Opencv fill contourOn 25.10.2020 by Arashisida
The boundaries of the shapes can be rendered with antialiasing implemented only for 8-bit images for now. For color images, the channel ordering is normally Blue, Green, Red. This is what imshowimreadand imwrite expect. So, if you form a color using the Scalar constructor, it should look like:.
The drawing functions process each channel independently and do not depend on the channel order or even on the used color space. If a drawn figure is partially or completely outside the image, the drawing functions clip it. Also, many drawing functions can handle pixel coordinates specified with sub-pixel accuracy. This means that the coordinates can be passed as fixed-point numbers encoded as integers.
The number of fractional bits is specified by the shift parameter and the real point coordinates are calculated as.
This feature is especially effective when rendering antialiased shapes. The functions do not support alpha-transparency when the target image is 4-channel. In this case, the color is simply copied to the repainted pixels. Thus, if you want to paint semi-transparent shapes, you can paint them in a separate buffer and then blend it with the main image.
The function circle draws a simple or filled circle with a given center and radius. The functions clipLine calculate a part of the line segment that is entirely within the specified rectangle. They return false if the line segment is completely outside the rectangle.
Otherwise, they return true. The functions ellipse with less parameters draw an ellipse outline, a filled ellipse, an elliptic arc, or a filled ellipse sector. A piecewise-linear curve is used to approximate the elliptic arc boundary. If you need more control of the ellipse rendering, you can retrieve the curve using ellipse2Poly and then render it with polylines or fill it with fillPoly.
The figure below explains the meaning of the parameters. The function ellipse2Poly computes the vertices of a polyline that approximates the specified elliptic arc. It is used by ellipse. The function fillConvexPoly draws a filled convex polygon. This function is much faster than the function fillPoly.Contours can be explained simply as a curve joining all the continuous points along the boundaryhaving same color or intensity.
The contours are a useful tool for shape analysis and object detection and recognition. See, there are three arguments in cv. And it outputs a modified image, the contours and hierarchy.
Each individual contour is a Numpy array of x,y coordinates of boundary points of the object. To draw the contours, cv. It can also be used to draw any shape provided you have its boundary points.
Its first argument is source image, second argument is the contours which should be passed as a Python list, third argument is index of contours useful when drawing individual contour. To draw all contours, pass -1 and remaining arguments are color, thickness etc. This is the third argument in cv. What does it denote actually? Above, we told that contours are the boundaries of a shape with same intensity.
It stores the x,y coordinates of the boundary of a shape. But does it store all the coordinates? That is specified by this contour approximation method. If you pass cv. But actually do we need all the points? For eg, you found the contour of a straight line. Do you need all the points on the line to represent that line?
No, we need just two end points of that line. This is what cv. It removes all redundant points and compresses the contour, thereby saving memory. Below image of a rectangle demonstrate this technique. Just draw a circle on all the coordinates in the contour array drawn in blue color.
First image shows points I got with cv. See, how much memory it saves!!! Goal Understand what contours are. Learn to find contours, draw contours etc You will see these functions : cv. For better accuracy, use binary images. So before finding contours, apply threshold or canny edge detection. Since OpenCV 3. In OpenCV, finding contours is like finding white object from black background. So remember, object to be found should be white and background should be black.
Let's see how to find contours of a binary image: import numpy as np.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.
The function cv2. See below:. From this moments, you can extract useful data like area, centroid etc. Centroid is given by the relations, and. This can be done as follows:.
Contour area is given by the function cv2. It is also called arc length. It can be found out using cv2. Second argument specify whether shape is a closed contour if passed Trueor just a curve.
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. Now you can use this function to approximate the shape.
In this, second argument is called epsilonwhich is maximum distance from contour to approximated contour. It is an accuracy parameter. A wise selection of epsilon is needed to get the correct output.
Third argument specifies whether curve is closed or not.
Convex Hull will look similar to contour approximation, but it is not Both may provide same results in some cases. Here, cv2.
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.
To understand it, we will take the rectangle image above.
Find and Draw Contours using OpenCV | Python
First I found its contour as cnt. These are the indices of corresponding points in contours. There is a function to check if a curve is convex or not, cv2.
