmentation needs to label different individual aspects of the same object [11–13].
On the basis of instance segmentation, panoptic segmentation needs to detect and segment
all objects in the image, including the background [14–16]. It can be found that the instance
segmentation and panoptic segmentation tasks are more complicated. At present, there are
still many problems in the research of instance segmentation and panoptic segmentation,
such as object occlusion and image degradation [10,11]. Although there are many new
research branches of semantic segmentation, this does not mean that the basic semantic
segmentation research is worthless. The semantic segmentation of remote sensing images
plays an important role in many applications such as resource surveys, disaster detection,

3. Remote Sens. 2023, 15, 1229 3 of 22
and urban planning [17]. However, due to the complexity of remote sensing images, there
are many problems to be solved in basic semantic segmentation. For example, how to
improve the accuracy and execution speed of remote sensing image semantic segmentation
is still a problem worth studying. When the performance of the basic remote sensing
semantic segmentation model is satisfactory, it is reasonable to further consider the instance
segmentation and panoptic segmentation of remote sensing images.
The results of remote sensing image segmentation are mainly determined by multiscale
feature extraction, spatial context information, and boundary information [18]. First of
all, rich features are conducive to determining the object categories in images in complex
environments. In particular, feature extraction at different scales can effectively alleviate
the segmentation impact caused by the differences between object sizes in the image.
Second, the global spatial context information can help determine the category of adjacent
pixels, because it is not easy to obtain satisfactory segmentation results using only local
information. Finally, since the satellite is not stationary when taking remote sensing images,
the boundaries of objects in the image are often unclear, so more attention should be paid
to the edge details of objects during image segmentation.
Traditional remote sensing image segmentation models generally use manual feature
extractors for feature extraction and then use traditional machine learning classification
models for the final pixel classification operations. Traditional feature extraction models
include oriented FAST and rotated brief (ORB) [19], local binary pattern (LBP) [20], speeded
up robust features (SURF) [21], etc. Traditional classification methods consist of support
vector machine [22], logistic regression [23], etc. However, due to the complex background
environment in remote sensing images, the performance of traditional remote sensing
image segmentation models is often unsatisfactory. Additionally, the abilities of traditional
feature extraction models cannot meet the needs of practical applications.
Deep learning has been widely used in image recognition [24,25], image classifica-
tion [26,27], video prediction [28,29], and other fields. In particular, deep convolutional
neural networks (DCNNs) with strong feature extraction capabilities are popular in com-
puter vision [30–37]. For instance, Reference [38] designed a YOLOv3 model with four
scale detection layers for pavement crack detection, using a multiscale fusion structure and
an efficient cross-linking (EIoU) loss function. Recently, DCNNs have been gradually used
in image semantic segmentation tasks. For example, Reference [39] built a segmentation
model called U-Net, which fuses the features of different scales to obtain segmentation
results. However, the U-net model does not consider the spatial context information in
the image. The PSPNet [40] model obtains contextual information through a pyramid
pooling module, which improves the segmentation performance. However, it has high
computational complexity and poor efficiency. Reference [41] designed a high-resolution
network (HRNet), which fuses structural features and high-level semantic information at
the output end, improving the spatial information extraction ability of images, but ignoring
local information. For the problem of HRNet, Reference [42] used the attention mechanism
to improve the recognition performance of the model for local regions. However, this model
is very resource-intensive and has a large amount of redundant information. Additionally,
Reference [18] proposed a high-resolution context extraction network (HRCNet) based
on a high-resolution network for the semantic segmentation of remote sensing images.
Reference [43] built a multi-attention network (MANet) to extract contextual dependencies
in images through multiple efficient attention modules. However, the multiscale context
fusion effect of HRCNet and MANet needs to be improved, and both have the problem
of high computational complexity. In summary, during the semantic segmentation task of
remote sensing images in complex environments, the current CNN-based model does not
pay enough attention to global context information, and its performance is not completely
convincing [44]. In addition, networks with a superior segmentation performance often
have the problem of excessive computing resource consumption.
Considering the main factors affecting the semantic segmentation of remote sensing
images and the problems of deep learning models in the semantic segmentation of remote

