dsift.h File Reference
Dense SIFT. More...
#include "generic.h"
Data Structures | |
struct | VlDsiftKeypoint |
Dense SIFT keypoint. More... | |
struct | VlDsiftDescriptorGeometry |
Dense SIFT descriptor geometry. More... | |
struct | VlDsiftFilter |
Dense SIFT filter. More... | |
Functions | |
VlDsiftFilter * | vl_dsift_new (int width, int height) |
Create a new DSIFT filter. | |
VlDsiftFilter * | vl_dsift_new_basic (int width, int height, int step, int binSize) |
Create a new DSIFT filter (basic interface) | |
void | vl_dsift_delete (VlDsiftFilter *self) |
Delete DSIFT filter. | |
void | vl_dsift_process (VlDsiftFilter *self, float const *im) |
Compute keypoints and descriptors. | |
void | vl_dsift_transpose_descriptor (float *dst, float const *src, int numBinT, int numBinX, int numBinY) |
Transpose descriptor. | |
void | _vl_dsift_update_buffers (VlDsiftFilter *self) |
Updates internal buffers to current geometry. | |
Setting parameters | |
void | vl_dsift_set_steps (VlDsiftFilter *self, int stepX, int stepY) |
Set steps. | |
void | vl_dsift_set_bounds (VlDsiftFilter *self, int minX, int minY, int maxX, int maxY) |
Set bounds. | |
void | vl_dsift_set_geometry (VlDsiftFilter *self, VlDsiftDescriptorGeometry const *geom) |
Set SIFT descriptor geometry. | |
void | vl_dsift_set_flat_window (VlDsiftFilter *self, vl_bool useFlatWindow) |
Set flat window flag. | |
void | vl_dsift_set_window_size (VlDsiftFilter *self, double windowSize) |
Set SIFT descriptor Gaussian window size. | |
Retrieving data and parameters | |
float const * | vl_dsift_get_descriptors (VlDsiftFilter const *self) |
Get descriptors. | |
int | vl_dsift_get_descriptor_size (VlDsiftFilter const *self) |
Get descriptor size. | |
int | vl_dsift_get_keypoint_num (VlDsiftFilter const *self) |
Get number of keypoints. | |
VlDsiftKeypoint const * | vl_dsift_get_keypoints (VlDsiftFilter const *self) |
Get keypoints. | |
void | vl_dsift_get_bounds (VlDsiftFilter const *self, int *minX, int *minY, int *maxX, int *maxY) |
Get bounds. | |
void | vl_dsift_get_steps (VlDsiftFilter const *self, int *stepX, int *stepY) |
Get steps. | |
VlDsiftDescriptorGeometry const * | vl_dsift_get_geometry (VlDsiftFilter const *self) |
Get SIFT descriptor geometry. | |
vl_bool | vl_dsift_get_flat_window (VlDsiftFilter const *self) |
Get flat window flag. | |
double | vl_dsift_get_window_size (VlDsiftFilter const *self) |
Get SIFT descriptor Gaussian window size. |
Detailed Description
Dense Scale Invariant Feature Transform
This module implements a dense version of SIFT. This is an object that can quickly compute descriptors for densely sampled keypoints with identical size and orientation. It can be reused for multiple images of the same size.
Overview
- See also:
- The SIFT module, Technical details
This module implements a fast algorithm for the calculation of a large number of SIFT descriptors of densely sampled features of the same scale and orientation. See the SIFT module for an overview of SIFT.
The feature frames (keypoints) are indirectly specified by the sampling steps (vl_dsift_set_steps) and the sampling bounds (vl_dsift_set_bounds). The descriptor geometry (number and size of the spatial bins and number of orientation bins) can be customized (vl_dsift_set_geometry, VlDsiftDescriptorGeometry).

Dense SIFT descriptor geometry
By default, SIFT uses a Gaussian windowing function that discounts contributions of gradients further away from the descriptor centers. This function can be changed to a flat window by invoking vl_dsift_set_flat_window. In this case, gradients are accumulated using only bilinear interpolation, but instad of being reweighted by a Gassuain window, they are all weighted equally. However, after gradients have been accumulated into a spatial bin, the whole bin is reweighted by the average of the Gaussian window over the spatial support of that bin. This “approximation” substantially improves speed with little or no loss of performance in applications.
Keypoints are sampled in such a way that the centers of the spatial bins are at integer coordinates within the image boundaries. For instance, the top-left bin of the top-left descriptor is centered on the pixel (0,0). The bin immediately to the right at (binSizeX
,0), where binSizeX
is a paramtere in the VlDsiftDescriptorGeometry structure. vl_dsift_set_bounds can be used to further restrict sampling to the keypoints in an image.
Usage
DSIFT is implemented by a VlDsiftFilter object that can be used to process a sequence of images of a given geometry. To use the DSIFT filter:
- Initialize a new DSIFT filter object by vl_dsift_new (or the simplified vl_dsift_new_basic). Customize the descriptor parameters by vl_dsift_set_steps, vl_dsift_set_geometry, etc.
- Process an image by vl_dsift_process.