It just return whether True or False. Not a big deal. It is found by the function cv2. Here, bounding rectangle is drawn with minimum area, so it considers the rotation also. The function used is cv2. It returns a Box2D structure which contains following detals - top-left corner x,ywidth, heightangle of rotation. But to draw this rectangle, we need 4 corners of the rectangle.In this tutorial we will learn how to fill holes in a binary image.
Filling holes in an image using OpenCV ( Python / C++ )
Consider the image on the left in Figure 1. In this tutorial the circular region that contains the coin will also be referred to as the foreground. Notice that the boundary of the coin is dark and distinct from its white background.
So, we use simple image thresholding to separate the boundary from the background. In other words, we say pixels with intensities above a certain value threshold are the background and the rest are the foreground. The center image shows this thresholded image black represents background, and white represents foreground. Unfortunately, even though the boundary has been nicely extracted it is solid whitethe interior of the coin has intensities similar to the background.
Therefore, the thresholding operation cannot distinguish it from the background. How do we fill all pixels inside the circular boundary with white? There is no imfill function in OpenCV, but we can surely write one! The idea is rather simple.
We know the pixel 0,0 is connected to the background. So we can extract the background, by simply doing a floodfill operation from pixel 0, 0. Pixels that are not affected by the floodfill operation are necessarily inside the boundary. The flood-filled image when inverted and combined with the thresholded image gives the foreground mask!
There are other ways to solve the same problem.
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One way is to use morphological close operation. However, for morphological operations to work you will need to know the maximum size of the hole. Another way is to use findContours to find the contours and then fill it in using drawContours. I prefer the simplicity and speed of the technique described in this post. If you liked this article, please subscribe to our newsletter to download all code and images used in this post.
You will also receive a free Computer Vision Resource guide. Subscribe Now. Skip to primary navigation Skip to main content Skip to primary sidebar Figure 1. Left : Image of a nickel. Center : Thresholded and inverted. Right : Holes filled. Figure 2. Read in the image. Threshold the input image to obtain a binary image.Can you please explain how to access the edges of a polygon. I mean x,y values of edges. Sorry, Are you asking about sides of polygon or its vertices? Also read the part-2, part-3, part-4 of this series on contours for more info.
It may be because, when you approximate the contour, it contains only 3 points. Just check it out. You will have to adjust the parameter in cv2. Check reducing its value. I can't tell more without the image you are working with. When I try to draw the contours using cv2. Any idea? Cheers, Joe. Me too get the same error. Problem is with version 2. I have filed a bug report to OpenCV. Meanwhile, you have to work with version 2. Congrats for this amazing blog! I've got a little problem with 'drawContours' function.
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Filling holes in an image using OpenCV ( Python / C++ )
What I need now is effective method to fill all polylines. I haven't found such method in opencv, but maybe it exists. Alternatively, maybe I could implement algorithm to do the job but fast one- I have HD ready pictures. Please share your thoughts. I think what you are looking for is cv2. This is a simple snippet, I generate a contour of four points representing vertices of a square, then I fill the polygon with a white color. While using cv2.
Learn more. Asked 6 years, 6 months ago. Active 1 year, 1 month ago. Viewed 46k times. I have binary image with polylines created with: cv2. Active Oldest Votes. Barracuda 2, 1 1 gold badge 17 17 silver badges 37 37 bronze badges. Ash Ketchum Ash Ketchum 7 7 silver badges 5 5 bronze badges.
Does this fill the contour and it's border or does it leave the border out? Mahm00d Mahm00d 3, 7 7 gold badges 33 33 silver badges 68 68 bronze badges.I am currently using drawContours to return a Mat with a hand detected through back projection. However the result is not perfect, and the regions between fingers as shown fill, which is erroneous. What might be a fix for this?
I am only using external contours as I don't need the information contained inside the palm. To tell what's going on, you should include the same image as just outlines. Otherwise we can't see what's wrong. Asked: How to draw irregular outline of MSER region? Ghosting like problem with image operations. About contours of the blob,area of blob's hole and contours hierarchy. The contours in drawcontours is closed or not? Split contours into many small rectangles.
First time here? Check out the FAQ! Hi there! Please sign in help. Contour filling with drawContours fills area inside contours. Hi everyone, I am currently using drawContours to return a Mat with a hand detected through back projection.
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