4. Remote Sens. 2023, 15, 1229 4 of 22
sensing images, this paper proposes the mmsDCNN-DenseCRF model, in which a modified
multiscale deformable convolutional neural network (mmsDCNN) is used for multiscale
feature extraction, while DenseCRF is utilized to capture semantic information and global
context dependency and optimize rough segmentation results such as edge refinement.
First, a lightweight multiscale deformable convolution neural network is proposed, which
is based on the modified multiscale convolutional neural network (mmsCNN) designed
in our previous work [45]. The mmsDCNN model adds an offset to the sampling posi-
tion of mmsCNN convolution, allowing for the convolutional kernel to adaptively choose
the receptive field size with only a slight increase in computing resources. In addition,
the mmsDCNN can achieve a balance between strong multiscale feature extraction per-
formance and low computational complexity. We used the mmsDCNN to extract rich
multiscale features and obtain preliminary prediction results. Subsequently, a multi-level
DenseCRF based on the superpixel and pixel level is proposed as a back-end optimization
module, which can make full use of spatial context information at different granularities.
The multi-level DenseCRF model considers the relationship between each pixel and all
other pixels, establishes a dependency between all pixel pairs in the image, and uses the
interdependencies between pixels to introduce the global information of the image, which
is suitable for the semantic segmentation task of remote sensing images in complex envi-
ronments [46]. Although DenseCRF improves the robustness of processing images with
semantic segmentation, it can only determine the region position and approximate shape
contour of the object of interest in the image marking results. As a result, the real boundary
of the segmented region cannot be accurately obtained, since there are still fuzzy edge
categories or segmentation mistakes. To solve the above problems, we used the Sketch
token edge detection algorithm [47] to extract the edge contour features of the image and
fuse the edge contour features into the potential function of the DenseCRF model. In
summary, this paper proposes a new idea and specific implementation for the problems
existing in the existing models based on convolutional neural networks in remote sensing
image segmentation tasks.
The main contributions are as follows:
• A framework of the mmsDCNN-DenseCRF combined model for the semantic seg-
mentation of remote sensing images is proposed.
• We designed a lightweight mmsDCNN model, which incorporates deformable con-
volution into the mmsCNN proposed in our previous work [45]. Notably, the mms-
DCNN adds an offset to the sampling position of the mmsCNN convolution, which
enables the convolutional kernel to adaptively determine the receptive field size. Com-
pared with the mmsCNN, the mmsDCNN can achieve a satisfactory performance
improvement with only a tiny increase in the computational complexity.
• The multi-level DenseCRF model based on the superpixel level and pixel level is
proposed. We combined the pixel-level potential function with the superpixel-based
potential function to obtain the final Gaussian potential function, which enables our
model to consider features of various scales and the context information of the image
and prevents poor superpixel segmentation results from affecting the final result.
• To solve the problem of blurring edge categories or segmentation errors in the semantic
segmentation task of DenseCRF, we utilized a Sketch token edge detection algorithm
to extract the edge contour features of the image and integrated them into the Gaussian
potential function of the DenseCRF model.
The remainder of the article is organized as follows. Section 2 introduces the modified
multiscale deformable convolutional neural network (mmsDCNN), and the multi-level
DenseCRF model. In Section 3, we conduct some comparative experiments on the public
dataset, i.e., the International Society for Photogrammetry and Remote Sensing (ISPRS) [48].
Finally, Section 5 provides a summary of the article.

5. Remote Sens. 2023, 15, 1229 5 of 22
2. Methodology
The pipeline of the proposed model is shown in Figure 2. The following two parts are
included: the modified multiscale deformable convolutional neural network (mmsDCNN)
and the multi-level DenseCRF model.
Figure 2. The figure shows the basic framework of the proposed model.
2.1. mmsDCNN
At present, multiscale CNNs can be divided into two types: the multiscale image and
the multiscale convolutional kernel. The CNN based on multiscale images feeds images
of different resolutions into the neural network with the same size of the convolutional
kernel to obtain multiscale features. This method consumes many memory resources due
to the need to process multiple images at the same time. The CNN based on the multiscale
convolutional kernel feeds the image into several convolutional kernels of different sizes to
obtain multiscale features. Due to the introduction of the multiscale convolutional kernels
in this structure, the parameter amount of the network is greatly increased, and gradient
vanishing is prone to occur during model training. To address the above problems, we
proposed a modified multiscale convolutional neural network (mmsCNN) in [45]. In this
paper, we combined the previous work and improved it to propose the modified multiscale
deformable convolutional neural network (mmsDCNN) model, as shown in Figure 3.

6. Remote Sens. 2023, 15, 1229 6 of 22
Figure 3. The framework of the modified multiscale deformable convolutional neural network
(mmsDCNN).
Recently, Reference [49] provided a new perspective: compared with the CNN with a
deep large-scale convolutional kernel, the CNN with a shallow large-scale convolutional
kernel has a larger receptive field, which is more similar to the human perception mode. In
addition, compared with the deep network, the shallow network has fewer parameters, and
the problem of gradient disappearance is less likely to occur. Based on this, we integrated
the mode of shortcut connections in the residual neural network into the msCNN based
on a multiscale convolutional kernel. The proposed model consists of several various
deformable convolutional kernel CNNs, i.e., De f Conv[(1, 1), 0, 1], De f Conv[(3, 3), 1, 1],
De f Conv[(5, 5), 2, 1], and De f Conv[(7, 7), 3, 1]. Concretely, De f Conv[(m, n), i, j] denotes
the CNN with deformable convolutional kernel size = m × n, padding = i, and stride = j.
Then, we invalidated the first 1, 2, and 3 layers of De f Conv[(3, 3), 1, 1], De f Conv[(5, 5), 2, 1],
and De f Conv[(7, 7), 3, 1], respectively, which are shown in the yellow box in Figure 3. We
refer to the network layer in the yellow box as the invalid layer, which is where the
shortcut connections are performed. Subsequently, we flexibly set the pooling step size
of the network layer after the invalid layer, which ensures that the final output scales of
different convolutional networks are consistent. To be specific, we set the pooling layers to
Pooling[(4, 4), 0, 4], Pooling[(8, 8), 0, 8], Pooling[(16, 16), 0, 16] according to the number of
invalid layers, as shown in Figure 3. Pooling[(m, n), i, j] represents the pooling layers with
filtersize = m × n, padding = i, and stride = j.
For pixel-level image segmentation tasks, we fused deformable convolution with the
mmsCNN model, which can further expand the model’s receptive field. The deformable
convolution adds an offset to each convolutional sampling point, which is equivalent to the
scalable change in each block of the convolutional kernel, thereby changing the range of
the receptive field [50]. Figure 4 compares standard convolutional kernels and deformable
convolutional kernels. In the training phase, the convolutional kernel for generating

