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Version: 0.1 alpha2 Project Page:

HOG and Spatial Convolution on SIMD Architecture

Authors: Ishan Misra, Abhinav Shrivastava, Martial Hebert.

The details and aim of this project can be found in our tech report. Please cite it if you use this code for any purpose. The code can be downloaded freely from our github project page.

Ishan Misra, Abhinav Shrivastava, Martial Hebert - ”HOG and Spatial Convolution on SIMD Architecture” CMU Tech Report XXXX (2013)


esvmTestCPP is an implementation of the MATLAB Exemplar-SVM testing pipeline written in C++. Computationally intensive parts were written using ISPC for SIMD performance.

One may also use this code just for computing HOG features or performing spatial-convolution (without anything to do with Exemplar SVMs). The code was designed to be modular to allow for such a use-case.

If you are using this pipeline for testing, it is assumed that you have basic familiarity with the MATLAB esvm code.

Disclaimer: This is alpha quality software. (Notice the alpha2 in the version number). Alpha is Latin for “doesn’t work and may burn your computer”. The code hasn’t been tested very thoroughly, and we will try to fix any bugs that you report.


The code depends on the following open-source projects

  1. ISPC ( : For optimized SIMD code generation
  2. OpenCV ( : For Image I/O.
  3. OpenMP and pthreads : For spawning threads (in ISPC as well as omp parallel for constructs).

It requires a x86 or x86-64 ISA compatible machine. The code is released for a GNU/Linux compatible Operating System. There are a few dependencies on the GNU C Compiler (GCC) mainly due to __expect macros defined in esvm_utils.h. The dependencies on GNU/Linux and GCC will be resolved in future releases.


  1. ISPC Setup : The code base includes the ISPC binary for x86-64. (version 1.3.0 as of this writing). So nothing special needs to be done for this setup. If you are using a 32 bit Operating System, you will need to compile ISPC from source. As of this writing pre-built 32-bit binaries are not available from the ISPC github page.
  2. OpenCV : Any version 2.x should suffice. In reality, any version above 1.2 will be fine, but may need a change in the includes (since OpenCV 2.x has a different way of including header files).
  3. demos/Makefile: The Makefile in the demos directory compiles demos. One may need to change architecture specific flags like mavx, corei7-avx etc. depending on the exact CPU model. These flags are marked separately for convenience (as ARCH flags). Discarding architecture specific flags generally affects performance, but maybe useful if you just want to try out the code.

Directory structure

##+begin_src sh :results output #tree -d ##+end_src

├── common: ISPC 64-bit Linux binary and internal files (Task system)
├── demos: contains demo files for HOG, Exemplar testing.
├── internal
├── matlab-files: easy conversion between HOG format in MATLAB/C++ codebases
└── sample-data: sample data for demo files to run
    └── exemplars
        ├── exemplar-mat-files
        └── exemplar-txt-files

8 directories

Getting Started

Input format for Exemplars

Suppose you have $C$ classes, and for each class you have $Ni$ ($i=1\ldots C$) exemplars. The input format as of this version is

  1. descFile: This is a file containing names of $C$ classes followed by the name of a classDescFile. The format is ClassName<space>classDescFileName<newline>.
  2. classDescFile: This file contains 4 fields per line. The first field is the path to the txt file containing the exemplar data, the second and third field are the number of rows and columns respectively. The fourth field is the offset ($b$ in the SVM decision function $wTx+b$) which is exemplar-specific.

Generating the exemplar data files

The exemplar data is written in ASCII files. These files can be generated by using the writeHogTxt function in C++. It is also expected that the user will have trained exemplars in the form of .mat files from the MATLAB esvm code. In order to convert these .mat files and generate the necessary descFile and classDescFiles, there are helper scripts in the matlab-files directory. The script convert_mat_txt.m is provided for reference. The functions readHogTxt.m and writeHogTxt.m are used for reading and writing HOG features or exemplars (since exemplars and HOG features are both 3D arrays of the form $m× n× 31$).


