How can I grow bright pixel in grey region?
Input:
image
Output: image
My answer is somewhat less helpful than my usual efforts, but it is hard to get up enthusiasm for questions with so little effort...
You can solve your issue by using OpenCV findContours() - documentation here. You will need to be sure to use the retrieval mode CV_RETR_TREE.
You then need to write a loop, iterating through all the contours found. In the loop, you need to look for a contour that:
a) has a colour of white and,
b) which has a parent with colour grey.
There is a decent explanation of how the hierarchy works here.
Mat im = imread("ask.png", 0);
Mat mat;
mat = im==255;
findContours( mat, contours, hierarchy, RETR_TREE, CHAIN_APPROX_SIMPLE);
for( size_t i = 0; i< contours.size(); i++ )
{
floodFill(mat, contours[i].at(0), 255, 0, Scalar(128), Scalar(255), FLOODFILL_FIXED_RANGE);
}
mat = mat==255; // output image
Related
I have a camera under a glass with IR light to detect objects. I can find the contours and draw them using the following code (I just found some examples online and modified it to my need so I am not a master at all!).
using namespace cv;
cvtColor(mat, mat, COLOR_BGR2GRAY);
blur(mat, mat, Size(3,3));
erode(mat, mat, NULL, Point(-1,-1), 2);
dilate(mat, mat, NULL, Point(-1,-1), 2);
Canny(mat, mat, 100, 200);
auto contours = std::vector<std::vector<Point>>();
auto hierarchy = std::vector<Vec4i>();
findContours(mat, contours, hierarchy, CV_RETR_TREE,
CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
Mat drawing = Mat::zeros(mat.size(), CV_8UC3);
for( int i = 0; i< contours.size(); i++ ) {
Scalar color = Scalar(rng.uniform(0, 255), rng.uniform(0,255),
rng.uniform(0,255));
drawContours(drawing, contours, i, color, 2, 8, hierarchy, 0, Point());
}
putText(mat,
(QString("Blobs: %1").arg(contours.size())).toStdString(),
Point(25,175), cv::FONT_HERSHEY_PLAIN, 10, CV_RGB(0, 0, 255), 2);
This code results in a nice finding of the contours that I am quite happy with. Except the fact that my IR light somehow makes artifacts at the corners and bottom of the image.
You can see that I have used gimp to highlight the areas that I want to ignore while searching for contours. Under the gray shade you see the white pixels that my original code detects as contours. These areas are problematic and I want to exclude them from the either contour search or contour drawing (whichever is easier!)
I was thinking of cropping the image to get the ROI but the cropping is a rectangle while I (for example) could have things to be detected i.e. exactly at leftmost area.
I think there should be some data in the contour that tells me where are the pixels but I could not figure it out yet...
The easiest way would be to simply crop the image. Areas of the image are known as ROIs in OpenCV, which stands for Region of Interest.
So, you could simply say
cv::Mat image_roi = image(cv::Rect(x, y, w, h));
This basically makes a rectangular crop, with the top left corner at x,y, width w and height h.
Now, you might not want to reduce the size of the image. The next easiest way is to remove the artifacts is to set the borders to 0. Using ROIs, of course:
image(cv::Rect(x, y, w, h)).setTo(cv::Scalar(0, 0, 0));
This sets a rectangular region to black. You then have to define the 4 rectangular regions on the borders of your image that you want dark.
Note that all of the above is based on manual tuning and some experimentation, and it would work provided that your system is static.
I have image with curved line like this :
I couldn't find a technique to straighten curved line using OpenCV. It is similar to this post Straightening a curved contour, but my question is specific to coding using opencv (in C++ is better).