7. Remote Sens. 2023, 15, 1229 7 of 22
the output features and the convolutional kernel for generating the offsets are learned
synchronously. The experiments proved that the mmsDCNN model can consume fewer
resources, obtain better experimental results, and effectively avoid gradient disappearance.
Figure 4. The figure compares standard convolutional kernels and deformable convolutional kernels.
Since the feature map gradually becomes smaller during the convolution process, to
obtain a feature map with the same size as the original image, we needed to upsample the
feature map. An intuitive idea is to perform bilinear interpolation, which is easily imple-
mented with a fixed kernel via deconvolution. Therefore, at the output of the mmsDCNN,
we replaced the fully connected network of the mmsCNN model with a deconvolutional
network. In addition, before deconvolution, we needed to fuse the multiscale features and
then utilize a simple convolution to make the number of channels of the output feature
map the same as the predetermined number of categories of the pixels. Notably, the output
of the mmsDCNN model is a feature map of the same size as the input image, which will
be further processed by DenseCRF to obtain the final prediction map. Table 1 shows the
parameter settings of our mmsDCNN model.
Table 1. Structure parameters of our mmsDCNN model.
Type Number Filter Size Pad Stride
Conv1 + ReLU 32 1 × 1 0 1
Max Pooling − 2 × 2 0 2
Conv2 + ReLU 64 (1/3) × (1/3) 0/1 1
Max Pooling − (2/4) × (2/4) 0 2/4
Conv3 + ReLU 128 (1/3/5) × (1/3/5) 0/1/2 1
Max Pooling − (2/4/8) × (2/4/8) 0 2/4/8
Conv4 + ReLU 256 (1/3/5/7) × (1/3/5/7) 0/1/2/3 1
Max Pooling − (2/4/8/16) × (2/4/8/16) 0 2/4/8/16
Conv5 + ReLU 6 1 × 1 0 1
Deconv Layer 6 32 × 32 8 16
2.2. The Multi-Level DenseCRF Model
The conditional random field (CRF) model is a probabilistic undirected graph model,
which has recently made great progress in the application of remote sensing image semantic
segmentation. In this section, we attempt to utilize DenseCRF to further refine the rough
segmentation results output by the mmsDCNN model.
2.2.1. Edge Constraint
Compared with the pairwise CRF (PCRF) model, which only considers local neighbor-
hood relations, the DenseCRF model is more suitable for image semantic segmentation [46].
It further considers the relationship between each pixel and all other pixels, establishes
the dependency on all pixel pairs in the image, and uses the interdependence between

8. Remote Sens. 2023, 15, 1229 8 of 22
pixels to introduce the global information of the image. Although DenseCRF improves
the performance of processing image semantic segmentation, it can only determine the
region position and approximate shape contour of the object of interest in the image mark-
ing results, so there will still be fuzzy edge categories, making it difficult to obtain the
real boundary of the segmented region. In order to improve the accuracy of the image
segmentation edge, we utilized the Sketch token edge detection algorithm to extract the
edge contour features of the image and fused the edge contour features into the potential
function of DenseCRF model. The Sketch token edge detection algorithm is a method
for learning and detecting image boundary information for mid-level features. It designs
Sketch tokens to represent edge structure information in image patches. The input image
is first divided into image patches with a size of 35 × 35 pixels according to certain rules;
then, the Sketch token is used to obtain the channel index features and self-similar features
contained in the image patches, and they are finally input into the Structured Random
Forest [51] to classify the pixels. Figure 5 shows the edge detection results of some images
in the Vaihingen dataset using this algorithm. The Sketch token edge detection algorithm
can achieve a similar performance to that of state-of-the-art algorithms, and the global
probability of boundary (GPB) [52], sparse code gradients (SCGs) [53], and running speed
are greatly improved. For the image to be segmented, let S = {1, 2, · · · , N} represent the
set of image pixels and Ns represent the neighborhood area of the sth pixel. We define the
edge energy function as
E = ωp ∑
s
∑
r∈Ns
ψsr(bs, br)
= ωp ∑
s
∑
r∈Ns
exp
−α|bs − br|2
,
(1)
where ωp is the edge prior parameter, ψsr(bs, br) represents the edge constraint item, and
bs and br represent the image edge features obtained by the Sketch token edge detection
algorithm. For any two pixels, if both are points on the edge or not on the edge, then
the penalty of the edge constraint item is 0 to ensure that the segmentation result is not
over-segmented or over-smoothed. It is beneficial to maintain the edge constraint of the
image; otherwise, we added a marginal penalty to the point. The edge constraint item
can increase the constraint on the target segmentation boundary, thereby improving the
accuracy of target segmentation.
2.2.2. Combined Multilevel Potential Function
In Section 2.1, we used the mmsDCNN to perform feature extraction on the original
image to obtain a preliminary probability map of the same size. In this section, a multi-level
DenseCRF based on the superpixel level and pixel level is proposed, which takes the feature
map of the mmsDCNN as the input and further utilizes the contextual information of the
image at different levels. Notably, the potential energy function of multi-level DenseCRF
contains two parts: superpixel-level potential energy function and pixel-level potential
energy function.
Firstly, we performed superpixel segmentation on the original image according to the
simple linear iterative clustering (SILC) algorithm [54]. In particular, we considered the
label of a superpixel to be the category label of most of the pixels in the superpixel. Then,
we converted the pixel-level preliminary probability map (feature map) output by the
mmsDCNN into a superpixel-level prediction probability map according to the superpixel
segmentation map. To be specific, we have
Psuperpixel(xs) =
1
N
N
∑
n=1
Ppixel(xn), (2)
where Psuperpixel(xs) is the probability distribution of the superpixel, xs is a superpixel
block, Ppixel(xn) is the probability distribution of the nth pixel output by the mmsDCNN

