CudaSift - SIFT features with CUDA

This is the fourth version of a SIFT (Scale Invariant Feature Transform) implementation using CUDA for GPUs from NVidia. The first version is from 2007 and GPUs have evolved since then. This version is slightly more precise and considerably faster than the previous versions and has been optimized for Kepler and later generations of GPUs.

On a GTX 1060 GPU the code takes about 1.2 ms on a 1280x960 pixel image and 1.7 ms on a 1920x1080 pixel image. There is also code for brute-force matching of features that takes about 2.2 ms for two sets of around 1900 SIFT features each.

The code relies on CMake for compilation and OpenCV for image containers. OpenCV can however be quite easily changed to something else. The code can be relatively hard to read, given the way things have been parallelized for maximum speed.

The code is free to use for non-commercial applications. If you use the code for research, please refer to the following paper.

M. Björkman, N. Bergström and D. Kragic, "Detecting, segmenting and tracking unknown objects using multi-label MRF inference", CVIU, 118, pp. 111-127, January 2014. ScienceDirect

New version for Pascal (2018-10-26)

There is a new version optimized for Pascal cards, but it should work also on many older cards. Since it includes some bug fixes that changes slightly how features are extracted, which might affect matching to features extracted using an older version, the changes are kept in a new branch (Pascal). The fixes include a small change in ScaleDown that corrects an odd behaviour for images with heights not divisible by 2^(#octaves). The second change is a correction of an improper shift of (0.5,0.5) pixels, when pixel values were read from the image to create a descriptor.

Then there are some improvements in terms of speed, especially in the Laplace function, that detects DoG features, and the LowPass function, that is seen as preprocessing and is not included in the benchmarking below. Maybe surprisingly, even if optimizations were done with respect to Pascal cards, these improvements were even better for older cards. The changes involve trying to make each CUDA thread have more work to do, using fewer thread blocks. For typical images of today, there will be enough blocks to feed the streaming multiprocessors anyway.

Latest result of version under test:

		1280x960	1920x1080	GFLOPS	Bandwidth	Matching
Pascal	GeForce GTX 1080 Ti	0.6*	0.8*	10609	484	1.0
Pascal	GeForce GTX 1060	1.2*	1.7*	3855	192	2.2
Maxwell	GeForce GTX 970	1.3*	1.8*	3494	224	2.5
Kepler	Tesla K40c	2.4*	3.4*	4291	288	4.7

Matching is done between two sets of 1911 and 2086 features respectively.

Benchmarking of new version (2018-08-22)

About every 2nd year, I try to update the code to gain even more speed through further optimization. Here are some results for a new version of the code. Improvements in speed have primarilly been gained by reducing communication between host and device, better balancing the load on caches, shared and global memory, and increasing the workload of each thread block.

		1280x960	1920x1080	GFLOPS	Bandwidth	Matching
Pascal	GeForce GTX 1080 Ti	0.7	1.0	10609	484	1.0
Pascal	GeForce GTX 1060	1.6	2.4	3855	192	2.2
Maxwell	GeForce GTX 970	1.9	2.8	3494	224	2.5
Kepler	Tesla K40c	3.1	4.7	4291	288	4.7
Kepler	GeForce GTX TITAN	2.9	4.3	4500	288	4.5

Matching is done between two sets of 1818 and 1978 features respectively.

It's questionable whether further optimization really makes sense, given that the cost of just transfering an 1920x1080 pixel image to the device takes about 1.4 ms on a GTX 1080 Ti. Even if the brute force feature matcher is not much faster than earlier versions, it does not have the same O(N^2) temporary memory overhead, which is preferable if there are many features.

Benchmarking of previous version (2017-05-24)

Computational cost (in milliseconds) on different GPUs:

		1280x960	1920x1080	GFLOPS	Bandwidth	Matching
Pascal	GeForce GTX 1080 Ti	1.7	2.3	10609	484	1.4
Pascal	GeForce GTX 1060	2.7	4.0	3855	192	2.6
Maxwell	GeForce GTX 970	3.8	5.6	3494	224	2.8
Kepler	Tesla K40c	5.4	8.0	4291	288	5.5
Kepler	GeForce GTX TITAN	4.4	6.6	4500	288	4.6

Matching is done between two sets of 1616 and 1769 features respectively.