So far, I'm only able to find the contour of the curved line.
int main()
{
Mat src; Mat src_gray;
src = imread("D:/2.jpg");
cvtColor(src, src_gray, COLOR_BGR2GRAY);
cv::blur(src_gray, src_gray, Size(1, 15));
Canny(src_gray, src_gray, 100, 200, 3);
/// Find contours
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
RNG rng(12345);
findContours(src_gray, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0));
/// Draw contours
Mat drawing = Mat::zeros(src_gray.size(), CV_8UC3);
for (int i = 0; i < contours.size(); i++)
{
drawContours(drawing, contours, i, (255), 1, 8, hierarchy, 0, Point());
}
imshow("Result window", drawing);
imwrite("D:/C_Backup_Folder/Ivan_codes/VideoStitcher/result/2_res.jpg", drawing);
cv::waitKey();
return 0;
}
But I have no idea how to determine which line is curved and not, and how to straighten it. Is it possible? Any help would be appreciated.
Here is my suggestion:
Before everything, resize your image into a much bigger image (for example 5 times bigger). Then do what you did before, and get the contours. Find the right-most pixel of each contour, and then survey all pixel of that contour and count the horizontal distance of each pixels to the right-most pixel and make a shift for that row (entire row). This method makes a right shift to some rows and left shift to the others.
If you have multiple contours, calculate this shift value for every one of them in every single row and compute their "mean" value, and do the shift according to that mean value for each row.
At the end resize back your image. This is the simplest and fastest thing I could think of.
I am trying to detect a rectangle using find contours, but I don't get any contours from the following image.
I cant detect any contours in the image. Is find contours is bad with the following image, or should I use hough transform.
UPDATE: I have updated the source code to use approximated polygon.
but I still I get the outlier bounding rect, I cant find the smallest rectangle that is in the screenshot.
I have another case which the current solution it doesnt work even when adding erosion or dilation.
image 2
and here is the code
using namespace cm;
using namespace cv;
using namespace std;
cv::Mat input = cv::imread("heightmap.png");
RNG rng(12345);
// convert to grayscale (you could load as grayscale instead)
cv::Mat gray;
cv::cvtColor(input,gray, CV_BGR2GRAY);
// compute mask (you could use a simple threshold if the image is always as good as the one you provided)
cv::Mat mask;
cv::threshold(gray, mask, 0, 255,CV_THRESH_OTSU);
cv::namedWindow("threshold");
cv::imshow("threshold",mask);
// find contours (if always so easy to segment as your image, you could just add the black/rect pixels to a vector)
std::vector<std::vector<cv::Point> > contours;
std::vector<cv::Vec4i> hierarchy;
cv::findContours(mask,contours, CV_RETR_EXTERNAL, CV_CHAIN_APPROX_SIMPLE);
cv::Mat drawing = cv::Mat::zeros( mask.size(), CV_8UC3 );
vector<vector<cv::Point> > contours_poly( contours.size() );
vector<vector<cv::Point> > ( contours.size() );
vector<cv::Rect> boundRect( contours.size() );
for( int i = 0; i < contours.size(); i++ )
{
approxPolyDP( cv::Mat(contours[i]), contours_poly[i], 3, true );
boundRect[i] = boundingRect( cv::Mat(contours_poly[i]) );
}
for( int i = 0; i< contours.size(); i++ )
{
cv::Scalar color = cv::Scalar( rng.uniform(0, 255), rng.uniform(0,255), rng.uniform(0,255) );
rectangle( drawing, boundRect[i].tl(), boundRect[i].br(), color, 2, 8, 0 );
}
// display
cv::imshow("input", input);
cv::imshow("drawing", drawing);
cv::waitKey(0);
The code you are using looks like its from this question.
It uses BinaryInv threshold because its detecting a black shape on white background.
Your example is the opposite so you should tweak your code to use Binary threshold type instead (or negate the image).
Without this fix, FindContours will detect the perimeter of the image which will be the biggest contour.
So I don't think the code is failing to detect contours, just not the "biggest contour" you expect.
Even with that fixed, the code you posted won't fit a rectangle to the rectangle in your example image, as the most obvious rectangular feature doesn't have a clean border. The approxPolyDP suggestion in the linked question might help but you'll have to improve the source image.
See this question for a comparison of this and Hough methods for finding rectangles.