9. Remote Sens. 2023, 15, 1229 9 of 22
model, and N represents the number of most pixels of the same category in a superpixel
block. Next, we defined the superpixel-level unary potential function of the DenseCRF
model according to the superpixel-level prediction probability map:
ψsu(xs) = − log Psuperpixel(xs). (3)
Figure 5. lThe figure shows the edge detection results of some images in the Vaihingen dataset using
Sketch token edge detection algorithm.
Subsequently, we extracted the color feature Is, spatial feature Ps, and texture feature
Ts of the superpixels according to the superpixel segmentation map. The color feature Is
is the average vector of pixels in the CIE-LAB space; the spatial feature Ps is the average
vector of the spatial features of all pixels in the superpixel; the texture feature Ts is extracted
from the superpixel using the local binary pattern [55]. Then, we defined the superpixel-
level binary potential function of the DenseCRF model according to the above superpixel
features:
ψsb xsi, xsj
= µ xsi, xsj
K
∑
m=1
ωsm exp
βm fsi − fsj
2
, (4)
where xsi and xsj are two different superpixel blocks, ωsm is the weight parameter on each
kernel function, and f = (Is, Ps, Ts) denotes the superpixel-level feature vectors. The label
compatibility function is defined as follows:
µ xsi, xsj
=
(
0, if xsi = xsj
1, otherwise.

10. Remote Sens. 2023, 15, 1229 10 of 22
To be more exact, the superpixel-level binary potential function of the DenseCRF
model can be expanded into
ψsb xsi, xsj
= µ xsi, xsj
ωs1 exp −
Psi − Psj
2
2θ2
s1
!
+ωs2 exp −
Psi − Psj
2
2θ2
s2
−
Isi − Isj
2
2θ2
s3
!
+ωs3 exp −
Psi − Psj
2
2θ2
s4
−
Tsi − Tsj
2
2θ2
s5
!#
,
where θs is the parameter corresponding to the feature.
Superpixel segmentation can not only obtain local feature information, but also better
fit the edge contour of the objects. However, the CRF model based on superpixels usually
enforces the consistency of the classification labels of all pixels in a superpixel, meaning that
the segmentation results are seriously dependent on the quality of the clustering algorithm.
To handle this problem, we added the pixel-level potential function constraint term to the
superpixel-based Gaussian potential function to obtain a combined Gaussian potential
function, which enabled our model to consider features of different granularities and
prevent poor superpixel segmentation results from making the output of the model worse.
We defined a pixel-level unary potential function based on the pixel-level probability
prediction map that was output by the mmsDCNN model:
ψpu(xi) = − log Ppixel(xi), (5)
where Ppixel(xi) is the probability distribution of pixel xi. Only the characteristics of a single
independent pixel are considered in the unary potential energy term, so the result of image
segmentation is often not smooth and contains noise. Therefore, we extracted pixel-level
color feature I and spatial feature P from the original image and then constructed a pixel-
level binary potential function. Similarly, the pixel-level binary potential function can be
written as
ψpb xi, xj
= µ xi, xj
ωp exp −
bi − bj
2
2θ2
p
!
+ ωp1 exp −
Pi − Pj
2
2θ2
p1
!
+ωp2 exp −
Pi − Pj
2
2θ2
p2
−
Ii − Ij
2
2θ2
p3
!#
,
where θp is the parameter corresponding to the feature and ωp exp −
bi − bj
2
2θ2
p
!
is the
edge energy function defined in Equation (1).
To more accurately model the high-dimensional complex features of images, we
introduced a high-order potential energy term based on the robust Pn Potts model [56]:
ψpn(xc) =
(
Ni(xc) 1
Q γmax, if Ni(xc) ≤ Q
γmax, otherwise,
where
Ni(xc) = min
l∈L
(|c| − nl(xc))
is the number of pixels that do not take the main part of the label in subcluster c, |c|
indicates the number of pixels in subcluster c, nl(xc) is the number of pixels in subcluster c
with class value l:
γmax = |c|θα
(θ1 + θ2G(c)),

