OpenMV is a small camera board for machine vision. I use it to direct my robot. The software library is adequate, but lacks some algorithms I need. Out comes this project, in which I implement a few missing pieces myself. I hope it could save time for someone who tries to do the same.
Everything is tested on OpenMV Cam H7
Vector and matrix are mainstay in machine vision. Finding no satisfactory libraries, I have again implemented my own.
Along with this project's rv package, the OpenMV camera's SD card should
contain:
/
├── mtx.py
├── rv
│ ├── __init__.py
│ ├── moments.py
│ ├── planar.py
│ └── quickshiftpp.py
└── vec
├── distance.py
└── __init__.py
In addition, you need some theoretical backgrounds to use vision algorithms effectively. This page does not give you those backgrounds. Study them yourself.
Hu moments is a shape descriptor invariant to translation, scale, and rotation. That means it can recognize the same shape no matter its location, size, and orientation in the picture.
The last element of Hu moments is somewhat of an oddball. It indicates reflection rather than the general shape. For matching, the last element should be dropped.
Use the function vec.distance.euclidean() to see how close two vectors are.
import rv.moments
import vec.distance
import image
a = image.Image('/images/a.pgm')
b = image.Image('/images/b.pgm')
ha = rv.moments.hu(a)
hb = rv.moments.hu(b)
print(vec.distance.euclidean(ha[:-1], hb[:-1]))Remark: Although accepting gray-level images, this implementation treats pixels as either 0 or 1. Pixels having a non-zero brightness are treated as 1. This speeds up calculation.
More: test_moments.py
Map points from one coordinate system to another. For example, a red ball sits at (90, 50) on the image and you know it is on the floor (not floating in air), planar homography can map the image point (90, 50) to a position on the floor, telling you how far the red ball is in front of the robot and how much left or right. In this case, points are essentially mapped from the image coordinate system to the floor coordinate system.
Once you can map points, finding out the size of objects is straight-forward.
It works only when two coordinate systems (i.e. the two planes) are fixed relative to each other. In other words, the camera's height and orientation relative to the floor cannot change.
Once you have the matrix, the rest is easy.
import rv.planar
H = [[ 3.14916496e+01, -9.79038178e+02, 1.03951636e+05],
[ 7.57939015e+02, -3.31912533e+01, -5.86807545e+04],
[ 2.06572544e-01, 2.03579263e+00, 1.00000000e+00]]
p = rv.planar.Planar(H)
image_points = [[83, 109],
[70, 100],
[51, 92]]
print(p.project(image_points))More: test_planar.py
The latest member of the Meanshift family of clustering algorithms, Quickshift++ accepts a bunch of points and group them. I use it to "discover" the colors of disks on the floor, before using colors to pick out the disks. This saves me from hard-coding the colors beforehand, and makes the robot adaptive.
It is not optimized to handle a large number of points. OpenMV's limited memory precludes handling a lot of points anyway. Don't expect to use it to segment an entire image.
import rv.quickshiftpp
points = [
# cluster 1
[1, 1],
[1.0, 1.2],
[0.9, 1.1],
[0.95, 0.99],
# cluster 2
[3.3, 3.0],
# cluster 3
[5.0, 8.2],
[5.5, 7.9],
[4.8, 8.1],
[5.1, 7.7],
]
print(rv.quickshiftpp.cluster(points,
k=2,
beta=0.2))The parameter k determines how density is estimated. It uses distance to the
k-th nearest neighbor to estimate density around each point.
The parameter beta determines how much density is allowed to vary within
cluster cores. Here is not the place to explain what "cluster core" means. Some
theoretical understanding cannot be avoided.
In short, use k and beta to tune the clustering.
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