The goal of this project was to augment the standard CTC decoder, implemented in TensorFlow, to produce output, restricted by a pre-defined lexicon. Given such a restriction, nonsensical, or "almost-correct" decodes should be pruned from the network output.
Our approach to efficiently implement such a dictionary restriction relies on storing the lexicon within a trie, which is then referenced on each step within the CTC decoder's Beam Search. Beam Search is a path-based algorithm, which considers only the n most likely moves on each ply. In this case, each move represents performing a state expansion to a specific label, based on the label's logit scores.
In the case of our lexicon-augmented decoder, during each state expansion, upon considering moving to the label in question, we check to see if that label is a child of our current location within the trie. If the label is a child, then we know that performing a state expansion to that label would be expanding our frontier within a prefix of some dictionary word. This, such an expansion would be valid, as restricted by our dictionary. If the label in question does not have a corresponding child of our current location within the trie, then we do set a score for that expansion to be the 0 log probability (-inf) and that state is not added to our beam, thus ending our beam search in that path through the input logits.
Upon reaching the end of the beam search, we determine if our current location within the dictionary trie signifies the end of a lexicon word. If it is not, we set the score for the decode at that state to be the 0 log probability, so that decode path is not outputted by the CTC beam search decoder.
The beam search repeatedly interacts with the trie in three locations within the
TrieBeamScorer: InitializeState, ExpandState, and ExpandStateEnd. In
InitializeState, this scorer function is called by the beam search decoder
with a beam search root state as a parameter. Accordingly, we set the trie node
root to be equal to root of the dictionary trie. On each state expansion, if the
to_label is different from the from-label and the to_label is not blank,
then we search for the to_label within the children of our current trie node
of our from_state to see if that new label continues a prefix from our
dictionary. Subsequently, at the end of the beam search, the ExpandStateEnd
function references the endWord flag of the current state's trie node to set
that decode path's score appropriately. Each of these functions are called for
any non-trivial decode, so the trie must be referenced within this scorer
implementation.
So, if we have an input dictionary of
root
|
1
|\
1 3
| |
3 1
and an input pre-log-scored logit sequence of
{{{0, 0.4, 0, 0.6, 0}}, // timestep 0
{{0, 0.4, 0, 0.4, 0.2}}, // timestep 1
{{0, 1, 0, 0, 0}}, // timestep 2
{{0, 0.6, 0, 0.4, 0}}, // timestep 3
{{0, 0.5, 0, 0.5, 0}}} // timestep 4
On timestep 0, while label 1 has a lower log-score than label 3, we must select
label 1, since it is the only child of our root within the trie. Within
ExapndState, given a to_label of 3, we would set the score to be log 0, and
that state expansion would not be added to leaves_: the data structure that
maintains the current frontiers of the beam search.
Suppose that at timestep 4, we have a current path of [1, 4, 1, 1], where 4
signifies a blank label. Our beam state must maintain a reference to the trie
node with label 1 on the second level below the root of our trie. A to_label
of 0, 1, or 2, would not be valid, as none of those labels are children of that
node within the trie. Appropriately the scores of each of those 'potential
state expansions' should be log 0, and none would be considered candidate
path expansions to be added to leaves_. Only a state expansion to label 3
would be considered valid for timestep 4 and our new frontier state would
maintain a reference to this node. Subsequently, in the call to
ExpandStateEnd at after this last timestep, we check to see if this frontier
node denotes the end of a word and see that it does and thus the decode
[1, 4, 1, 1, 3] is considered as a potential output path. More cases similar to
this are listed within ctc_trie_beam_search_test.h
This MAP involved the creating the additional files ctc_trie_node.h and
ctc_vocabulary.h, as well as creating large additions to the files
ctc_beam_entry.h and ctc_beam_scorer.h. For some small-scale testing
purposes, a slight modification to ctc_beam_search.h was made. Tests were
constructed within the files ctc_trie_node_test.h and
ctc_trie_beam_seach_test.h.
Additionally, an operation implementing these CTC additions is defined by an operation, which serves as a header for the implementation, which is constructed within the kernel.
Moreover, to call the back-end C++ op from python code, I made an additional definition for this call within the python operation definition.
Lastly, I made a change to Professor Weinman's
cnn_lstm_ctc_ocr
repository, such that the lexicon-restricted decoder may be called with the flag
--lexicon /path/to/lexicon. At the time of writing this report, this change
lies within the branch update-to-tf-1.8.
The core utilities for CTC live within the directory
//tensorflow/core/util/ctc/.
The op definition lives within the file
//tensorflow/core/ops/ctc_ops.cc and the kernel lives within the file
//tensorflow/core/kernels/ctc_decoder_ops.cc.