11. Remote Sens. 2023, 15, 1229 11 of 22
G(c) represents the segmentation quality of subcluster c, θ is the model parameter, and Q
is the truncation parameter.
Finally, we obtained the multi-level DenseCRF based on the superpixel level and pixel
level, and its potential function can be expressed as
E(x) = ∑
i∈V
ψsu(xsi) + ∑
i∈V,j∈Ni
ψsb xsi, xsj
+ ∑
i∈S
ψpu(xi) + ∑
i∈S,j∈Ni
ψpb xi, xj
+ ∑
c∈C
ψpn(xc),
(6)
where V is the set of all superpixels in the image, Ni represents the neighborhood area of
the ith pixel, S is the set of all pixels in the image, and C is a set of all subclusters c in the
image. The multi-level DenseCRF can consider features of different scales for segmentation,
increase the effective constraints on the image target boundary, and improve the accuracy of
image segmentation edges. Ultimately, DenseCRF optimizes the rough segmentation result
of the mmsDCNN, such as edge thinning and precision machining, to obtain a refined
segmentation image.
3. Experiment
To demonstrate the performance of the proposed model, we conducted a series of
experiments using the publicly available Potsdam and Vaihingen datasets. In this section,
we first provide a brief introduction to the Potsdam and Vaihingen datasets. Next, we
describe the experimental parameter settings and evaluation criteria in detail. Finally, we
compare the performance with several state-of-the-art models according to the evaluation
criteria.
3.1. Datasets
The International Society for Photogrammetry and Remote Sensing (ISPRS) [48] dataset
provides two state-of-the-art aerial image datasets for the urban classification and 3D
building reconstruction test projects. This dataset employs digital surface models (DSMs)
generated from high-resolution orthophotos and corresponding dense image matching
techniques. Both datasets cover urban scenes, and each dataset is manually classified
into six common land cover classes: impervious surfaces, buildings, low vegetation, trees,
cars, and backgrounds. The background class includes water and objects distinct from
other defined classes (e.g., containers, tennis courts, swimming pools), which usually
belong to uninteresting semantic objects in urban scenes. The Vaihingen dataset contains
33 remote sensing images of different sizes; each image was extracted from a larger, top-
level orthophoto image with a spatial resolution of 9 cm. The Potsdam dataset contains
38 image regions of the same size with a spatial resolution of 5 cm. We present several
images from the Potsdam and Vaihingen datasets and their corresponding ground truth
images in Figure 6, respectively. Due to the memory limitations of the experimental
equipment, it was not possible to directly train and infer on the entire image. This paper
used the technique in [57] to cut the original data into small pieces before use, keeping the
structure of the objects in the images unchanged. We cut the image into a size of 384 × 384
with an overlap of 72 and 192 pixels in the training and testing datasets to prevent the loss
of information.

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Figure 6. (a) Sample image of Potsdam dataset. (b) Sample image of Vaihingen dataset. (c) Ground
truth of sample image of Potsdam dataset. (d) Ground truth of sample image of Vaihingen dataset.
3.2. Setting of the Experiments and Evaluation Metrics
For the training and testing setting, 19 images in the Potsdam dataset were selected
as the training set, 5 images as the validation set, and 14 images as the testing set. In
addition, 13 images were chosen in the Vaihingen dataset as the training set, 3 images as the
validation set, and the remaining 17 images as the testing set. We obtained the classification
results of 10 groups of parameters on the training set and the validation set and selected
the group with the best effect as the final parameters of our model. Table 2 reports the
parameters of experimental datasets. The experimental environment is shown in Table 3.
Table 2. Parameters of the experimental dataset.
Item
Potsdam
Training
Potsdam Testing
Vaihingen
Training
Vaihingen
Testing
size (pixel) 384 × 384 384 × 384 384 × 384 384 × 384
number 6931 13454 654 2219
overlap pixels 72 192 72 192
p pixels 72 192 72 192

