Merge branch 'master' of github.com:taddeus/licenseplates

docs/verslag.tex → docs/report.tex

 \documentclass[a4paper]{article}
 
+\usepackage{amsmath}
 \usepackage{hyperref}
+\usepackage{graphicx}
 
 \title{Using local binary patterns to read license plates in photographs}
 
@@ -19,6 +21,8 @@ Tadde\"us Kroes\\
 Fabi\'en Tesselaar
 
 \tableofcontents
+\pagebreak
+
 \setcounter{secnumdepth}{1}
 
 \section{Problem description}
@@ -30,13 +34,9 @@ conditions.
 Reading license plates with a computer is much more difficult. Our dataset
 contains photographs of license plates from various angles and distances. This
 means that not only do we have to implement a method to read the actual
-characters, but also have to determine the location of the license plate and
-its transformation due to different angles.
-
-We will focus our research on reading the transformed characters on the
-license plate, of which we know where the letters are located. This is because
-Microsoft recently published a new and effective method to find the location of
-text in an image.
+characters, but given the location of the license plate and each individual
+character, we must make sure we transform each character to a standard form. 
+This has to be done or else the local binary patterns will never match!
 
 Determining what character we are looking at will be done by using Local Binary
 Patterns. The main goal of our research is finding out how effective LBP's are
@@ -45,19 +45,31 @@ in classifying characters on a license plate.
 In short our program must be able to do the following:
 
 \begin{enumerate}
-    \item Use perspective transformation to obtain an upfront view of license
+    \item Use a perspective transformation to obtain an upfront view of license
           plate.
-    \item Reduce noise where possible.
-    \item Extract each character using the location points in the info file.
-    \item Transform character to a normal form.
-    \item Create a local binary pattern histogram vector.
-    \item Match the found vector with a learning set.
+    \item Reduce noise where possible to ensure maximum readability.
+    \item Extracting characters using the location points in the xml file.
+    \item Transforming a character to a normal form.
+    \item Creating a local binary pattern histogram vector.
+    \item Matching the found vector with a learning set.
+    \item And finally it has to check results with a real data set.
 \end{enumerate}
 
-\section{Solutions}
+\section{Language of choice}
+
+The actual purpose of this project is to check if LBP is capable of recognizing
+license plate characters. We knew the LBP implementation would be pretty
+simple. Thus an advantage had to be its speed compared with other license plate 
+recognition implementations, but the uncertainity of whether we could get some
+results made us pick Python. We felt Python would not restrict us as much in 
+assigning tasks to each member of the group. In addition, when using the
+correct modules to handle images, Python can be decent in speed.
+
+\section{Implementation}
+
+Now we know what our program has to be capable of, we can start with the
+implementations.
 
-Now that the problem is defined, the next step is stating our basic solutions.
-This will come in a few steps as well.
 
 \subsection{Transformation}
 
@@ -65,22 +77,9 @@ A simple perspective transformation will be sufficient to transform and resize
 the plate to a normalized format. The corner positions of license plates in the
 dataset are supplied together with the dataset.
 
-\subsection{Reducing noise}
-
-Small amounts of noise will probably be suppressed by usage of a Gaussian
-filter. A real problem occurs in very dirty license plates, where branches and
-dirt over a letter could radically change the local binary pattern. A question
-we can ask ourselves here, is whether we want to concentrate ourselves on these
-exceptional cases. By law, license plates have to be readable. Therefore, we
-will first direct our attention at getting a higher score in the 'regular' test
-set before addressing these cases. Considered the fact that the LBP algorithm
-divides a letter into a lot of cells, there is a good change that a great
-number of cells will still match the learning set, and thus still return the
-correct character as a best match. Therefore, we expect the algorithm to be
-very robust when dealing with noisy images.
-
 \subsection{Extracting a letter}
 
+NO LONGER VALID!
 Because we are already given the locations of the characters, we only need to
 transform those locations using the same perspective transformation used to
 create a front facing license plate. The next step is to transform the
@@ -100,14 +99,86 @@ The resulting image will always have the same size, the character contained
 will always be of the same height, and the character will alway be positioned
 at either the left of right side of the image.
 