The parameters for exemplar testing can be put together in the struct esvmParameters. A user can get default parameters by calling the function esvmDefaultParameters. These default parameters correspond to default parameters from the MATLAB esvm code. The following are the main fields to be concerned with

  1. levelsPerOctave: Defines the number of times an image is resized between two scalings of 1/2. A larger value means tighter bounding box (in terms of “where exactly is the object ?”). An empirical maximum and minimum are between 10 and 3. The actual value is application specific.
  2. maxHogLevels: Maximum number of HOG levels computed. The actual value also depends on minHogDim and minImageScale.
  3. minHogDim: Minimum dimension of HOG before any sort of zero-padding.
  4. minImageScale: A number between 0 and 1. Determines the minimum scaling factor for resizing the image.
  5. useMexResize: A boolean parameter. When set to true (the default) image resizing uses a C++ version from the original MATLAB esvm code. Setting this to false, uses the native OpenCV image resizing which is faster.
  6. detectionThreshold: A number between 0 and 1. The threshold for exemplar detection. A higher threshold means lesser false positives (but also a lower detection rate).
  7. nmsOverlapThreshold: A number between 0 and 1. The non-maximal-suppression threshold. Decides when to consider two overlapping detections as two different detections.
  8. maxWindowsPerExemplar: Maximum number of detections per exemplar.
  9. maxTotalBoxesPerExemplar: This value is used for pre-allocation of memory. It should be greater than maxWindowsPerExemplar.
  10. userTasks: Maximum number of threads to spawn. Usually setting this number equal to 1 or 2 times the number of physical cores gives a reasonable performance.

Bounding box information

The bounding boxes are stored in struct esvmBoxes. It internally stores them in a float array. It is recommended to use pre-defined macros for accessing/copying the bounding boxes. These are defined in esvm_utils.h. The demos directory contains an example showing how to use them.

Precision issues

Detection precision depends on which image resize function is used. As far as we can tell, it is best to use the same resize function for training and testing. The default option of useMexResize, uses the resize function from the MATLAB implementation of Exemplar-SVM. If speed is an issue, then one can switch over to the OpenCV resize function, but the detection results will differ.

Another thing to note is that the HOG implementation uses float precision for computing the features (as opposed to double in the MATLAB HOG implementation of Pedro Felzenszwalb).

Performance characteristics

Read the Tech-Report for more details on how the performance compares to the MATLAB testing pipeline.


The detection demos aren’t even close to perfect

Yes. It is just a demo. You will need to adjust the thresholds depending on your particular dataset/exemplars.

I am getting an Illegal Instruction when running demos on a Virtual Machine

This happens because a lot of the VMs do not support avx or sse4-2 instructions. In the Makefile set the variables ISPCARCHFLAGS and ARCHFLAGS to blank (i.e. just delete whatever is front of them, but keep the “=” sign). This should generally resolve the issue. This of course means that you are not using SIMD code now.

You can try setting the ARCHFLAGS to blank and ISPCARCHFLAGS to --target=sse2 or --target=sse4.

You keep mentioning C++, but all of your programming is C style!

Correct. I mention C++ because I did use a few STL libraries. There were a few headaches using C++ classes and our flavor of SIMD optimizations (ISPC).

Can I use this for HOG computation only ?

Yes. Check out examples (demo01, demo02) in the demo directory.

Can I use this for Convolution computation only ?

Yes. Check out examples (demo00) in the demos directory.

What HOG feature do you compute ?

It is based on the paper

  • P. F. Felzenszwalb, R. B. Girshick, D. McAllester, and D. Ramanan, ”Object detection with discriminatively trained part based models”, PAMI 2010

It is different from the HOG popularized by the “Pedestrian detection” application from Navneet Dalal’s paper (N. Dalal and B. Triggs, ”Histograms of oriented gradients for human detection”, CVPR 2005).

This latest reincarnation of the HOG feature is generally considered to be more discriminative than the earlier versions, for object detection tasks.

Is this library thread-safe ?

Unfortunately, no. The reason has to do with the ISPC task implementation. A request for changing this has been filed (!topic/ispc-users/FgQgCVFMWTs) and as soon as this gets fixed, the library should be thread-safe.

Code TODOs

High priority

  • BLOCK convolution: Model convolution as matrix multiplication and use ATLAS for performing the matrix multiplication. Hope to achieve speeds comparable to the MATLAB design.
  • Reduce memory reads/writes in NMS
  • Better I/O format for Exemplars. This will involve changing the read/write functions in MATLAB and C++. No changes expected in the API. I need feedback from users as to what they would like!

Low priority

  • Fix dependency issues on GCC and Linux. The __expect macros, and memalign calls need to be changed.
  • Include a 32 bit binary for ISPC ?
  • parameters->flipImage to be implemented.


Testing pipeline for Exemplar-SVM in C++







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