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Table 3. The experimental environment.
The Experimental Environment Experimental Configuration
Processor AMD Ryzen 7 − 4800H
GPU Titan X 12G
Memory 32G
Operating system ubantu 16.04.4
Compiler Pycharm
For the parameter settings, the learning rate was initialized to 0.001. The batch size
was set to 8. The kernel parameters of multi-level DenseCRF and the weight parameters
corresponding to each kernel function were obtained by the grid search method [46].
To more accurately and objectively evaluate the performance of different models, the
overall accuracy (OA), precision, recall, F1 [58], and mean intersection over union (mIoU)
were selected as our evaluation criteria. The above evaluation parameters can be calculated
by the following formulas:
OA =
TP + TN
FP + FN + TP + TN
(7)
precision =
TP
FP + TP
(8)
recall =
TP
FN + TP
(9)
F1 = 2 ×
precision × recall
precision + recall
(10)
IoU =
TP
FN + FP + TP
(11)
where TP, TN, FP, and FN are the number of true positives, true negatives, false positives,
and false negatives of pixels in the model output image, respectively.
3.3. Experimental Results
3.3.1. Experimental Results on the Potsdam Dataset
First of all, the experiments were conducted on the Potsdam dataset. The methods
FCN [59], PSPNet [40], FPN [60], UNet [39], DeepLabv3 [61], DANet [62], LWRefineNet [63],
and HRCNet [18] were chosen for comparison. Table 4 reports their overall accuracies
(OA), precision, recall, and F1. In particular, the bold font represents the best indicator.
Table 4. Results for the Potsdam dataset using the above models.
Model Recall (%) Precision (%) F1 Score (%) OA (%)
FCN [59] 86.07 85.70 85.75 85.64
PSPNet [40] 90.13 88.95 89.45 88.78
FPN [60] 88.59 89.19 88.72 88.27
UNet [39] 89.42 88.13 88.67 87.76
DeepLabv3 [61] 90.29 89.23 89.66 88.97
DANet [62] 90.13 88.80 89.37 88.82
LWRefineNet [63] 89.10 89.01 89.02 88.43
HRCNet-W48 [18] 90.69 89.90 90.20 89.50
The proposed model 91.51 90.65 90.92 90.42
It is clear from the results our proposed model showed the best performance on the
Potsdam dataset. Compared with the HRCNet-W48 model with excellent performance,

14. Remote Sens. 2023, 15, 1229 14 of 22
the model proposed in this paper improved by 0.82% the recall index, 0.75% the precision
index, 0.72% the F1 scores, and 0.92% the overall accuracies (OAs). The experimental
results verified that the combined model of the multiscale deformable convolution network
and multi-level DenseCRF proposed by us can accurately extract low-level details and
high-level semantic features from complex remote sensing images and obtain excellent
segmentation results.
Table 5 shows the IoU results for each category for all experimental models on the
Potsdam dataset. In particular, the bold font represents the best indicator. Based on the
HRCNet-W48 model, the mmsDCNN-DenseCRF model proposed in this paper had an
IoU that improved by 1.04% for the “Impervious Surfaces” category, for the “Building”
category by 0.68%, for the “Low Vegetation” category by 0.85%, for the “Tree” category by
0.59%, and for the “Car”; category by 0.83%. In sum, the mean IoU (mIoU) of our model
was 0.8% higher than that of the HRCNet-W48 model, proving the excellent performance
of the proposed model.
Table 5. The mean intersection over union (%) on the Potsdam dataset with the above models.
Model ImSurface Building
Low
Vegetation
Tree Car mIoU (%)
FCN [59] 81.67 88.99 71.24 72.80 79.91 78.92
PSPNet [40] 82.71 90.21 72.62 74.22 81.11 80.18
FPN [60] 81.63 89.22 71.37 73.05 79.09 78.87
UNet [39] 82.58 90.08 72.58 74.22 81.38 80.17
DeepLabv3 [61] 82.81 89.95 72.22 74.11 82.16 80.25
DANet [62] 82.27 89.15 71.77 73.70 81.72 79.72
LWRefineNet [63] 80.20 87.15 71.02 71.54 75.14 77.01
HRCNet-W48 [18] 83.58 91.15 73.07 74.88 83.32 81.20
The proposed model 84.62 91.83 73.92 75.47 84.15 82.00
Figure 7 shows the experimental results of all experimental models in the Potsdam
dataset. Compared with the ground truth given in the dataset, the mmsDCNN-DenseCRF
model proposed in this paper not only considers the edge information of large objects
in the graph, but can also distinguish the details of small objects, which has an indelible
relationship with the edge constraint added in the multi-level DenseCRF.
3.3.2. Experimental Results on the Vaihingen Dataset
In the second part of the experiment, we replaced the dataset with the Vaihingen
dataset in the ISPRS. Analogously, the models FCN [59], PSPNet [40], FPN [60], UNet [39],
DeepLabv3 [61], DANet [62], LWRefineNet [63], and HRCNet [18] were selected for com-
parison. Table 6 summarizes the overall accuracies (OAs), precision, recall, and F1 of those
models. In particular, bold font represents the best indicator.
Table 6. Results for the Vaihingen dataset with the above models.
Model Recall (%) Precision (%) F1 Score (%) OA (%)
FCN [59] 80.89 83.14 81.46 87.11
PSPNet [40] 84.10 84.86 84.15 87.17
FPN [60] 82.21 84.50 82.94 86.70
UNet [39] 85.01 84.46 84.44 87.14
DeepLabv3 [61] 84.38 85.09 84.42 87.32
DANet [62] 83.89 84.65 83.95 87.07
LWRefineNet [63] 84.45 85.32 84.81 87.23
HRCNet-W48 [18] 86.53 85.96 85.97 88.33
The proposed model 87.45 87.03 86.82 89.12

15. Remote Sens. 2023, 15, 1229 15 of 22
Figure 7. The figure shows the experimental results of all experimental models and the ground truth
on the Potsdam dataset.