The improvements in this version involved a slight adaptation for Pascal, changing from textures to global memory (mostly through L2) in the most costly function LaplaceMulti. The medium-end card GTX 1060 is impressive indeed.

Usage

There are two different containers for storing data on the host and on the device; SiftData for SIFT features and CudaImage for images. Since memory allocation on GPUs is slow, it's usually preferable to preallocate a sufficient amount of memory using InitSiftData(), in particular if SIFT features are extracted from a continuous stream of video camera images. On repeated calls ExtractSift() will reuse memory previously allocated.

#include <opencv2/core/core.hpp>
#include <opencv2/highgui/highgui.hpp>
#include <cudaImage.h>
#include <cudaSift.h>

/* Reserve memory space for a whole bunch of SIFT features. */
SiftData siftData;
InitSiftData(siftData, 25000, true, true);

/* Read image using OpenCV and convert to floating point. */
cv::Mat limg;
cv::imread("image.png", 0).convertTo(limg, CV32FC1);
/* Allocate 1280x960 pixel image with device side pitch of 1280 floats. */ 
/* Memory on host side already allocated by OpenCV is reused.           */
CudaImage img;
img.Allocate(1280, 960, 1280, false, NULL, (float*) limg.data);
/* Download image from host to device */
img.Download();

int numOctaves = 5;    /* Number of octaves in Gaussian pyramid */
float initBlur = 1.0f; /* Amount of initial Gaussian blurring in standard deviations */
float thresh = 3.5f;   /* Threshold on difference of Gaussians for feature pruning */
float minScale = 0.0f; /* Minimum acceptable scale to remove fine-scale features */
bool upScale = false;  /* Whether to upscale image before extraction */
/* Extract SIFT features */
ExtractSift(siftData, img, numOctaves, initBlur, thresh, minScale, upScale);
...
/* Free space allocated from SIFT features */
FreeSiftData(siftData);

Parameter setting

The requirements on number and quality of features vary from application to application. Some applications benefit from a smaller number of high quality features, while others require as many features as possible. More distinct features with higher DoG (difference of Gaussians) responses tend to be of higher quality and are easier to match between multiple views. With the parameter thresh a threshold can be set on the minimum DoG to prune features of less quality.

In many cases the most fine-scale features are of little use, especially when noise conditions are severe or when features are matched between very different views. In such cases the most fine-scale features can be pruned by setting minScale to the minimum acceptable feature scale, where 1.0 corresponds to the original image scale without upscaling. As a consequence of pruning the computational cost can also be reduced.

To increase the number of SIFT features, but also increase the computational cost, the original image can be automatically upscaled to double the size using the upScale parameter, in accordings with Lowe's recommendations. One should keep in mind though that by doing so the fraction of features that can be matched tend to go down, even if the total number of extracted features increases significantly. If it's enough to instead reduce the thresh parameter to get more features, that is often a better alternative.

Results without upscaling (upScale=False) of 1280x960 pixel input image.

thresh	#Matches	%Matches	Cost (ms)
1.0	4236	40.4%	5.8
1.5	3491	42.5%	5.2
2.0	2720	43.2%	4.7
2.5	2121	44.4%	4.2
3.0	1627	45.8%	3.9
3.5	1189	46.2%	3.6
4.0	881	48.5%	3.3

Results with upscaling (upScale=True) of 1280x960 pixel input image.

thresh	#Matches	%Matches	Cost (ms)
2.0	4502	34.9%	13.2
2.5	3389	35.9%	11.2
3.0	2529	37.1%	10.6
3.5	1841	38.3%	9.9
4.0	1331	39.8%	9.5
4.5	954	42.2%	9.3
5.0	611	39.3%	9.1