Table Detection Using Deep Learning

Abstract

Table detection is a crucial step in many document analysis applications as tables are used for presenting essential information to the reader in a structured manner. It is a hard problem due to varying layouts and encodings of the tables. Researchers have proposed numerous techniques for table detection based on layout analysis of documents. Most of these techniques fail to generalize because they rely on hand engineered features which are not robust to layout variations. In this paper, we have presented a deep learning based method for table detection. In the proposed method, document images are first pre-processed. These images are then fed to a Region Proposal Network followed by a fully connected neural network for table detection. The proposed method works with high precision on document images with varying layouts that include documents, research papers, and magazines. We have done our evaluations on publicly available UNLV dataset where it beats Tesseract's state of the art table detection system by a significant margin.

Full text

Table Detection using Deep Learning
Azka Gilani∗, Shah Rukh Qasim∗, Imran Malik†and Faisal Shafait‡
∗National University of Sciences and Technology (NUST), Islamabad, Pakistan
Email: agilani(dot)mscs15seecs,14beesqasim,malik(dot)imran,faisal(dot)shafait(at)seecs.edu.pk
Abstract— Table detection is a crucial step in many document
analysis applications as tables are used for presenting essential
information to the reader in a structured manner. It is a
hard problem due to varying layouts and encodings of the
tables. Researchers have proposed numerous techniques for table
detection based on layout analysis of documents. Most of these
techniques fail to generalize because they rely on hand engineered
features which are not robust to layout variations. In this paper,
we have presented a deep learning based method for table
detection. In the proposed method, document images are first
pre-processed. These images are then fed to a Region Proposal
Network followed by a fully connected neural network for table
detection. The proposed method works with high precision on
document images with varying layouts that include documents,
research papers, and magazines. We have done our evaluations
on publicly available UNLV dataset where it beats Tesseract's
state of the art table detection system by a significant margin.
I. INTRODUCTION
Tables are widely used for presenting structural and func-
tional information. They are present in diverse classes of doc-
uments including newspapers, research articles and scientific
documents, etc. Tables enable readers to rapidly compare,
analyse and understand facts present in documents. Table
detection in documents is significant in the field of documents
analysis and recognition; hence it has attracted a number of
researchers to make their contributions in this domain.
Table detection is carried out by layout and content analysis
of documents. Tables have varying layouts and variety of
encodings. Because of this reason, writing a general algorithm
for table detection is very hard. Hence, table detection is
considered a hard problem in scientific society. Large number
of researches have been carried out in this field but most of
them have their limitations. Existing commercial and open
source techniques for document analysis including Tesseract
lack the capability to completely detect table regions from
document images [1].
In the recent years, deep learning techniques have greatly
improved the results on various computer vision problems.
Recently, Hao et al. [2] presented an approach for table
detection in documents using deep learning. Their proposed
method employs combination of custom algorithms and ma-
chine learning in order to generate region proposals and to
detect whether a table exists in the proposed region or not.
The major limitation of this method is that it is limited to
PDF (Portable Document Format) documents only which are
non-raster. Another limitation is that it works well on the tables
that have ruling lines but fails to detect those without ruling
lines and those which are spanned across multiple columns.
Hence in order to improve the performance of table detection
and to make up for the limitations of prior techniques, this
paper proposes a methodology for table detection based on
purely based on deep learning without using extensive pre or
post processing. To explain it further, the document image
is transformed into a new image. Then, this paper uses
Faster Recurrent Convolutional Neural Network (Faster R-
CNN) as the deep learning module. In contrary to Hao et
al. [2] technique, the Faster-RCNN computes region proposals
itself and then helps in determining whether the selected area
is a table or not. Our approach has a major advantage of
being invariant to changes in table structure and layout as it
can be fined-tuned to work on any dataset very easily. This
capability is not present in any of the existing approaches.
Hence, we make a significant contribution to table detection
problem by making it data-driven. Additionally, we have
used publicly available UNLV dataset for evaluation of our
proposed methodology [3] where it gives better results than
Tesseract’s table detection system. We have also compared
our results with the commercial market leading OCR Engine,
Abbyy Cloud OCR SDK [4].
The rest of the paper is organized as follows: Section
II describes researches related to table detection. Section 3
describes our proposed methodology that consists of pre-
processing and detection module. Section IV explains de-
scribes performance measures that have been used to eval-
uate our system and explains experimental results. Section V
concludes the paper and provides some directions for future
research.
II. LITERATURE REVIEW
Several researchers have reported their work regarding table
detection in document images. Kieninger et al. [5]–[7] pro-
posed an algorithm for table spotting and structure extraction
from documents called T-Recs. This system takes word bound-
ing boxes as input. They are clustered to form segmentation
graph using bottom-up approach. The key problem with this
technique is that it depends entirely on word bounding boxes
and is unable to perform well in presence of multi-column
layouts.
Another approach was proposed by Wang et al. [8]. It
detects table lines depending on distance between consecutive
words. After that, horizontal consecutive words are grouped
together with vertical adjacent lines in order to propose
table entity candidates. This statistical approach assumes that
maximum number of columns in the document is two and
designs the algorithm according to three layout templates
(single column, double column, mixed column). Then, column