16. Remote Sens. 2023, 15, 1229 16 of 22
Similarly, the data in the Table 6 make it clear that the model designed in this paper
performed best on the Vaihingen dataset. To be specific, compared with the HRCNet-
W48 model with a splendid performance, the model proposed in this paper improved
by 0.92% the recall index, 1.07% the precision index, 0.85% the F1 scores, and 0.79% the
overall accuracies (OAs). The experimental results showed that the multiscale deformable
convolutional network model (mmsDCNN) proposed by us can extract rich multiscale
features from remote sensing images, and the multi-level DenseCRF further considers the
spatial context information of images and, finally, obtained a satisfactory segmentation
image.
Table 7 reports the IoU results for each category for all experimental models on the
Vaihingen dataset. In particular, the bold font represents the best indicator. Based on the
HRCNet-W48 model, the mmsDCNN-DenseCRF model proposed in this paper showed
an IoU that improved by 1.07% for the “Impervious Surfaces” category, for the “Building”
category by 0.88%, for the “Low Vegetation” category by 0.91%, for the “Tree” category
by 0.82%, and for the “Car” category by 0.93%. In summary, the mean IoU (mIoU) of our
model was 0.92% higher than that of the HRCNet-W48 model, which fully verified that our
model can also perform image segmentation tasks in Vaihingen dataset.
Table 7. The mean intersection over union (%) on the Vaihingen dataset with the above models.
Model ImSurface Building
Low
Vegetation
Tree Car mIoU (%)
FCN [59] 77.68 82.33 63.94 74.35 51.83 70.02
PSPNet [40] 78.90 84.26 65.34 75.14 56.18 71.96
FPN [60] 77.65 82.72 64.34 74.43 52.11 70.25
UNet [39] 79.02 84.46 65.23 75.13 57.07 72.18
DeepLabv3 [61] 79.23 83.70 64.88 75.14 57.54 72.10
DANet [62] 78.55 82.10 64.18 74.80 56.36 71.20
LWRefineNet [63] 79.21 85.13 65.32 76.51 53.38 71.91
HRCNet-W48 [18] 81.05 86.65 66.91 76.63 59.31 74.11
The proposed model 82.12 87.53 67.82 77.45 60.24 75.03
Figure 8 shows the experimental results of all experimental models on the Vaihingen
dataset. Compared with all the experimental models, the proposed mmsDCNN-DenseCRF
model not only focused on the overall contour of large objects in the image, but can also
consider the fine-grained information of small objects, which showed that the mmsDCNN
and multi-level DenseCRF can extract multiscale features and grasp the context of the
information in the image. In addition, Figure 9 compares the initial segmentation map
of the mmsDCNN and the semantic segmentation map of multi-level DenseCRF, which
further verified the importance of the multi-level DenseCRF.
3.4. Robustness Verification
To further evaluate the stability of the proposed model, the robustness verification
experiment was performed on the Potsdam dataset. According to the method proposed
in [35], salt-and-pepper noise and Gaussian noise were randomly added to the testing
image, where the salt-and-pepper noise ratio was 0.05 and the mean value and variance of
the Gaussian noise were 0 and 0.05. Table 8 reports the experimental results for the damaged
Potsdam testing set. According to the experimental results, although the performance of the
proposed model declined, it was still acceptable. As a consequence, the robustness of the
proposed model was verified by this experiment, and the importance of image denoising
algorithm research was also illustrated.

17. Remote Sens. 2023, 15, 1229 17 of 22
Figure 8. The figure shows the initial segmentation of the mmsDCNN and the segmentation map as
estimated from DenseCRF.

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Table 8. Results on the damaged Potsdam testing set.
Model mIoU (%) Precision (%) F1 Score (%) OA (%)
Proposed model 82.00 90.65 90.92 90.42
Proposed model
(salt-and-pepper noise)
79.65 88.33 88.82 88.25
Proposed model
(Gaussian noise)
79.78 88.72 89.03 88.49
Figure 9. The initial segmentation of the mmsDCNN and the segmentation map as estimated from
the DenseCRF.
4. Discussion
4.1. Complexity Analysis
In this part, we compared the number of parameters of all models in the experiment.
In addition, we used the floating point operations per seconds (FLOPs) to compare the
complexity of all experimental models. Table 9 reports the number of parameters and
FLOPs of all experimental models on the Potsdam dataset. Particularly, the bold fonts
indicate the best indicator.
Table 9. Complexity comparison on the Potsdam dataset.
Methods OA
Parameter
Quantities
FLOPs
FCN [59] 85.64 14.6 M 45.3 G
PSPNet [40] 88.78 46.5 M 104.0 G
FPN [60] 88.27 26.4 M 25.7 G
UNet [39] 87.76 27 M 70.0 G
DeepLabv3 [61] 88.97 226 M 96.4 G
DANet [62] 88.82 47.3 M 115.6 G
LWRefineNet [63] 88.43 176 M 2.1 G
HRCNet-W48 [18] 89.50 59.8 M 52.8 G
Proposed 90.42 29 M 11.2 G
According to Table 9, although the parameter quantities of the model proposed in this
paper were not the lowest, they were superior to most of the models. We had 14.4 M, 2.6 M,
and 2 M more parameters than FCN [59], FPN [60], and UNet [39], respectively, but the
overall accuracy (OA) of the proposed model was the highest. Additionally, compared
with DeepLabv3 [61], LWRefineNet [63], and HRCNet-W48 [18] the parameter quantities
of the proposed model were reduced. Compared with PSPNet [40] and DANet [62], the
parameter quantities of our model were nearly 17 M less. As shown in Table 9, the FLOPs
of the proposed model were superior to most of the experimental methods. In conclusion,
the mmsDCNN-DenseCRF can achieve a satisfactory balance between the segmentation
performance and the complexity of the model.