Edit
You should be able to separate the rectangle in your example image from the other blob by calling Erode (3x3) twice.
You'll have to replace selecting the biggest contour with selecting the squarest.
I'm fairly new to OpenCV, and very excited to learn more. I've been toying with the idea of outlining edges, shapes.
I've come across this code (running on an iOS device), which uses Canny. I'd like to be able to render this in color, and circle each shape. Can someone point me in the right direction?
Thanks!
IplImage *grayImage = cvCreateImage(cvGetSize(iplImage), IPL_DEPTH_8U, 1);
cvCvtColor(iplImage, grayImage, CV_BGRA2GRAY);
cvReleaseImage(&iplImage);
IplImage* img_blur = cvCreateImage( cvGetSize( grayImage ), grayImage->depth, 1);
cvSmooth(grayImage, img_blur, CV_BLUR, 3, 0, 0, 0);
cvReleaseImage(&grayImage);
IplImage* img_canny = cvCreateImage( cvGetSize( img_blur ), img_blur->depth, 1);
cvCanny( img_blur, img_canny, 10, 100, 3 );
cvReleaseImage(&img_blur);
cvNot(img_canny, img_canny);
And example might be these burger patties. OpenCV would detect the patty, and outline it.
Original Image:
Color information is often handled by conversion to HSV color space which handles "color" directly instead of dividing color into R/G/B components which makes it easier to handle same colors with different brightness etc.
if you convert your image to HSV you'll get this:
cv::Mat hsv;
cv::cvtColor(input,hsv,CV_BGR2HSV);
std::vector<cv::Mat> channels;
cv::split(hsv, channels);
cv::Mat H = channels[0];
cv::Mat S = channels[1];
cv::Mat V = channels[2];
Hue channel:
Saturation channel:
Value channel:
typically, the hue channel is the first one to look at if you are interested in segmenting "color" (e.g. all red objects). One problem is, that hue is a circular/angular value which means that the highest values are very similar to the lowest values, which results in the bright artifacts at the border of the patties. To overcome this for a particular value, you can shift the whole hue space. If shifted by 50° you'll get something like this instead:
cv::Mat shiftedH = H.clone();
int shift = 25; // in openCV hue values go from 0 to 180 (so have to be doubled to get to 0 .. 360) because of byte range from 0 to 255
for(int j=0; j<shiftedH.rows; ++j)
for(int i=0; i<shiftedH.cols; ++i)
{
shiftedH.at<unsigned char>(j,i) = (shiftedH.at<unsigned char>(j,i) + shift)%180;
}
now you can use a simple canny edge detection to find edges in the hue channel:
cv::Mat cannyH;
cv::Canny(shiftedH, cannyH, 100, 50);
You can see that the regions are a little bigger than the real patties, that might be because of the tiny reflections on the ground around the patties, but I'm not sure about that. Maybe it's just because of jpeg compression artifacts ;)
If you instead use the saturation channel to extract edges, you'll end up with something like this:
cv::Mat cannyS;
cv::Canny(S, cannyS, 200, 100);
where the contours aren't completely closed. Maybe you can combine hue and saturation within preprocessing to extract edges in the hue channel but only where saturation is high enough.
At this stage you have edges. Regard that edges aren't contours yet. If you directly extract contours from edges they might not be closed/separated etc:
// extract contours of the canny image:
std::vector<std::vector<cv::Point> > contoursH;
std::vector<cv::Vec4i> hierarchyH;
cv::findContours(cannyH,contoursH, hierarchyH, CV_RETR_TREE , CV_CHAIN_APPROX_SIMPLE);
// draw the contours to a copy of the input image:
cv::Mat outputH = input.clone();
for( int i = 0; i< contoursH.size(); i++ )
{
cv::drawContours( outputH, contoursH, i, cv::Scalar(0,0,255), 2, 8, hierarchyH, 0);
}
you can remove those small contours by checking cv::contourArea(contoursH[i]) > someThreshold before drawing. But you see the two patties on the left to be connected? Here comes the hardest part... use some heuristics to "improve" your result.
