Method for producing a histogram of oriented gradients
View Patent ↗A method for producing a histogram of oriented gradients (HOG) for at least a portion of an image comprises dividing the image portion into cells, each cell comprising a plurality of image pixels. Then, for each image pixel of a cell, obtaining a horizontal gradient component, gx, and a vertical gradient component, gy, based on differences in pixel values along at least a row of the image and a column of the image respectively including the pixel; and allocating a gradient to one of a plurality of sectors, where n is a sector index, each sector extending through a range of orientation angles and at least some of the sectors being divided from adjacent sectors according to the inequalities: b*16<a*(n+1) where 0≤n≤5, b=gy and a=gx; and b*16<a*(13−n) where 7≤n≤12, a=gx+gy and b=gx−gy. At least one sector is associated with a bin; and each instance of a pixel gradient of a cell associated with a bin is accumulated to provide a HOG for the cell.
1. A method for producing a histogram of oriented gradients (HOG) for a cell of at least a portion of an image, the method comprising:
for individual image pixels of the cell,
obtaining a horizontal gradient component, gx, and a vertical gradient component, gy, of a pixel gradient; and
allocating a gradient weight W 1 to a sector of a pair of sectors bounding the pixel gradient, wherein each of the pair of sectors corresponds to a respective angle within a full circle; and
allocating the gradient weight for a sector to a bin associated with the sector according to an angular relationship between the sector and the associated bin; and
accumulating a weighted pixel gradient for the individual image pixels of the cell associated with a bin to provide a HOG for the cell.
2. A method according to claim 1 wherein the allocating a gradient weight W 1 to a sector comprises calculating the gradient weight according to the formula:
W
1
=
a
1
a
1
+
a
2
=
1
-
W
0
where a 1 and a 2 are the intra-sector angles of the pixel gradient, and where W 0 comprises the weight for the other sector of the pair.
3. A method according to claim 2 wherein the angle a 1 is approximated with the vertical gradient component gy, and the angle a 1 +a 2 is approximated with a maximum vertical gradient component for the sector.
4. A method according to claim 3 further comprising employing a lookup table Kw to approximate the angle a 1 +a 2 as a function of the horizontal gradient component gx and calculating the weights W 1 , W 0 according to the formula:
W
1
=
gy
(
x
,
y
)
*
Kw
c
=
1
-
W
0
where c is a constant and a power of 2.
5. A method according to claim 1 wherein sectors extend along lines including gx=2 n *gy, where n is any integer value with a magnitude greater than or equal to 1.
6. A method according to claim 1 , wherein sectors each having an index n extend along lines: b*16=a*(n+1) where 0≤n≤5, b=gy and a=gx; and b*16=a*(13−n) where 7≤n≤12, a=gx+gy and b=gx−gy.
7. An image processing system including digital logic for implementing the method of claim 1 .
8. A method according to claim 1 , further comprising associating the sector with the bin.
9. A method according to claim 1 , wherein allocating the gradient weight W 1 to the sector is according to a relative angle of the pixel gradient to a sector of the pair of sectors bounding the pixel gradient.
10. A method according to claim 1 wherein the allocating a gradient weight W 1 to a sector comprises calculating the gradient weight based at least in part on intra-sector angles of the pixel gradient.
11. A method according to claim 10 further comprising employing a lookup table to approximate the intra-sector angles as a function of the horizontal gradient component gx.
12. An image processing system, comprising:
memory configured to store a cell of at least a portion of an image;
digital logic configured to perform operations including:
for individual image pixels of the cell,
obtaining a horizontal gradient component, gx, and a vertical gradient component, gy, of a pixel gradient; and
allocating a gradient weight W 1 to a sector of a pair of sectors bounding the pixel gradient, wherein each of the pair of sectors corresponds to a respective angle within a full circle; and
allocating the gradient weight for a sector to a bin associated with the sector according to an angular relationship between the sector and the associated bin; and
accumulating a weighted pixel gradient for the individual image pixels of the cell associated with a bin to provide a HOG for the cell.
13. An image processing system according to claim 12 wherein the allocating a gradient weight W 1 to a sector comprises calculating the gradient weight according to the formula:
W
1
=
a
1
a
1
+
a
2
=
1
-
W
0
where a 1 and a 2 are the intra-sector angles of the pixel gradient, and where WO comprises the weight for the other sector of the pair.
14. An image processing system according to claim 13 wherein the angle a 1 is approximated with the vertical gradient component gy, and the angle a 1 +a 2 is approximated with a maximum vertical gradient component for the sector.
15. An image processing system according to claim 14 the operations further comprising employing a lookup table Kw to approximate the angle a 1 +a 2 as a function of the horizontal gradient component gx and calculating the weights W 1 , W 0 according to the formula:
W
1
=
gy
(
x
,
y
)
*
Kw
c
=
1
-
W0
where c is a constant and a power of 2.
16. An image processing system according to claim 12 wherein sectors extend along lines including gx=2 n *gy, where n is any integer value with a magnitude greater than or equal to 1.
17. An image processing system according to claim 12 , wherein sectors each having an index n extend along lines: b*16=a*(n+1) where 0≤n≤5, b=gy and a=gx; and b*16=a*(13−n) where 7≤n≤12, a=gx+gy and b=gx−gy.
18. An image processing system according to claim 12 , the operations further comprising associating the sector with the bin.
19. An image processing system according to claim 12 , wherein allocating the gradient weight W 1 to the sector is according to a relative angle of the pixel gradient to a sector of the pair of sectors bounding the pixel gradient.
20. An image processing system according to claim 12 wherein the allocating a gradient weight W 1 to a sector comprises calculating the gradient weight based at least in part on intra-sector angles of the pixel gradient.