19. Remote Sens. 2023, 15, 1229 19 of 22
4.2. Improvements and Future Work
Considering the problems of deep learning models in the semantic segmentation of re-
mote sensing images, this paper proposed the mmsDCNN-DenseCRF model. The proposed
model mainly has the following contributions and improvements. First of all, the modified
multiscale deformable convolutional neural network (mmsDCNN) was proposed for mul-
tiscale feature extraction. The mmsDCNN model can achieve a balance between strong
multiscale feature extraction performance and low computational complexity. Secondly, a
multi-level DenseCRF model integrating edge information was designed to complete the
optimization task of complex remote sensing image semantic segmentation. Specifically, it
can further consider the global context information in the remote sensing image at different
granularities, which makes up for the shortcomings of the neural network. Besides, the
DenseCRF model incorporating edge information can more accurately restore the outline of
objects. The mmsDCNN-DenseCRF model has a clear division of labor and fully considers
the main factors that affect the semantic segmentation effect of remote sensing images, such
as multiscale feature extraction, spatial context information, and boundary information.
Finally, this paper provided a new idea for the semantic segmentation of remote sensing
images. Concretely, in the semantic segmentation task of remote sensing images, the learn-
ing and reasoning capabilities in the probabilistic graphical model (PGM) can be fully
utilized to reduce the burden of deep neural networks and improve the performance of the
segmentation model. The numerical experiments on the Potsdam and Vaihingen datasets
verified that the segmentation accuracy of the proposed model was about 1% higher than
that of the most-advanced model.
In addition to realizing and demonstrating this effect using a specific model, this
article hopes to provide a way to solve the problem. In fact, if a feature extractor with
better performance appears in the future, the experimental effect may be better, but the idea
proposed in this paper is still applicable at that time. Therefore, our next step is to improve
and integrate the feature extraction model with a stronger multiscale feature extraction
capability and less computational complexity with the semantic segmentation model used
in this paper, such as the new modified YOLOv3 model with four scale detection layers [38],
which shows an excellent multiscale feature extraction performance. Furthermore, in future
experiments, we will further extend the model experiments to explore multi-spectral remote
sensing data.
5. Conclusions
Considering the main factors affecting the semantic segmentation of remote sensing
images and the problems of deep learning models in the semantic segmentation of remote
sensing images, this paper proposed the mmsDCNN-DenseCRF model. First of all, a
lightweight multiscale deformable convolution network (mmsDCNN) was designed to
generate a preliminary prediction probability map. The mmsDCNN model was improved
on the basis of the CNN and can achieve a balance between strong multiscale feature
extraction capabilities and less computational complexity. Then, a multi-level DenseCRF
based on the superpixel level and pixel level was proposed as an optimization module,
which can make full use of the image context information at different granularities in the
decoding process to obtain more precise semantic segmentation results. In addition, to
better recover the contour of the object, the Sketch token edge detection algorithm was
utilized to extract the edge contour features of the image, and the edge contour features
were fused into the Gaussian edge potential function of the DenseCRF model. In sum,
the mmsDCNN-DenseCRF model comprehensively considers several main factors that
affect the semantic segmentation effect of remote sensing images, including multiscale
feature extraction, spatial context information, and boundary information. In particular,
the proposed model can strike a balance between satisfactory segmentation performance
and low complexity. Finally, the numerical experiments on the Potsdam and Vaihingen
datasets verified that the proposed model is superior to the most-advanced model.

20. Remote Sens. 2023, 15, 1229 20 of 22
Author Contributions: Methodology, X.C.; formal analysis, X.C.; investigation, X.C.; writing—
original draft preparation, X.C.; writing—review and editing, X.C. and H.L.; visualization, X.C.
and H.L.; supervision, H.L.; project administration, H.L. All authors have read and agreed to the
published version of the manuscript.
Funding: This research received no external funding.
Data Availability Statement: Datasets relevant to our paper are available online.
Conflicts of Interest: The authors declare no conflict of interest.
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