### Video Content Analysis at Google

* "Ease of use coupled with quite remarkable improvements in
performance made the Jacket system extremely attractive."
*

Video content analysis is the basis for categorizing videos and enabling search by content. Using sparse-coding methods to extract motion features in video in support of video content analysis is of growing interest. Several approaches rely on solving an L1-regularized least-squares optimization problem to apply the basis vectors which are used to extract these features. The number and size of the basis vectors required for video are substantially larger than the corresponding basis vectors learned from static images due to the wide diversity of visual transformations characteristic of video. In this application, Jacket and GPUs were used to improve performance by substantially accelerating the solution of the L1-regularized least-squares optimization problem.

The authors re-implemented a version of the coordinate-descent algorithm of Friedman et al [2007] developed by Raina et al [2009]. Raina et al implemented their version of coordinate descent in C and in CUDA. The authors took the C code and rewrote it as vectorized MATLAB. The vectorized code clearly mirrors the linear-algebra formulation of the algorithm. The transition to MATLAB enabled the use of the Jacket platform for the prototyping and mapping of the algorithm onto GPU resources. The Raina et al CUDA code exploited potential structure in the original coordinate-descent algorithm but serially coded each input vector. The vectorized code running under Jacket exploited both opportunities for parallelism. Moreover, the authors were able to experiment with exploiting additional opportunities for parallelism by making simple changes to the vectorized code, changes that would require a major rewrite of the CUDA code. Ease of use coupled with quite remarkable improvements in performance made the Jacket system extremely valuable.

**Authors:** Thomas Dean (Google), Rich Washington (Google) and Greg Corrado
(Stanford)

**Speedup:** 10-20x versus multi-core CPU implementations

**System:** MATLAB R2009a, an NVIDIA GeForce GTX 280, in a Dell Precision
workstation with Two Intel 2.40 GHz Core 2 Quad Q6600

## Comparisons

The research compared the results from six implementations each based on one of two algorithms:

- I. Feature-sign algorithm [Lee et al, 2007]:
- 1. Implementation in MATLAB
- II. Coordinate-descent algorithm [Friedman et al, 2007]:
- 2. Multi-threaded implementation in C++
- 3. CUDA implementation by [Raina et al, 2009]
- 4. Vectorized re-implementation of [2] in MATLAB
- 5. Vectorized re-implementation of [2] in Eigen/C
- 6. Same code as [4] but running under Jacket

The flexibility of Jacket made it easy to experiment with how best to use the GPU. The Jacket and GPU system easily outperformed the other implementations.

## Results

Note: All runs were performed using 864 basis vectors, each vector with a spatial extent of 13 x 13 and a temporal extent of 7 frames and 1024 input vectors. The reported times are averaged over multiple runs. All implementations used a sample of 1024 input vectors extracted from the first thirty seconds of a video. This is the basis for computing descriptors used to categorize videos in our experimental video categorization research.

- [1] J. Friedman, T. Hofling, and R. Tibshirani, "Pathwise coordinate optimization," Annals of Applied Statistics, vol. 1, no. 2, pp. 302.332, 2007.
- [2] H. Lee, A. Battle, R. Raina, and A. Y. Ng, "Efficient sparse coding algorithms," in Advances in Neural Information Processing Systems 19, B. Scholkopf, J. Platt, and T. Hofmann, Eds. Cambridge, MA: MIT Press, 2007, pp. 801.808.
- [3] R. Raina, A. Madhavan, and A. Ng, "Large-scale deep unsupervised learning using graphics processors," in Proceedings of the 25th Annual International Conference on Machine Learning, 2009.

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