# Frequently Asked Questions

Do not post a question like "I'm using dlib, and it doesn't work?" or "I'm using the object detector and it doesn't work, what do I do?". If this is all you say then I have no idea what is wrong. 99% of the time it's some kind of user error. 1% of the time it's some problem in dlib. But again, without more information it's impossible to know. So please don't post questions like this.

If you think you found some kind of bug or problem in dlib then feel free to post on sourceforge or github. But include the version of dlib you are using, what you are trying, what happened, what you expected to have happened instead, etc.

On the other hand, if you haven't found a bug or problem in dlib, but instead are looking for machine learning/computer vision/programming consulting then you can still post your question on sourceforge. But be clear that this is what you are asking for. Maybe someone will help you or you can find someone to pay money in exchange for a solution to your problem.

However, don't try to get someone to write your code for you by repeatedly asking a question like "ok, what do I type next to make a program that does X?". I get this question all the time from people who obviously don't know how to program. If you are not familiar with C++ it's much better to learn it by reading one of the excellent books on the topic than by an infinite sequence of questions posted in the dlib forums.

There are instructions on the How to Compile page. If you do not understand the instructions in the "Compiling on Windows Using Visual Studio" section or are getting errors then follow the instructions in the "Compiling on Any Operating System Using CMake" section. In particular, install CMake and then type these exact commands from within the root of the dlib distribution: cd examples mkdir build cd build del /F /S /Q * cmake .. cmake --build . --config Release That should compile the dlib examples in visual studio. The output executables will appear in the Release folder. The del /F /S /Q * command is to make sure you clear out any extraneous files you might have placed in the build folder and is not necessary if build begins empty.
Dlib isn't slow. I get this question many times a week and 95% of the time it's from someone using Visual Studio who has compiled their program in Debug mode rather than the optimized Release mode. So if you are using Visual Studio then realize that Visual Studio has these two modes. The default is Debug. The mode is selectable via a drop down:

Debug mode disables compiler optimizations. So the program will be very slow if you run it in Debug mode. So click the drop down,

and select Release.

Then when you compile the program it will appear in a folder named Release rather than in a folder named Debug.

Finally, you can enable either SSE4 or AVX instruction use. These will make certain operations much faster (e.g. face detection). You do this using CMake's cmake-gui tool. For example, if you execute these commands you will get the cmake-gui screen: cd examples mkdir build cd build cmake .. cmake-gui . Which looks like this:

Where you can select SSE4 or AVX instruction use. Then you click configure and then generate. After that when you build your visual studio project some things will be faster.

Finally, note that AVX is a little bit faster than SSE4 but if your computer is fairly old it might not support it. In that case, either buy a new computer or use SSE4 instructions.
If you use dlib in your research then please use the following citation:
Davis E. King. Dlib-ml: A Machine Learning Toolkit. Journal of Machine Learning Research 10, pp. 1755-1758, 2009

    @Article{dlib09,
      author = {Davis E. King},
      title = {Dlib-ml: A Machine Learning Toolkit},
      journal = {Journal of Machine Learning Research},
      year = {2009},
      volume = {10},
      pages = {1755-1758},
    }

Here are the possibilities:

- You are using a file stream and forgot to put it into binary mode. You need to do something like this: std::ifstream fin("myfile", std::ios::binary); or std::ofstream fout("myfile", std::ios::binary);

  If you don't give std::ios::binary then the iostream will mess with the binary data and cause serialization to not work right.
- The iostream is in a bad state. You can check the state by calling mystream.good(). If it returns false then the stream is in an error state such as end-of-file or maybe it failed to do the I/O. Also note that if you close a file stream and reopen it you might have to call mystream.clear() to clear out the error flags.