Additionally, the python definition, which calls the back-end C++ op lives within
the file //tensorflow/python/ops/ctc_ops.py.
The CTC logic for decoding and all the supporting classes and functions exist as
utilities, since they simply used operation kernels, as sorts of 'helper
functions'. Following the standard already in place from the base beam search
decoder tests, I decided to include the tests for the additional logic within
the same directory. Since these are simply additions that rely on the same
dependencies as the built-in base beam-search decoder, I decided it was
appropriate to include these additions within the core directory, rather than
some contrib repository.
Originally, I wanted to place definition and kernel for ctc_beam_search_trie
within a separate contrib repository, in order to adhere to the general
TensorFlow contribution guidelines. However, after much headache trying to get
everything to compile, I found in some research that two of the dependencies of
the kernel implementation of the op, namely //tensorflow/core:lib and
//tensorflow/core:framework are not able to be included within the build
path of files within contrib.
So, I placed the new op definition and kernel within the core op files, as
simple additions appended to the end of these libraries. Note that in doing this
workaround, these additions are not currently following the standard
contribution guidelines. If a pull request to the TensorFlow master branch is to
be made, then this explanation should be referenced.
To implement the trie as a storage and navigation mechanism for the lexicon, I
created the additional files ctc_trie_node.h and ctc_vocabulary.h.
ctc_vocabulary.h provides helper functions
The core augmentation utilizing the trie is an additional beam scorer class
within ctc_beam_scorer.h called TrieBeamScorer, which references the
trie at each time-step within the path state expansion. If the character is not
valid, as restricted by the trie, then the scorer assigns a log 0 score to that
beam state. Therefore, the contributing score of that state expansion sets the
score of the whole path to be log 0.
To maintain information of the trie at each state within the beam search, a new
state structure, TrieBeamState, maintains a reference to a node within the
trie, as well as a vector containing the labels which compose the trie
navigation path up to that point. This 'prefix' vector is primarily for
debugging purposes, as it is not used in beam search navigation decisions,
but is useful for tracking intermediate progress within the search. Note, that
the 'word' contained within this vector forms the post-merge-repeated sequence
of labels, as the trie only contains complete words, without label repeats as
would be generally present among adjacent timesteps within the logit input.
All said, this new beam state within ctc_beam_entry.h serves as an interface
between the beam search and a backing trie structure, generated from a lexicon
before entering the beam search.
TensorFlow works by building relatively high level operations, or ops in the
core back-end, and then invoking these operations through front-end APIs (in
our case, in Python). So, if we want to create a way for someone building their
model in python to use CTC decoding, we must create such an operation. Every
operation, requires a definition within an 'op' file, as well as a 'kernel'
which servers as the implementation of the function definition. An op
definition, CTCBeamSearchDecoderTrie, is defined within ctc_ops.cc and
serves as a function definition for this operation, and also does some input
checking to ensure correct input tensor rank and dimension equality, as well as
set up the output tensors. The kernel implementation,
CTCBeamSearchDecoderTrieOp, within ctc_decoder_ops.cc, provides the
necessary compute function, which sets up the dictionary trie and uses the CTC
utilities to decode an input logit sequence. This is the core operation that
is called by the Python API function in order to perform CTC decoding.
Lastly, from within the Python directory within TensorFlow, we implement a
function that calls our backend C++ operation, and export this function as a
module under nn so that the user may call this function in python code.
Currently, all this code lies within my personal TensorFlow fork, within the master branch. First, to get each of these files, simply clone a copy of this repository using the command:
git clone https://github.com/murphymatt/tensorflow.gitto your machine and build from there. Additionally, to run Professor Weinman's model which uses the augmented trie, clone the repository using the following command:
git clone https://github.com/weinman/cnn_lstm_ctc_ocr.gitTensorFlow also requires some third-party dependencies to run. Note, that on the mathlan, or some other machines, you may or may not have sudo access, so I like to use virtualenv to create a local, isolated environment which permits installing pip packages to without sudo access. Installing the dependencies should then proceed as follows:
virtualenv ~/tf_custom_env
source ~/tf_custom_env/bin/activate
pip install six numpy wheel After that, simply build the custom TF package from within the repository clone and then proceed. Once the build process is completed, outside of the TensorFlow repository, install the custom package
First, to build TensorFlow from source, you must install Bazel. System-specific Bazel installation instructions can be found here.
Subsequently, you must simply configure and then build with Bazel. From within
the repository, call ./configure from the root of the repository clone. Some
installation preferences may pop up, but all that we have really used for
testing purposes this semester has been CUDA support. So in configuration, you
can simply say 'yes' to that option, and 'no' to everything else. All default
file paths may be used when prompted.