classification algorithm is applied to find out column layout
of the page and use this information as prior knowledge for
table spotting. Major limitation of this technique is that it can
only work on those templates for which it has been designed.
Hu et al. [9] presented an approach for table detection while
assuming that input images are single columned. Like previous
methods, this technique can not be applied on multi-column
layouts. Shafait et al. [10] presented another approach for
table detection in heterogeneous documents. This system is
integrated into open source Tesseract OCR engine. It works
well on large variety of documents but major limitation is that
it is a traditional technique and not data-driven.
Tupaj et al. [11] proposed an OCR based table detection
technique. The system searches for sequences of table-like
lines based on the keywords that might be present in the
table headers. The line that contains keyword is regarded as
the starting line while subsequent lines are then analyzed to
match with predefined set of tokens which are then categorized
as table structure. The limitation of this technique is that it
depends highly on the keywords that might appear in table
headers.
Harit et al. [12] proposed a technique for table detection
based on the identification of unique table start and trailer
pattern. The major limitation of this method is that it will not
work properly whenever the table start patterns are not unique
in document images.
Gatos et al. [13] proposed an approach for table detection by
finding area of intersection between the horizontal and vertical
lines. Table are then reconstructed by drawing corresponding
horizontal and vertical lines that are connected to intersection
pairs. The limitation of this system is that it works only for the
documents in which the table rows and columns are separated
by ruling lines. Costa e Silva [14] presented a technique
for table detection using Hidden Markov Models (HMMs).
The system extracts text from PDF files using pdftotext Linux
utility. Feature vectors are then computed on basis of spaces
present between the text. The major limitation of this technique
is that it works only on non-raster PDF files that do not have
any noise.
Kasar [15] presented a method to locate tables by identify-
ing column and row line separators. This system then employs
run-length approach in order to detect horizontal and vertical
lines from input image. From each group of horizontal and
vertical lines, a set of 26 low level features are extracted and
passed to Support Vector Machine (SVM) which then detects
the table. The major limitation of this approach is that it will
fail on tables without ruling lines.
Jahan et al. [16] presented a method that uses local thresh-
olds for word spacing and line height for localization and
extraction of table regions from document images. The major
limitation of this method is that it detects table regions along
with surrounding text regions. Hence it cannot be used for
localization of table regions only.
Anh et al. [17] presented a hybrid approach for table
detection in document images. This system first classifies
document in text and non-text regions. On the basis of that,
it uses a hybrid method to find candidate table regions. These
regions are then examined to get table regions. This approach
will fail if table is spanned across multiple columns in the
document. Moreover, it will not work for scanned images as
it does not use any heuristic filter to cater for noisy images.
Hao et al. [2] presented deep learning based approach for
table detection. This system computes region proposals from
document images through some predefined set of rules. These
region proposals are then passed to the CNN that detects
whether a certain region proposal belongs to table region or
not. The major limitation is that it works well for tables with
ruling lines but fails to localize table regions if the table is
spanned across multiple columns. Another limitation is that it
works only on non-raster PDF documents.
In order to make up for the limitations of prior method-
ologies, this paper attempts to adapt Faster R-CNN, a deep
learning technique used for object detection in natural images,
to solve table detection problem.
III. PROP OS ED METHODOLOGY
The proposed method consists of two major modules:
Image transformation and table detection. Documents consist
of content region and blank spaces. Image transformation is
applied in order to separate these regions while the table
detection module uses Faster R-CNN as a basic element of
deep network. Faster R-CNN is highly dependant on combined
network that is composed of Region Proposal Networks (RPN)
and Fast R-CNN. In this section we will describe each module
in detail.
A. Image Transformation
Image Transformation is the initial step of our proposed
methodology. Faster R-CNN [18] was initially proposed for
natural images. Hence image transformation plays a pre-
liminary role in conversion of document images to natural
images as close as possible so that we can easily fine-tune
on existing Faster R-CNN models. Distance transform [19]–
[21] is a derived representation of digital image. It calculates
the precise distance between text regions and white spaces
present in the document image which can give a good estimate
about presence of a table region. In our proposed methodology,
we have used different types of distance transforms so that
different features can be stored in all three channels. Image
transformation is done using the following procedure:
procedure IMAGE TRANSFORMATION(I)
b←EuclideanDistanceTransform(I)
g←LinearDistanceTransform(I)
r←MaxDistanceTransform(I)
P←ChannelMerge(b,g,r)
return P
The transformation algorithm takes binary image as an
input. It then computes Euclidean distance transform, linear
distance transform and max distance transform [19]–[21] on
blue, green and red channels of the image respectively. Result

