SIFT (Scale Invariant Feature Transform (SIFT)) More...
Data Structures | |
struct | VlSiftKeypoint |
SIFT filter keypoint. More... | |
struct | VlSiftFilt |
SIFT filter. More... | |
Typedefs | |
typedef float | vl_sift_pix |
SIFT filter pixel type. | |
Functions | |
Create and destroy | |
VlSiftFilt * | vl_sift_new (int width, int height, int noctaves, int nlevels, int o_min) |
Create a new SIFT filter. More... | |
void | vl_sift_delete (VlSiftFilt *f) |
Delete SIFT filter. More... | |
Process data | |
int | vl_sift_process_first_octave (VlSiftFilt *f, vl_sift_pix const *im) |
Start processing a new image. More... | |
int | vl_sift_process_next_octave (VlSiftFilt *f) |
Process next octave. More... | |
void | vl_sift_detect (VlSiftFilt *f) |
Detect keypoints. More... | |
int | vl_sift_calc_keypoint_orientations (VlSiftFilt *f, double angles [4], VlSiftKeypoint const *k) |
Calculate the keypoint orientation(s) More... | |
void | vl_sift_calc_keypoint_descriptor (VlSiftFilt *f, vl_sift_pix *descr, VlSiftKeypoint const *k, double angle) |
Compute the descriptor of a keypoint. More... | |
void | vl_sift_calc_raw_descriptor (VlSiftFilt const *f, vl_sift_pix const *image, vl_sift_pix *descr, int widht, int height, double x, double y, double s, double angle0) |
Run the SIFT descriptor on raw data. More... | |
void | vl_sift_keypoint_init (VlSiftFilt const *f, VlSiftKeypoint *k, double x, double y, double sigma) |
Initialize a keypoint from its position and scale. More... | |
Retrieve data and parameters | |
int | vl_sift_get_octave_index (VlSiftFilt const *f) |
Get current octave index. More... | |
int | vl_sift_get_noctaves (VlSiftFilt const *f) |
Get number of octaves. More... | |
int | vl_sift_get_octave_first (VlSiftFilt const *f) |
Get first octave. More... | |
int | vl_sift_get_octave_width (VlSiftFilt const *f) |
Get current octave width. More... | |
int | vl_sift_get_octave_height (VlSiftFilt const *f) |
Get current octave height. More... | |
int | vl_sift_get_nlevels (VlSiftFilt const *f) |
Get number of levels per octave. More... | |
int | vl_sift_get_nkeypoints (VlSiftFilt const *f) |
Get number of keypoints. More... | |
double | vl_sift_get_peak_thresh (VlSiftFilt const *f) |
Get peaks treashold. More... | |
double | vl_sift_get_edge_thresh (VlSiftFilt const *f) |
Get edges threshold. More... | |
double | vl_sift_get_norm_thresh (VlSiftFilt const *f) |
Get norm threshold. More... | |
double | vl_sift_get_magnif (VlSiftFilt const *f) |
Get the magnification factor. More... | |
double | vl_sift_get_window_size (VlSiftFilt const *f) |
Get the Gaussian window size. More... | |
vl_sift_pix * | vl_sift_get_octave (VlSiftFilt const *f, int s) |
Get current octave data. More... | |
VlSiftKeypoint const * | vl_sift_get_keypoints (VlSiftFilt const *f) |
Get keypoints. More... | |
Set parameters | |
void | vl_sift_set_peak_thresh (VlSiftFilt *f, double t) |
Set peaks threshold. More... | |
void | vl_sift_set_edge_thresh (VlSiftFilt *f, double t) |
Set edges threshold. More... | |
void | vl_sift_set_norm_thresh (VlSiftFilt *f, double t) |
Set norm threshold. More... | |
void | vl_sift_set_magnif (VlSiftFilt *f, double m) |
Set the magnification factor. More... | |
void | vl_sift_set_window_size (VlSiftFilt *f, double x) |
Set the Gaussian window size. More... | |
Detailed Description
Function Documentation
◆ vl_sift_calc_keypoint_descriptor()
void vl_sift_calc_keypoint_descriptor | ( | VlSiftFilt * | f, |
vl_sift_pix * | descr, | ||
VlSiftKeypoint const * | k, | ||
double | angle0 | ||
) |
- Parameters
-
f SIFT filter. descr SIFT descriptor (output) k keypoint. angle0 keypoint direction.
