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A tool to visualize feature spaces commonly used in object detection.
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iHOG: Inverting Histograms of Oriented Gradients

This software package contains tools to invert and visualize HOG features. It implements the Paired Dictionary Learning algorithm described in our paper "Inverting and Visualizing Features for Object Detection" [1].


Before you can use this tool, you must compile iHOG. Execute the 'compile' script in MATLAB to compile the HOG feature extraction code and sparse coding SPAMS toolbox:

$ cd /path/to/ihog
$ matlab
>> compile

Remember to also adjust your path so MATLAB can find iHOG:

>> addpath(genpath('/path/to/ihog'))

If you want to use iHOG in your own project, you can simply drop the iHOG directory into the root of your project.

In order to use iHOG, you must have a learned paired dictionary. By default, iHOG will attempt to download a pretrained one from MIT for you on the first execution. If you wish to download it manually, simply do:

$ wget

Inverting HOG

To invert a HOG point, use the 'invertHOG()' function:

>> feat = features(im, 8);
>> ihog = invertHOG(feat);
>> imagesc(ihog); axis image;

Computing the inverse should take no longer than a second for a typical sized image on a modern computer. (It may slower the first time you invoke it as it caches the paired dictionary from disk.)

Visualizing HOG

iHOG has several functions to visualize HOG. The most basic is 'visualizeHOG()':

>> feat = features(im, 8);
>> visualizeHOG(feat);

The above displays a figure with the HOG glyph and the HOG inverse. This visualization is a drop-in replacement for more standard visualizations, and should work with existing code bases.

The de-facto HOG has signed components, unsigned components, as well as texture components. 'dissectHOG()' visualizes each of these components invidually:

>> dissectHOG(feat);

A similar visualization 'spreadHOG()' shows each dimension individually:

>> spreadHOG(feat);

More visualizations are available. Check out the 'visualizations/' folder and read the comments for more.


We provide a prelearned dictionary in 'pd.mat', but you can learn your own if you wish. Simply call the 'learnpairdict()' function and pass it a directory of images:

>> pd = learnpairdict('/path/to/images/', 1000000, 1000, 5, 5);

The above learns a 5x5 HOG patch paired dictionary with 1000 elements and a training set size of one million window patches. Depending on the size of the problem, it may take minutes or hours to complete.

Bundled Libraries

The iHOG package contains source code from the SPAMS sparse coding toolbox ( We have modified their code to better support 64 bit machines.

In addition, we have included a select few files from the discriminatively trained deformable parts model ( We use their HOG computation code and glyph visualization code.

Questions and Comments

If you have any feedback, please write to Carl Vondrick at


The conference paper for this software is currently under submission. In the mean time, please see our technical report:

[1] Carl Vondrick, Aditya Khosla, Tomasz Malisiewicz, Antonio Torralba. "Inverting and Visualizing Features for Object Detection." Technical Report. 2013.

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