-\subsection{Local binary patterns}
+\subsection{Reducing noise}
+
+Small amounts of noise will probably be suppressed by usage of a Gaussian
+filter. A real problem occurs in very dirty license plates, where branches and
+dirt over a letter could radically change the local binary pattern. A question
+we can ask ourselves here, is whether we want to concentrate ourselves on these
+exceptional cases. By law, license plates have to be readable. Therefore, we
+will first direct our attention at getting a higher score in the 'regular' test
+set before addressing these cases. Considered the fact that the LBP algorithm
+divides a letter into a lot of cells, there is a good change that a great
+number of cells will still match the learning set, and thus still return the
+correct character as a best match. Therefore, we expect the algorithm to be
+very robust when dealing with noisy images.
 
+\subsection{Local binary patterns}
 Once we have separate digits and characters, we intent to use Local Binary
-Patterns to determine what character or digit we are dealing with. Local Binary
+Patterns (Ojala, Pietikäinen \& Harwood, 1994) to determine what character
+or digit we are dealing with. Local Binary
 Patters are a way to classify a texture based on the distribution of edge
 directions in the image. Since letters on a license plate consist mainly of
 straight lines and simple curves, LBP should be suited to identify these.
 
+\subsubsection{LBP Algorithm}
+The LBP algorithm that we implemented is a square variant of LBP, the same
+that is introduced by Ojala et al (1994). Wikipedia presents a different
+form where the pattern is circular. 
+\begin{itemize}
+\item Determine the size of the square where the local patterns are being
+registered. For explanation purposes let the square be 3 x 3. \\
+\item The grayscale value of the middle pixel is used a threshold. Every value
+of the pixel around the middle pixel is evaluated. If it's value is greater
+than the threshold it will be become a one else a zero.
+
+\begin{figure}[h!]
+\center
+\includegraphics[scale=0.5]{lbp.png}
+\caption{LBP 3 x 3 (Pietik\"ainen, Hadid, Zhao \& Ahonen (2011))}
+\end{figure}
+
+Notice that the pattern will be come of the form 01001110. This is done when a
+the value of the evaluated pixel is greater than the threshold, shift the bit
+by the n(with i=i$_{th}$ pixel evaluated, starting with $i=0$).
+
+This results in a mathematical expression:
+Let I($x_i, y_i$) an Image with grayscale values and $g_n$ the grayscale value
+of the pixel $(x_i, y_i)$. Also let $s(g_i - g_c)$ with $g_c$ = grayscale value
+of the center pixel.
+
+$$
+  s(v, g_c) = \left\{
+  \begin{array}{l l}
+    1 & \quad \text{if v $\geq$ $g_c$}\\
+    0 & \quad \text{if v $<$ $g_c$}\\
+  \end{array} \right.
+$$
+
+$$LBP_{n, g_c = (x_c, y_c)} = \sum\limits_{i=0}^{n-1} s(g_i, g_c)^{2i} $$
+
+The outcome of this operations will be a binary pattern.
+
+\item Given this pattern, the next step is to divide the pattern in cells. The
+amount of cells depends on the quality of the result, so trial and error is in
+order. Starting with dividing the pattern in to cells of size 16. 
+
+\item Compute a histogram for each cell.
+
+\begin{figure}[h!]
+\center
+\includegraphics[scale=0.7]{cells.png}
+\caption{Divide in cells(Pietik\"ainen et all (2011))}
+\end{figure}
+
+\item Consider every histogram as a vector element and concatenate these. The
+result is a feature vector of the image.
+
+\item Feed these vectors to a support vector machine. This will ''learn'' which
+vector indicate what letter. 
+
+\end{itemize}
+
 To our knowledge, LBP has yet not been used in this manner before. Therefore,
 it will be the first thing to implement, to see if it lives up to the
 expectations. When the proof of concept is there, it can be used in the final
@@ -138,11 +209,41 @@ choices we made.
 In order to retrieve the license plate from the entire image, we need to
 perform a perspective transformation. However, to do this, we need to know the 
 coordinates of the four corners of the licenseplate. For our dataset, this is
-stored in XML files. So, the first step is to read these XML files.\\
-\\
-\paragraph*{XML reader}
+stored in XML files. So, the first step is to read these XML files.
 