The function computes the SIFT descriptor of the keypoint k of orientation angle0. The function fills the buffer descr which must be large enough to hold the descriptor.
The function assumes that the keypoint is on the current octave. If not, it does not do anything.
◆ vl_sift_calc_keypoint_orientations()
int vl_sift_calc_keypoint_orientations | ( | VlSiftFilt * | f, |
double | angles[4], | ||
VlSiftKeypoint const * | k | ||
) |
- Parameters
-
f SIFT filter. angles orientations (output). k keypoint.
The function computes the orientation(s) of the keypoint k. The function returns the number of orientations found (up to four). The orientations themselves are written to the vector angles.
- Remarks
- The function requires the keypoint octave k->o to be equal to the filter current octave vl_sift_get_octave. If this is not the case, the function returns zero orientations.
-
The function requires the keypoint scale level
k->s
to be in the ranges_min+1
ands_max-2
(where usuallys_min=0
ands_max=S+2
). If this is not the case, the function returns zero orientations.
- Returns
- number of orientations found.
◆ vl_sift_calc_raw_descriptor()
void vl_sift_calc_raw_descriptor | ( | VlSiftFilt const * | f, |
vl_sift_pix const * | grad, | ||
vl_sift_pix * | descr, | ||
int | width, | ||
int | height, | ||
double | x, | ||
double | y, | ||
double | sigma, | ||
double | angle0 | ||
) |
- Parameters
-
f SIFT filter. grad image gradients. descr SIFT descriptor (output). width image width. height image height. x keypoint x coordinate. y keypoint y coordinate. sigma keypoint scale. angle0 keypoint orientation.
The function runs the SIFT descriptor on raw data. Here image is a 2 x width x height array (by convention, the memory layout is a s such the first index is the fastest varying one). The first width x height layer of the array contains the gradient magnitude and the second the gradient angle (in radians, between 0 and \( 2\pi \)). x, y and sigma give the keypoint center and scale respectively.
In order to be equivalent to a standard SIFT descriptor the image gradient must be computed at a smoothing level equal to the scale of the keypoint. In practice, the actual SIFT algorithm makes the following additional approximation, which influence the result:
- Scale is discretized in
S
levels. - The image is downsampled once for each octave (if you do this, the parameters x, y and sigma must be scaled too).
◆ vl_sift_delete()
void vl_sift_delete | ( | VlSiftFilt * | f | ) |
- Parameters
-
f SIFT filter to delete.
The function frees the resources allocated by vl_sift_new().
◆ vl_sift_detect()
void vl_sift_detect | ( | VlSiftFilt * | f | ) |
The function detect keypoints in the current octave filling the internal keypoint buffer. Keypoints can be retrieved by vl_sift_get_keypoints().
- Parameters
-
f SIFT filter.
Index GSS
Index matrix A
◆ vl_sift_get_edge_thresh()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- threshold.
◆ vl_sift_get_keypoints()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- pointer to the keypoints list.
◆ vl_sift_get_magnif()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- magnification factor.
◆ vl_sift_get_nkeypoints()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- number of keypoints.
◆ vl_sift_get_nlevels()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- number of leves per octave.
◆ vl_sift_get_noctaves()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- number of octaves.
◆ vl_sift_get_norm_thresh()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- threshold.
◆ vl_sift_get_octave()
|
inline |
- Parameters
-
f SIFT filter. s level index.
The level index s ranges in the interval s_min = -1
and s_max = S + 2
, where S
is the number of levels per octave.
- Returns
- pointer to the octave data for level s.
◆ vl_sift_get_octave_first()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- index of the first octave.
◆ vl_sift_get_octave_height()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- current octave height.
◆ vl_sift_get_octave_index()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- index of the current octave.
◆ vl_sift_get_octave_width()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- current octave width.
◆ vl_sift_get_peak_thresh()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- threshold ;
◆ vl_sift_get_window_size()
|
inline |
- Parameters
-
f SIFT filter.
- Returns
- standard deviation of the Gaussian window (in spatial bin units).
◆ vl_sift_keypoint_init()
void vl_sift_keypoint_init | ( | VlSiftFilt const * | f, |
VlSiftKeypoint * | k, | ||
double | x, | ||
double | y, | ||
double | sigma | ||
) |
- Parameters
-
f SIFT filter. k SIFT keypoint (output). x x coordinate of the keypoint center. y y coordinate of the keypoint center. sigma keypoint scale.