- Retrieve the number of keypoints (vl_dsift_get_keypoint_num), the keypoints (vl_dsift_get_keypoints), and their descriptors (vl_dsift_get_descriptors).
- Optionally repeat for more images.
- Delete the DSIFT filter by vl_dsift_delete.
Technical details
This section extends the SIFT descriptor section and specialzies it to the case of dense keypoints.
Dense descriptors
When computing descriptors for many keypoints differing only by their position (and with null rotation), further simplifications are possible. In this case, in fact,
Since many different values of T are sampled, this is conveniently expressed as a separable convolution. First, we translate by and we use the symmetry of the various binning and windowing functions to write
Then we define kernels
and obtain
Furthermore, if we use a flat rather than Gaussian windowing function, the kernels do not depend on the bin, and we have
(here is the side of the flat window).
- Note:
- In this case the binning functions
are triangular and the convolution can be computed in time independent on the filter (i.e. descriptor bin) support size by integral signals.
Sampling
To avoid resampling and dealing with special boundary conditions, we impose some mild restrictions on the geometry of the descriptors that can be computed. In particular, we impose that the bin centers are always at integer coordinates within the image boundaries. This eliminates the need for costly interpolation. This condition amounts to (expressed in terms of the x coordinate, and equally applicable to y)
Notice that for this condition to be satisfied, the descriptor center needs to be either fractional or integer depending on
being even or odd. To eliminate this complication, it is simpler to use as a reference not the descriptor center T, but the coordinates of the upper-left bin
. Thus we sample the latter on a regular (integer) grid
and we impose that the bin size is integer as well.
Function Documentation
void vl_dsift_delete | ( | VlDsiftFilter * | self ) |
- Parameters:
-
self DSIFT filter.
void vl_dsift_get_bounds | ( | VlDsiftFilter const * | self, |
int * | minX, | ||
int * | minY, | ||
int * | maxX, | ||
int * | maxY | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. minX bounding box minimum X coordinate. minY bounding box minimum Y coordinate. maxX bounding box maximum X coordinate. maxY bounding box maximum Y coordinate.
int vl_dsift_get_descriptor_size | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
- Returns:
- size of a descriptor.
float const * vl_dsift_get_descriptors | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
- Returns:
- descriptors.
int vl_dsift_get_flat_window | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
- Returns:
TRUE
if the DSIFT filter uses a flat window.
VlDsiftDescriptorGeometry const * vl_dsift_get_geometry | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
- Returns:
- DSIFT descriptor geometry.
int vl_dsift_get_keypoint_num | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
VlDsiftKeypoint const * vl_dsift_get_keypoints | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
void vl_dsift_get_steps | ( | VlDsiftFilter const * | self, |
int * | stepX, | ||
int * | stepY | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. stepX sampling step along X. stepY sampling step along Y.
double vl_dsift_get_window_size | ( | VlDsiftFilter const * | self ) | [inline] |
- Parameters:
-
self DSIFT filter object.
- Returns:
- window size.
VlDsiftFilter* vl_dsift_new | ( | int | imWidth, |
int | imHeight | ||
) |
- Parameters:
-
imWidth width of the image. imHeight height of the image
- Returns:
- new filter.
VlDsiftFilter* vl_dsift_new_basic | ( | int | imWidth, |
int | imHeight, | ||
int | step, | ||
int | binSize | ||
) |
- Parameters:
-
imWidth width of the image. imHeight height of the image. step sampling step. binSize bin size.
- Returns:
- new filter.
The descriptor geometry matches the standard SIFT descriptor.
void vl_dsift_process | ( | VlDsiftFilter * | self, |
float const * | im | ||
) |
- Parameters:
-
self DSIFT filter. im image data.
void vl_dsift_set_bounds | ( | VlDsiftFilter * | self, |
int | minX, | ||
int | minY, | ||
int | maxX, | ||
int | maxY | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. minX bounding box minimum X coordinate. minY bounding box minimum Y coordinate. maxX bounding box maximum X coordinate. maxY bounding box maximum Y coordinate.
void vl_dsift_set_flat_window | ( | VlDsiftFilter * | self, |
vl_bool | useFlatWindow | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. useFlatWindow true
if the DSIFT filter should use a flat window.
void vl_dsift_set_geometry | ( | VlDsiftFilter * | self, |
VlDsiftDescriptorGeometry const * | geom | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. geom descriptor geometry parameters.
void vl_dsift_set_steps | ( | VlDsiftFilter * | self, |
int | stepX, | ||
int | stepY | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. stepX sampling step along X. stepY sampling step along Y.
void vl_dsift_set_window_size | ( | VlDsiftFilter * | self, |
double | windowSize | ||
) | [inline] |
- Parameters:
-
self DSIFT filter object. windowSize window size.
void vl_dsift_transpose_descriptor | ( | float * | dst, |
float const * | src, | ||
int | numBinT, | ||
int | numBinX, | ||
int | numBinY | ||
) | [inline] |
- Parameters:
-
dst destination buffer. src source buffer. numBinT numBinX numBinY The function writes to dst the transpose of the SIFT descriptor src. Let I
be an image. The transpose operator satisfies the equationtranspose(dsift(I,x,y)) = dsift(transpose(I),y,x)