Long answer, read the matrix example program.
Short answer, here are some examples: matrix<double> mat; mat.set_size(4,5); matrix<double,0,1> column_vect; column_vect.set_size(6); matrix<double,0,1> column_vect2(6); // give size to constructor matrix<double,1> row_vect; row_vect.set_size(5);

If you can't find something then check the index.
Also, the bulk of the documentation can be found by following the links.
There should never be anything in dlib that prevents you from using or interacting with other libraries. Moreover, there are some additional tools in dlib to make some interactions easier:

- BLAS and LAPACK libraries are used by the matrix automatically if you #define DLIB_USE_BLAS and/or DLIB_USE_LAPACK and link against the appropriate library files. Note that the CMakeLists.txt file that comes with dlib will do this for you automatically in many instances.
- Armadillo and Eigen libraries have matrix objects which can be converted into dlib matrix objects by calling dlib::mat() on them.
- OpenCV image objects can be converted into a form usable by dlib routines by using cv_image. You can also convert from a dlib matrix or image to an OpenCV Mat using dlib::toMat().
- Google Protocol Buffers can be serialized by the dlib serialization routines. This means that, for example, you can pass protocol buffer objects through a bridge.
- libpng and libjpeg are used by load_image whenever DLIB_PNG_SUPPORT and DLIB_JPEG_SUPPORT are defined respectively. You must also tell your compiler to link against these libraries to use them. However, CMake will try to link against them automatically if they are installed.
- SQLite is used by the database object. In fact, it is just a wrapper around SQLite's C interface which simplifies its use (e.g. makes resource management use RAII).

The optimization algorithm is somewhat unpredictable. Sometimes it is fast and sometimes it is slow. What usually makes it really slow is if you use a radial basis kernel and you set the gamma parameter to something too large. This causes the algorithm to start using a whole lot of relevance vectors (i.e. basis vectors) which then makes it slow. The algorithm is only fast as long as the number of relevance vectors remains small but it is hard to know beforehand if that will be the case.

You should try kernel ridge regression instead since it also doesn't take any parameters but is always very fast.

This function makes a copy of your training data for each thread. So you are probably running out of memory. To avoid this, use the randomly_subsample function to reduce the amount of data you are using or use fewer threads.

For example, you could reduce the amount of data by saying this: // reduce to only 1000 samples cross_validate_trainer_threaded(trainer, randomly_subsample(samples, 1000), randomly_subsample(labels, 1000), 4, // num folds 4); // num threads

See the Using Custom Kernels example program.

Picking the right kernel all comes down to understanding your data, and obviously this is highly dependent on your problem.

One thing that's sometimes useful is to plot each feature against the target value. You can get an idea of what your overall feature space looks like and maybe tell if a linear kernel is the right solution. But this still hides important information from you. For example, imagine you have two diagonal lines which are very close together and are both the same length. Suppose one line is of the +1 class and the other is the -1 class. Each feature (the x or y coordinate values) by itself tells you almost nothing about which class a point belongs to but together they tell you everything you need to know.

On the other hand, if you know something about the data you are working with then you can also try and generate your own features. So for example, if your data is a bunch of images and you know that one of your classes contains a lot of lines then you can make a feature that attempts to measure the number of lines in an image using a hough transform or sobel edge filter or whatever. Generally, try and think up features which should be highly correlated with your target value. A good way to do this is to try and actually hand code N solutions to the problem using whatever you know about your data or domain. If you do a good job then you will have N really great features and a linear or rbf kernel will probably do very well when using them.

Or you can just try a whole bunch of kernels, kernel parameters, and training algorithm options while using cross validation. I.e. when in doubt, use brute force :) There is an example of that kind of thing in the model selection example program.

This happens when you use the radial_basis_kernel and you set the gamma value to something highly inappropriate. To understand what's happening lets imagine your data has just one feature and its value ranges from 0 to 7. Then what you want is a gamma value that gives nice Gaussian bumps like the one in this graph:
However, if you make gamma really huge you will get this (it's zero everywhere except for one place):
Or if you make gamma really small then it will be 1.0 everywhere:

So you need to pick the gamma value so that it is scaled reasonably to your data. A good rule of thumb (i.e. not the optimal gamma, just a heuristic guess) is the following:

const double gamma = 1.0/compute_mean_squared_distance(randomly_subsample(samples, 2000));