+\paragraph*{XML reader}
 
+The XML reader will return a 'license plate' object when given an XML file. The
+licence plate holds a list of, up to six, NormalizedImage characters and from
+which country the plate is from. The reader is currently assuming the XML file
+and image name are corresponding. Since this was the case for the given
+dataset. This can easily be adjusted if required. 
+
+To parse the XML file, the minidom module is used. So the XML file can be
+treated as a tree, where one can search for certain nodes. In each XML
+file it is possible that multiple versions exist, so the first thing the reader
+will do is retrieve the current and most up-to-date version of the plate. The
+reader will only get results from this version.
+
+Now we are only interested in the individual characters so we can skip the
+location of the entire license plate. Each character has 
+a single character value, indicating what someone thought what the letter or
+digit was and four coordinates to create a bounding box. To make things not to
+complicated a Character class and Point class are used. They
+act pretty much as associative lists, but it gives extra freedom on using the
+data. If less then four points have been set the character will not be saved.
+
+When four points have been gathered the data from the actual image is being
+requested. For each corner a small margin is added (around 3 pixels) so that no
+features will be lost and minimum amounts of new features will be introduced by
+noise in the margin. 
+
+In the next section you can read more about the perspective transformation that
+is being done. After the transformation the character can be saved: Converted
+to grayscale, but nothing further. This was used to create a learning set. If
+it doesn't need to be saved as an actual image it will be converted to a
+NormalizedImage. When these actions have been completed for each character the
+license plate is usable in the rest of the code.
 
 \paragraph*{Perspective transformation}
 Once we retrieved the cornerpoints of the license plate, we feed those to a
@@ -283,8 +384,67 @@ commercial license plate recognition software score about $90\%$ to $94\%$,
 under optimal conditions and with modern equipment. Our program scores an
 average of blablabla.
 
+\section{Difficulties}
+
+During the implementation and testing of the program, we did encounter a
+number of difficulties. In this section we will state what these difficulties
+were and whether we were able to find a proper solution for them.
+
+\subsection*{Dataset}
+
+We did experience a number of problems with the provided dataset. A number of
+these are problems to be expected in a real world problem, but which make
+development harder. Others are more elemental problems.\\
+The first problem was that the dataset contains a lot of license plates which
+are problematic to read, due to excessive amounts of dirt on them. Of course,
+this is something you would encounter in the real situation, but it made it
+hard for us to see whether there was a coding error or just a bad example.\\
+Another problem was that there were license plates of several countries in
+the dataset. Each of these countries has it own font, which also makes it
+hard to identify these plates, unless there are a lot of these plates in the
+learning set.\\
+A problem that is more elemental is that some of the characters in the dataset
+are not properly classified. This is of course very problematic, both for
+training the SVM as for checking the performance. This meant we had to check
+each character whether its description was correct.
+
+\subsection*{SVM}
+
+We also had trouble with the SVM for Python. The standard Python SVM, libsvm,
+had a poor documentation. There was no explanation what so ever on which
+parameter had to be what. This made it a lot harder for us to see what went
+wrong in the program.
+
+\section{Workload distribution}
+
+The first two weeks were team based. Basically the LBP algorithm could be
+implemented in the first hour, while some talked and someone did the typing.
+Some additional 'basics' where created in similar fashion. This ensured that
+every team member was up-to-date and could start figuring out which part of the
+implementation was most suited to be done by one individually or in a pair.
+
+\subsection{Who did what}
+Gijs created the basic classes we could use and helped the rest everyone by 
+keeping track of what required to be finished and whom was working on what. 
+Tadde\"us and Jayke were mostly working on the SVM and all kinds of tests
+whether the histograms were matching and alike. Fabi\"en created the functions
+to read and parse the given xml files with information about the license
+plates. Upon completion all kinds of learning and data sets could be created.
+
+%Richard je moet even toevoegen wat je hebt gedaan :P:P
+%maar miss is dit hele ding wel overbodig. Ik dacht dat Rein het zei tijdens
+%gesprek van ik wil weten hoe het ging enzo
+
+\subsection{How it went}
+
+Sometimes one cannot hear the alarm bell and wake up properly. This however was
+not a big problem as no one was affraid of staying at Science Park a bit longer
+to help out. Further communication usually went through e-mails and replies
+were instantaneous! A crew to remember.
+
 \section{Conclusion}
 
+Awesome
 
 
 \end{document}