1. Implementation and Optimization of SIFT on a OpenCL GPU 6.869-6.338 Final Project 5/5/2010 Guy-Richard Kayombya

2. Overview Motivation Quick Intro to OpenCL Implementation Results

3. Motivation Learn OpenCL Adapt the SIFT algorithm to yet another parallel architecture Maybe achieve some speedup

4. Quick Intro to OpenCL New Standard from Khronos for Heterogeneous Parallel Computing (v1.0 Released Dec 2008) Initiated by Apple Open and royalty free Cross-Vendor and Cross-Platform Make use of all available processing entities CPUs, GPUs and other Processors Scales from Embedded to HPC solutions

5. Quick Intro to OpenCL(2) One Host, Multiple Devices Each Device has multiple Compute Units Each Compute Unit has multiple Processing Elements E.g: GT200 has 30 Compute units/Streaming processors and 8 Processing Elements/Scalar SIMD processors = 240 Processing elements

6. Quick Intro to OpenCL(3) NDRange = size of the problem to solve 1D or 2D Work-Group = block of work-items Work-Item ~ lightweight thread

7. Quick Intro to OpenCL(4) Global : per device Local : per Work- Group Private : Per Work- Item

8. Quick Intro to OpenCL(4) __kernel void vec_inc ( __global float *a, __global const float b) { int gid = get_global_id(0); a[gid] = a[gid] + b; }

9. Implementation Abstraction Layer (85 %) Gaussian/DoG Pyramids (100 % semi- optimized) Keypoint Detection (95 % - Naive) Keypoint Refinement (90 % - Naive) Orientation Assignment (10 %) Descriptor generation(0 %)

10. Abstraction Layer Problem : Host device code is cumbersome Requires dozens of repetitive lines to setup device contexts kernels,buffers,etc... Solution: OpenCL wrapper Simplifies creation and management of hybrid Host/Client buffers and execution of kernels Facilitates transition from serial to parallel execution  Host/Client Synchronization Memory management issues still need to be fixed

11. Gaussian Pyramid Separable convolution 2 1D filters Indirect filtering to reduce kernel size sigma_diff = sqrt(sig_dst^2 – sigma_src^2) Use convolutionSeperable() provided by Nvidia for efficient 2D seperable convolution on the GPU

12. Keypoint detection Each pixels is processed by one work item independently No state sharing Worst case 26 comparisons / per work Item

13. Keypoint Refinement Each Keypoint is processed independently by one work item Kernel is a slightly modified version of the keypoint refinement Matlab Mex module by Vedaldi

14. Preliminary Results (Time) All the measurements are performed on an input image of size 512x512. Gaussian Filtering (sigma 4.1): Vedaldi Matlab CPU = 0.19s – 100 % Naïve C++ CPU = 0.33s – 57% GPU = 0.0094s – 2000 % GPU with data transfer = 0.0133s – 1400 % Extrema Detection (octave 0 of pyramid): Vedaldi Matlab CPU = 0.179 s – 100 % GPU = 0.035725s – 500 % Keypoint Refinement (octave 0 of pyramid): Vedaldi Matlab CPU = 0.004 s – 100 % GPU = 0.0689s – 6 %

15. Preliminary Results(performance) Refined Keypoints for octave 0 Blue: Matlab implementation Red: OpenCL Green: Common 85% Correspondence