cv::dilate(cannyH, cannyH, cv::Mat());
cv::dilate(cannyH, cannyH, cv::Mat());
cv::dilate(cannyH, cannyH, cv::Mat());
Dilation before contour extraction will "close" the gaps between different objects but increase the object size too.
if you extract contours from that it will look like this:
If you instead choose only the "inner" contours it is exactly what you like:
cv::Mat outputH = input.clone();
for( int i = 0; i< contoursH.size(); i++ )
{
if(cv::contourArea(contoursH[i]) < 20) continue; // ignore contours that are too small to be a patty
if(hierarchyH[i][3] < 0) continue; // ignore "outer" contours
cv::drawContours( outputH, contoursH, i, cv::Scalar(0,0,255), 2, 8, hierarchyH, 0);
}
mind that the dilation and inner contour stuff is a little fuzzy, so it might not work for different images and if the initial edges are placed better around the object border it might 1. not be necessary to do the dilate and inner contour thing and 2. if it is still necessary, the dilate will make the object smaller in this scenario (which luckily is great for the given sample image.).
EDIT: Some important information about HSV: The hue channel will give every pixel a color of the spectrum, even if the saturation is very low ( = gray/white) or if the color is very low (value) so often it is desired to threshold the saturation and value channels to find some specific color! This might be much easier and much more stavle to handle than the dilation I've used in my code.
I am working in C++ and opencv
I am detecting the big contour in an image because I have a black area in it.
In this case, the area is only horizontally, but it can be in any place.
Mat resultGray;
cvtColor(result,resultGray, COLOR_BGR2GRAY);
medianBlur(resultGray,resultGray,3);
Mat resultTh;
Mat canny_output;
vector<vector<Point> > contours;
vector<Vec4i> hierarchy;
Canny( resultGray, canny_output, 100, 100*2, 3 );
findContours( canny_output, contours, hierarchy, CV_RETR_TREE, CV_CHAIN_APPROX_SIMPLE, Point(0, 0) );
Vector<Point> best= contours[0];
int max_area = -1;
for( int i = 0; i < contours.size(); i++ ) {
Scalar color = Scalar( 0, 0, 0 );
if(contourArea(contours[i])> max_area)
{
max_area=contourArea(contours[i]);
best=contours[i];
}
}
Mat approxCurve;
approxPolyDP(Mat(best),approxCurve,0.01*arcLength(Mat(best),true),true);
Wiht this, i have the big contour and it approximation (in approxCurve). Now, I want to obtain the corners of this approximation and get the image inside this contour, but I dont know how can I do it.
I am using this How to remove black part from the image?
But the last part I dont understad very well.
Anyone knows how can I obtain the corners? It is another way more simple that this?
Thanks for your time,
One much simpler way you could do that is to check the image pixels and find the minimum/maximum coordinates of non-black pixels.
Something like this:
int maxx,maxy,minx,miny;
maxx=maxy=-std::numeric_limits<int>::max();
minx=miny=std::numeric_limits<int>::min();
for(int y=0; y<img.rows; ++y)
{
for(int x=0; x<img.cols; ++x)
{
const cv::Vec3b &px = img.at<cv::Vec3b>(y,x);
if(px(0)==0 && px(1)==0 && px(2)==0)
continue;
if(x<minx) minx=x;
if(x>maxx) maxx=x;
if(y<miny) miny=y;
if(y>maxy) maxy=y;
}
}
cv::Mat subimg;
img(cv::Rect(cv::Point(minx,miny),cv::Point(maxx,maxy))).copyTo(subimg);
In my opinion, this approach is more reliable since you don't have to detect any contour, which could lead to false detections depending on the input image.
In a very efficient way, you can sample the original image until you find a pixel on, and from there move along a row and along a column to find the first (0,0,0) pixel. It will work, unless in the good part of the image you can have (0,0,0) pixels. If this is the case (e.g.: dead pixel), you can add a double check checking the neighbourhood of this (0,0,0) pixel (it should contain other (0,0,0) pixels.