Fig. 1: Transformed Images
of the image transformation algorithm on document images is
shown in Figure 1.
B. Table detection
For detection, our approach employed Faster R-CNN [18].
Faster R-CNN was originally proposed for object detection
and classification in natural images. It is composed of two
modules. The first module is a RPN that propose regions.
The region proposals are fed to the second module which is
the detector module that was originally proposed in Fast R-
CNN [22]. The entire system is a unified network for object
detection. Figure 2 shows the architectural diagram for our
system.
1) Region Proposal Network: As described in [18], Re-
gion Proposal Network (RPN) takes the transformed image
as an input and returns an output of a set of rectangular
object proposals, each with an objectness score. RPN shares
common set of convolutional layers with detector module of
Faster-RCNN. Ren et al. has used Zeiler and Fergus model
(ZF) [23] and Simonyan and Zisserman model (VGG-16) in
their experiments.
In order to generate region proposals, a small network is
slided over the convolutional feature map output by the last
shared feature map. RPN takes an n×nspatial window of
the input convolutional feature map as an input. It maps each
sliding window to a lower dimensional (256-d for ZF model)
feature. The complete architecture of network is shown in
Figure 2. The feature map is then passed to two fully con-
nected layers that include a regression layer and a classification
layer. For this paper we have used default implementation
of Faster R-CNN that takes n=3. The fully connected layers
of the network are shared across all spatial locations. This
architecture [18] is naturally implemented with an n×ncon-
volutional layer followed by two 1×1convolutional layers for
regression and classification. At each sliding window, Faster
R-CNN simultaneously predicts multiple region proposal for
each location that can be denoted by k. So classification layer
has 2k output scores while regression layer has 4k outputs that
Fig. 2: Our approach: The document image is first transformed
and then fed into a fine-tuned CNN model. It outputs a feature
map which are fed into region proposal network for proposing
candidate table regions. These regions are finally given as
input to fully connected detection network along with the
convolutional feature map to classify them into tables or non-
tables.
are encoding coordinates of kboxes. The kregional proposals
are then parametrized in relevance to kreference boxes which
are known as anchors. Faster R-CNN yields k=9 anchors at
each sliding position.
The important fact is that Faster R-CNN generates region
proposals that are scale and translational invariant. The RPN is
then trained end-to-end by Stochastic Gradient Descent (SGD)
and back propagation.In this paper, all the layers are fine tuned
by ZF network.
2) Detection network: After the training of network for
region proposal generation, these proposals are then passed
to the region based object detection CNN’s module that will
utilize these proposals. Detection module is highly based on
the unified network that is composed of RPN and Fast R-CNN
with shared convolutional layers. Resultantly, it detect tables
from test set and returns the coordinates of bounding boxes of
predicted tables.
C. Training
We have used Caffe based implementation of Faster R-
CNN [18] to fine tune on our images. Momentum Optimizer
with learning rate of 0.001 and a momentum of 0.9 was
used. Number of training iterations was 10,000. We trained
our system on 2 classes i.e. background and table region.
Background class has been used as the negative example
(table region is missing) while table class has been used as
the positive example (containing table region). Due to this
reason, our proposed system doesn’t search aggressively for
table regions on negative samples.

