Motivation

Recently, while implementing Efficient Hierarchical Graph-Based Video Segmentation, I needed to compare different algorithms for computing dense optical flow. While there are several optical flow benchmarks available online, for example the Middlebury benchmark [1], the Sintel benchmark [2] or the KITTI benchmark [3], I wanted to visualize the results first. After some attempts to visualize 2-channel optical flow images using OpenCV, guided by this StackOverflow question, I finally wrote a simple OpenCV wrapper for the C code provided by the Sintel dataset [2]. Note that the provided C code is based on imageLib, a lightweight C++ image library based on the StereoMatcher [4] code.

The code is available on GitHub:

Compiling

The code can be compiled using CMake (tested on Ubuntu 12.04, 14.04 and 14.10) and is based on OpenCV and Boost.

$ cd flow-io-opencv
$ mkdir build
$ cd build
$ cmake ..
$ make

Usage

The provided command line tool can be used to convert .flo files to .txt in cv::FileStorage format and optionally visualize the optical flow files:

$ ./cli/cli --help
Allowed options:
  --help                produce help message
  --input-dir arg       input directory
  --output-dir arg      output directory
  --vis-dir arg         visualize results in a separate directory

The wrapper can also be used to read .flo files into a cv::Mat or visualize optical flow available as 2-channel cv::Mat:

cv::Mat flow = FlowIOOpenCVWrapper::read(flo_file);
cv::Mat image = FlowIOOpenCVWrapper::flowToColor(flow);

Example

The GitHub repository provides several frames on the alley_1 sequence and figure 1 shows a visualization:

References

• [1] S. Baker, D. Scharstein, J. P. Lewis, S. Roth, M. J. Black, R. Szeliski. A Database and Evaluation Methodology for Optical Flow. International Journal of Computer Vision, volume 92, 2011.
• [2] D. J. Butler, J. Wulff, G. B. Stanley, M. J. Black. A naturalistic open source movie for optical flow evaluation. European Conference on Computer Vision, 2012.
• [3] M. Menze, A. Geiger. Object Scene Flow for Autonomous Vehicles. Conference on Computer Vision and Pattern Recognition, 2015.
• [4] D. Scharstein, R. Szeliski. A taxonomy and evaluation of dense two-frame stereo correspondence algorithms. Technical Report MSR-TR-2001-81, Microsoft Research, 2001.