Subsequently, after configuration, installation of the entire package is simply
bazel build --config=opt --config=cuda //tensorflow/tools/pip_package:build_pip_packageDepending on the machine you are using, this build process may take up to an hour or more, especially when building initially. Patience is a virtue.
Once the compilation is done, create the pip package with the following:
bazel-bin/tensorflow/tools/pip_package/build_pip_package ~/tmp/tensorflow_pkgThis will create a pip package within a user-level directory that you can install with pip. After that is completed, outside of the directory, and with a custom virtualenv activated, simply call
pip install ~/tmp/tensorflow_pkg/tensorflow-X.X.X-cpPYTHONVERSION-SYSTEMSPEC.whlsubstituting the appropriate variables with what exists inside of the directory.
Now, the custom TensorFlow installation is usable any time you are inside this
virtualenv. To make incremental upgrades, simply repeat the compilation process.
The build_pip_package usually does not take too long, following small changes.
Before using the custom TF build on the mjsynth data, the user should first run the tests of the dictionary-restricted components. Running these tests is simply
bazel test --dbg //tensorflow/core/util/ctc:ctc_trie_node_testbazel test --dbg //tensorflow/core/util/ctc:ctc_trie_beam_search_testAny additional test cases regarding the trie structure should go within the
first test file, but changes to the beam search scorer, or entry, which utilize
the trie logic should go within ctc_trie_beam_search_test.cc.
Currently, there is a small bug in which sometimes incomplete prefixes are
emitted by the beam search decoder. This bug was observed when using the custom
build over mjsynth data, on select words. For example, running the end-to-end
model over 2210/3/72_FENCES_28542.jpg will output multiple 'FENCES' outputs,
but will also output 'ENCE' as one of the lower-ranked top paths output. While
'ENCE' is a strict-prefix of a dictionary word, it is not a complete word and
should not be emitted by the decoder. This is likely an issue within
ExapandStateEnd within the TrieBeamScorer. This function should check to
see if the input TrieBeamState's current node has the endWord flag set. If
this flag is false, then the scorer should set the state expansion score to be
log 0. Over my last week and a half of my work on this project, I tried various
changes and debugging techniques, but
It is also worth noting that beam search extensions should be made within
new beam scorers and beam entries, as opposed to adding to or modifying
ctc_beam_search.h. However, I made two small changes to ctc_beam_search.h,
in order to get everything running. Originally, in the TopPaths function,
if the number of top paths stored within the leaves_ structure (the structure
that holds candidate paths, updated on each timestep of the beam search) is less
than the number n of paths to return, then the function returns an error and
no top paths are returned. In my modified version, in order to get the
small-scale test cases to pass, I set n equal to the minimum of n and
the number of leaves stored. This way, the beam search then outputs at most n
top paths.
Additionally, something I found that may be a bug within the beam search had to
do with checking candidate values after performing the ExpandStateEnd function
at the end of the beam search, before determining the top paths. In the base
beam search, this function does not apply any changes to the beam state, but, in
this modified search, this function should set the scores of incomplete words to
log 0. Originally, the function does not check to see if the state is still a
candidate after this function, but I put in a quick fix to check to see if it is
still a candidate. I will be submitting a written up pull request to TensorFlow
master soon.
Lastly, you can debug developments to the core framework using gdb. For
example, if after the development of additional test cases in
ctc_trie_beam_search_test.cc, some tests are failing, you can compile the
test library with debugging flags as follows:
bazel build --config=dbg //tensorflow/core/util/ctc:ctc_trie_beam_search_testand then run the test using
gdb bazel-bin/tensorflow/core/util/ctc/ctc_trie_beam_search_testIt is worth noting that I was only ever able to make this work when compiling on a machine without CUDA support. Additionally, if there are some issues from running the mjsynth Python code, after compiling the pip package with debugging flags, you can debug the python code using IPython as follows:
ipython
!ps | grep -i ipythonto get the process id of the IPython process, then attach this process with a debugger and then continue the process within the debugger, then run the python code by hopping back to IPython and calling
run example_script.pyThis was very helpful for debugging issues that resulted when calling
validate.py over the mjsynth dataset.
If you are working on a branch of my fork or your own fork, you should regularly keep up to date with the master TensorFlow repository. To do this, add the master repo as an upstream repository as follows:
git remote add upstream https://github.com/tensorflow/tensorflow.gitYou should fetch and merge regularly as follows:
git fetch upstream
git merge upstream/masterIt is unlikely that there would be merge conflicts, but if they are, they may
be resolved appropriately. If any CTC utility file would be most likely to
obtain merge conflicts, it would likely be ctc_beam_search.h since this is
the utility that should generally not change. It may also be possible that the
kernel or op specifications may change, but if so, the format of the op or
kernel should just stay consistent with the others within their respective
files.
Documentation |
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