OpenCV  4.10.0-pre
Open Source Computer Vision
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Contour Features

Prev Tutorial: Contours : Getting Started
Next Tutorial: Contour Properties


  • To find the different features of contours, like area, perimeter, centroid, bounding box etc
  • You will learn plenty of functions related to contours.

1. Moments

Image moments help you to calculate some features like center of mass of the object, area of the object etc. Check out the wikipedia page on Image Moments

We use the function: cv.moments (array, binaryImage = false)

arrayraster image (single-channel, 8-bit or floating-point 2D array) or an array ( 1×N or N×1 ) of 2D points.
binaryImageif it is true, all non-zero image pixels are treated as 1's. The parameter is used for images only.

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From this moments, you can extract useful data like area, centroid etc. Centroid is given by the relations, \(C_x = \frac{M_{10}}{M_{00}}\) and \(C_y = \frac{M_{01}}{M_{00}}\). This can be done as follows:

let cx = M.m10/M.m00
let cy = M.m01/M.m00

2. Contour Area

Contour area is given by the function cv.contourArea() or from moments, M['m00'].

We use the function: cv.contourArea (contour, oriented = false)

contourinput vector of 2D points (contour vertices)
orientedoriented area flag. If it is true, the function returns a signed area value, depending on the contour orientation (clockwise or counter-clockwise). Using this feature you can determine orientation of a contour by taking the sign of an area. By default, the parameter is false, which means that the absolute value is returned.

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3. Contour Perimeter

It is also called arc length. It can be found out using cv.arcLength() function.

We use the function: cv.arcLength (curve, closed)

curveinput vector of 2D points.
closedflag indicating whether the curve is closed or not.

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4. Contour Approximation

It approximates a contour shape to another shape with less number of vertices depending upon the precision we specify. It is an implementation of Douglas-Peucker algorithm. Check the wikipedia page for algorithm and demonstration.

We use the function: cv.approxPolyDP (curve, approxCurve, epsilon, closed)

curveinput vector of 2D points stored in cv.Mat.
approxCurveresult of the approximation. The type should match the type of the input curve.
epsilonparameter specifying the approximation accuracy. This is the maximum distance between the original curve and its approximation.
closedIf true, the approximated curve is closed (its first and last vertices are connected). Otherwise, it is not closed.

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5. Convex Hull

Convex Hull will look similar to contour approximation, but it is not (Both may provide same results in some cases). Here, cv.convexHull() function checks a curve for convexity defects and corrects it. Generally speaking, convex curves are the curves which are always bulged out, or at-least flat. And if it is bulged inside, it is called convexity defects. For example, check the below image of hand. Red line shows the convex hull of hand. The double-sided arrow marks shows the convexity defects, which are the local maximum deviations of hull from contours.


We use the function: cv.convexHull (points, hull, clockwise = false, returnPoints = true)

pointsinput 2D point set.
hulloutput convex hull.
clockwiseorientation flag. If it is true, the output convex hull is oriented clockwise. Otherwise, it is oriented counter-clockwise. The assumed coordinate system has its X axis pointing to the right, and its Y axis pointing upwards.
returnPointsoperation flag. In case of a matrix, when the flag is true, the function returns convex hull points. Otherwise, it returns indices of the convex hull points.

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6. Checking Convexity

There is a function to check if a curve is convex or not, cv.isContourConvex(). It just return whether True or False. Not a big deal.


7. Bounding Rectangle

There are two types of bounding rectangles.

7.a. Straight Bounding Rectangle

It is a straight rectangle, it doesn't consider the rotation of the object. So area of the bounding rectangle won't be minimum.

We use the function: cv.boundingRect (points)

pointsinput 2D point set.

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7.b. Rotated Rectangle

Here, bounding rectangle is drawn with minimum area, so it considers the rotation also.

We use the function: cv.minAreaRect (points)

pointsinput 2D point set.

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8. Minimum Enclosing Circle

Next we find the circumcircle of an object using the function cv.minEnclosingCircle(). It is a circle which completely covers the object with minimum area.

We use the functions: cv.minEnclosingCircle (points)

pointsinput 2D point set. (img, center, radius, color, thickness = 1, lineType = cv.LINE_8, shift = 0)

imgimage where the circle is drawn.
centercenter of the circle.
radiusradius of the circle.
colorcircle color.
thicknessthickness of the circle outline, if positive. Negative thickness means that a filled circle is to be drawn.
lineTypetype of the circle boundary.
shiftnumber of fractional bits in the coordinates of the center and in the radius value.

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9. Fitting an Ellipse

Next one is to fit an ellipse to an object. It returns the rotated rectangle in which the ellipse is inscribed. We use the functions: cv.fitEllipse (points)

pointsinput 2D point set.

cv.ellipse1 (img, box, color, thickness = 1, lineType = cv.LINE_8)

boxalternative ellipse representation via RotatedRect. This means that the function draws an ellipse inscribed in the rotated rectangle.
colorellipse color.
thicknessthickness of the ellipse arc outline, if positive. Otherwise, this indicates that a filled ellipse sector is to be drawn.
lineTypetype of the ellipse boundary.

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10. Fitting a Line

Similarly we can fit a line to a set of points. We can approximate a straight line to it.

We use the functions: cv.fitLine (points, line, distType, param, reps, aeps)

pointsinput 2D point set.
lineoutput line parameters. It should be a Mat of 4 elements[vx, vy, x0, y0], where [vx, vy] is a normalized vector collinear to the line and [x0, y0] is a point on the line.
distTypedistance used by the M-estimator(see cv.DistanceTypes).
paramnumerical parameter ( C ) for some types of distances. If it is 0, an optimal value is chosen.
repssufficient accuracy for the radius (distance between the coordinate origin and the line).
aepssufficient accuracy for the angle. 0.01 would be a good default value for reps and aeps.

cv.line (img, pt1, pt2, color, thickness = 1, lineType = cv.LINE_8, shift = 0)

pt1first point of the line segment.
pt2second point of the line segment.
colorline color.
thicknessline thickness.
lineTypetype of the line,.
shiftnumber of fractional bits in the point coordinates.

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