MATLAB binding for MXNet

How to use

MXNet needs to be built so that the lib/libmxnet.so is available, which can be done by:

cd ..
make

The pre-trained Inception-BN should be downloaded to obtain the symbol and network parameters.

./get_inception_model.sh

This data will be saved in the ./data folder:

./data/
├── cat.png
├── Inception-BN-0126.params
├── Inception-BN-symbol.json
└── synset.txt

####Sample usage

Run the demo script from the command-line without invoking Matlab GUI:

matlab -nodisplay -nojvm -nosplash -nodesktop -r "run('./demo.m'), exit(0);"

or the script may be run from the Matlab GUI as usual.

The script has the following components:

Load model

model = mxnet.model;
model.load('data/Inception-BN', 126);

Load data and normalise. Here we assume a fixed value of 120 as ‘mean image’:
```
img = single(imresize(imread('./data/cat.png'), [224 224])) - 120;
```
Get prediction:
```
pred = model.forward(img);
```

Do feature extraction on CPU or GPU 0:

feas = model.forward(img, {'max_pool_5b_pool', 'global_pool', 'fc1'});           % CPU mode
feas = model.forward(img, 'gpu', 0, {'max_pool_5b_pool', 'global_pool', 'fc1'}); % GPU mode

See demo.m for more details

Note on Implementation

We use loadlibrary to load mxnet library directly into Matlab and calllib to call MXNet functions. Note that Matlab uses the column-major to store N-dim arrays while and MXNet uses the row-major. So assume we create an array in Matlab with

X = zeros([2,3,4,5]);

If we pass the memory of X into MXNet, then the correct shape will be [5,4,3,2] in MXNet. When processing images, MXNet assumes the data layout is

batchSize x channel x width x height

while in Matlab we often store images in

width x height x channel x batchSize

So we should permute the dimensions by X = permute(X, [2, 1, 3, 4]) before passing X into MXNet.

FAQ

You may get the error GLIBCXX_x.x.xx is not found. Such as on Ubuntu 14.04:

> In loadlibrary (line 359)
Error using loadlibrary (line 447)
There was an error loading the library "/home/muli/work/mxnet/lib/libmxnet.so"
/usr/local/MATLAB/R2015a/bin/glnxa64/../../sys/os/glnxa64/libstdc++.so.6:
version `GLIBCXX_3.4.18' not found (required by
/home/muli/work/mxnet/lib/libmxnet.so)

Caused by:
    Error using loaddefinedlibrary
    /usr/local/MATLAB/R2015a/bin/glnxa64/../../sys/os/glnxa64/libstdc++.so.6:
    version `GLIBCXX_3.4.18' not found (required by
    /home/muli/work/mxnet/lib/libmxnet.so)

One way to fix it is to link `MATLAB_ROOT/sys/os/glnxa64/libstdc++.so.6` to
your system's `libstdc++`. For example

muli@ghc:/usr/local/MATLAB/R2015a/sys/os/glnxa64$ sudo rm -r libstdc++.so.6
muli@ghc:/usr/local/MATLAB/R2015a/sys/os/glnxa64$ sudo ln -s /usr/lib/x86_64-linux-gnu/    libstdc++.so.6.0.19 libstdc++.so.6

Matlab binding has been tested with the following version:
R2016b (9.1.0.441655) 64-bit (glnxa64)