Fig. 3: Results showing: (a) Partial detection, (b) Missed, (c) Over-Segmented, and (c) False Positives
Fig. 4: Some sample images from the UNLV dataset showing detection results of proposed Table Detection approach. Ground
truth is blue while the detected regions are red.
IV. PERFORMANCE MEASURES
Different performance measures have been mentioned in the
literature for evaluation of table detection algorithms. These
measures include precision and recall [9], [24] that have been
used for evaluating various table detection algorithms [1], [8],
[24]–[26]. We have compared our proposed methodology with
Shafait et al. [10] and a commercial engine, Abbyy Cloud
OCR SDK [4]. The evaluation measures described in [10] have
been employed.
We have used open sourced UNLV dataset [3] as used
in [10] to make a fair comparison of both methodologies.
Due to this dataset, we didn’t compare our methodology with
systems that were proposed in ICDAR 2013 Table Detection
competition. So, it wouldn’t be a fair comparison to their
proposed approach. Most of the techniques that were proposed
in ICDAR 2013 [15], [27] are not data driven and are highly
dependent on table layout and extraction of custom features
from the images. This makes those techniques non robust to
varying layout.
Considering Ground truth bounding box is represent by Gi
while the bounding box detected by our system is represented
by Dj. The formula for finding the overlapped region between
two bounding boxes is given by [10].
A(Gi, Dj) = 2× |Gi∩Dj|
|Gi|+|Dj|, A ∈[0,1] (1)
A(Gi, Dj) represents the overlapped region between ground
truth and detected bounding boxes. Depending on the area of
intersecting region, its value will lie between zero and one.
Note that we are using the same threshold values as described
by Shafait et al. [10] to make a fair comparison with their
technique.
Figure 3 shows some of the errors (partial detection, over
segmentation, and false positive detection) that occurred dur-
ing table detection. Here the blue region represents the ground
truth bounding boxes while red region represents bounding
boxes of detected regions.