The function initializes a keypoint structure k from the location x and y and the scale sigma of the keypoint. The keypoint structure maps the keypoint to an octave and scale level of the discretized Gaussian scale space, which is required for instance to compute the keypoint SIFT descriptor.
- Algorithm
The formula linking the keypoint scale sigma to the octave and scale indexes is
\[ \sigma(o,s) = \sigma_0 2^{o+s/S} \]
In addition to the scale index s (which can be fractional due to scale interpolation) a keypoint has an integer scale index is too (which is the index of the scale level where it was detected in the DoG scale space). We have the constraints (Detector see also the "SIFT detector"):
- o is integer in the range \( [o_\mathrm{min}, o_{\mathrm{min}}+O-1] \).
- is is integer in the range \( [s_\mathrm{min}+1, s_\mathrm{max}-2] \). This depends on how the scale is determined during detection, and must be so here because gradients are computed only for this range of scale levels and are required for the calculation of the SIFT descriptor.
- \( |s - is| < 0.5 \) for detected keypoints in most cases due to the interpolation technique used during detection. However this is not necessary.
Thus octave o represents scales \( \{ \sigma(o, s) : s \in [s_\mathrm{min}+1-.5, s_\mathrm{max}-2+.5] \} \). Note that some scales may be represented more than once. For each scale, we select the largest possible octave that contains it, i.e.
\[ o(\sigma) = \max \{ o \in \mathbb{Z} : \sigma_0 2^{\frac{s_\mathrm{min}+1-.5}{S}} \leq \sigma \} = \mathrm{floor}\,\left[ \log_2(\sigma / \sigma_0) - \frac{s_\mathrm{min}+1-.5}{S}\right] \]
and then
\[ s(\sigma) = S \left[\log_2(\sigma / \sigma_0) - o(\sigma)\right], \quad is(\sigma) = \mathrm{round}\,(s(\sigma)) \]
In practice, both \( o(\sigma) \) and \( is(\sigma) \) are clamped to their feasible range as determined by the SIFT filter parameters.
◆ vl_sift_new()
VlSiftFilt* vl_sift_new | ( | int | width, |
int | height, | ||
int | noctaves, | ||
int | nlevels, | ||
int | o_min | ||
) |
- Parameters
-
width image width. height image height. noctaves number of octaves. nlevels number of levels per octave. o_min first octave index.
The function allocates and returns a new SIFT filter for the specified image and scale space geometry.
Setting O to a negative value sets the number of octaves to the maximum possible value depending on the size of the image.
- Returns
- the new SIFT filter.
- See also
- vl_sift_delete().
◆ vl_sift_process_first_octave()
int vl_sift_process_first_octave | ( | VlSiftFilt * | f, |
vl_sift_pix const * | im | ||
) |
- Parameters
-
f SIFT filter. im image data.
The function starts processing a new image by computing its Gaussian scale space at the lower octave. It also empties the internal keypoint buffer.
- Returns
- error code. The function returns VL_ERR_EOF if there are no more octaves to process.
- See also
- vl_sift_process_next_octave().
◆ vl_sift_process_next_octave()
int vl_sift_process_next_octave | ( | VlSiftFilt * | f | ) |
- Parameters
-
f SIFT filter.
The function computes the next octave of the Gaussian scale space. Notice that this clears the record of any feature detected in the previous octave.
- Returns
- error code. The function returns the error VL_ERR_EOF when there are no more octaves to process.
- See also
- vl_sift_process_first_octave().
◆ vl_sift_set_edge_thresh()
|
inline |
- Parameters
-
f SIFT filter. t threshold.
◆ vl_sift_set_magnif()
|
inline |
- Parameters
-
f SIFT filter. m magnification factor.
◆ vl_sift_set_norm_thresh()
|
inline |
- Parameters
-
f SIFT filter. t threshold.
◆ vl_sift_set_peak_thresh()
|
inline |
- Parameters
-
f SIFT filter. t threshold.
◆ vl_sift_set_window_size()
|
inline |
- Parameters
-
f SIFT filter. x Gaussian window size (in units of spatial bin).
This is the parameter \( \hat \sigma_{\text{win}} \) of the standard SIFT descriptor Standard SIFT descriptor.