A. Correct Detections
These are the number of ground truth tables that have a
major overlap (A ≥0.9) with one of the detected tables. The
area has been calculated using eq.1
B. Partial Detections
These are the number of ground truth tables that have a
partial overlap (0.1 < A < 0.9) with one of the detected tables.
C. Over-Segmented Tables
These are the number of ground truth tables that have
overlap (0.1 < A < 0.9) with more than one detected tables. It
means that different parts of the ground truth table have been
detected as separate tables.
D. Under-Segmented Tables
These are the number of ground truth tables that have major
overlap (0.1 < A <0.9) with detected table but that detected
table also overlaps with several other ground truth tables. It
means that more than one tables were merged during detection
and were reported as single table.
E. False Positive Tables
This indicates the number of detected tables that do not have
an overlap (A ≤0.1) with any of the ground truth tables. Such
tables are missed during detection.
F. Missed Tables
This indicates the number of ground truth tables that do not
have an overlap (A ≤0.1) with any of the detected tables. It
means that these tables are missed by the detecting algorithm.
G. Precision
Precision measure has been used for evaluating the overall
performance of table detection method. It finds the percentage
of detected tables that actually belong to table regions of
ground truth document image. Formula for calculating pre-
cision is as follows:
Area of Ground truth regions in Detected regions
Area of all Detected table regions (2)
H. Recall
It is evaluated by finding the percentage of ground truth
table regions that were marked as detected table regions.
Formula for calculating recall is as follows:
Area of Ground truth regions in Detected regions
Area of all Ground truth table regions (3)
I. F1 Score
It considers both precision and recall to compute the accu-
racy of methodology.
2×Precision ×Recall
Precision ×Recall (4)
Fig. 5: Visualization of various engines. Ground truth bound-
ing box is represented by blue color while the detected
bounding box by our method is represented by green color.
Magenta color represents bounding box of Abbyy Cloud OCR
SDK while maroon color shows the result of Tesseract.
V. EXPERIMENTS AND RESULTS
In order to evaluate the performance of the proposed
methodology, we chose publicly available UNLV dataset [3].
This dataset consists of wide variety of document images
ranging from business reports to research papers and mag-
azines that includes varying and very complex table layouts.
This dataset contains approximately 10,000 images at different
resolutions. For each scanned image, manually keyed ground
truth text is provided, along with manually determined zone
information. Each zone is further categorized depending on
the contents (text, half-tone, table, etc.) of that zone. Amongst
10,000 document images, only 427 contain table regions. We
have used all of these 427 images from UNLV dataset for
evaluating our proposed technique. As the dataset is small so
we have used transfer learning approach [28]. We have used
data augmentation approaches including rotation, scaling and
flipping to overcome over-fitting.
Performance comparison between open source Shafait et
al. [10] technique (Tesseract), a commercial engine (Abbyy
Cloud OCR SDK) and our method is shown in Table I. While
parsing table, row and column headers are often used as keys.
So even if they are missed, it is impossible to extract any
information; hence, the whole detected table becomes useless.
Thus, number of correct detections is the most expressive
performance measure. Tesseract and Abbyy fail to detect
tables in presence of complex layouts that consists of wide
white spaces. The results exhibit that our approach has better
performance as correct detections significantly improve from
44% to 60.5%.
Figure 5 visualizes results of all three engines with respect
to ground truth. Overall results of our proposed methodology
have been shown in Figure 4.
VI. CONCLUSION
This paper presented an approach for table detection based
on deep learning. The proposed system uses image transfor-
mation for separating text regions from non-text regions. It
then uses RPN followed by fully connected neural network for
detection of table regions in document images. Experimental

Accuracy (%)
Performance Measures Tesseract Abbyy Without Distance Transform Our Approach
Correct Detections 44.9 41.28 51.37 60.5
Partial Detections 28.4 32.1 42.2 30.2
Missed Tables 25.68 25.68 6.42 9.17
Over Segmented Tables 3.66 7.33 29.35 24.7
Under Segmented Table 3.66 7.33 42.20 30.27
False Positive Detections 22.72 7.21 5 10.17
Area Precision 93.2 95.0 84.5 82.3
Area Recall 64.29 64.3 89.17 90.67
F1 Score 76.09 76.69 86.77 86.29
TABLE I: Performance comparison of different engines
results show that deep learning based system is robust to layout
analysis for table detection as it is not dependent on hand
engineered features. The proposed system has been evaluated
on publicly available UNLV dataset. It gives better results
as compared to the Tesseract's state-of-the-art table detection
system. We plan to extend this work in the direction of table
structure and content extraction in future.
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