I am translating some matlab code to c++ using opencv. I want to get the values of a Matrix which satisfies a condition. I created a mask for this and when I apply it to the original Matrix I get the same size of the original matrix but with 0 values which are not there in the mask. But my question is how can I get only the values that are non-zero in the matrix and assign it to a different matrix.
My matlab code is:
for i= 1:size(no,1)
mask= labels==i;
op = orig(mask, :); //op only contains the values from the orig matrix which are in the mask. So orig size and op size is not the same
.....
end
The c++ translation that I have now is:
for (int i=0; i<n.rows; i++)
{
Mat mask;
compare(labels,i,mask,CMP_EQ);
Mat op;
orig.copyTo(op,mask); //Here the orig size and the op size is always same but values which are not in the mask are 0
}
So, how can I create a matrix which only has the values that the mask satisfies???
You might try to make use of cv::SparseMat (http://docs.opencv.org/modules/core/doc/basic_structures.html#sparsemat), which only keeps non-zero values in a hash.
When you assign a regular cv::Mat to a cv::SparseMat, it automatically captures the non-zero values. From that point, you can iterate through the non-zero values and manipulate them as you'd like.
Hope I got question correctly and it helps!
OpenCv does support Matrix Expresions like A > B or A <= Band so on.
This is stated in the Documentation off cv::Mat
If you're simply wanting to store values, the Mat object is probably not the best way, since it has been made for the purpose of containing images.
In that case, use an std::vector object instead of the cv::Mat object, and you can use the .push_back handle whenever you find an element that is non-zero, which will dynamically resize the vector.
If you're trying to create a new image, then you have to be specific about what kind of image you want to see, because if you don't know how many nonzero elements there are, how can you set the width and height? Also you might end up with an odd number of elements.
Related
I am trying to use an opencv function that accepts std::vector<cv::Point3f> among other parameters. In my program, I have an std::vector<cv::Point3f> worldPoints and another std::vector<int> mask, both of larger dimension than what I want to send.
What I want to do is pass to the opencv function only the entries that have a respective non-zero mask, as efficiently as possible.
std::vector<cv::Point3f> worldPointsSubset;
for (int i=0; i<mask.size(); i++) {
if (mask[i] != 0) {
worldPointsSubset.push_back(worldPoints[i]);
}
}
// Then use worldPointsSubset in function
Is there any other way around, possibly involving no copying of data?
EDIT 1: The function I am referring to is solvePnPRansac()
The function that you call requires a vector of Point3f, so if the only thing you have is a masked vector, then you have to copy the data first. There is no way around this if the function doesn't accept a vector and its mask.
To see if this copy is an issue, you must measure the drop in performance first and see if this copy is a bottleneck. If it is a bottleneck, the first thing is to count the number of points you need and reserve that capacity in worldPointsSubset.
There is no way to convert data from std::vector<int> to std::vector<cv::Point3f> without a copy because despite the fact you see the same values the size of data might be different.
But you can change the type of data you are working on (std::vector<int> to std::vector<cv::Point3f>) and work directly with cv::Point3f and when needed pass it to solvePnPRansac().
I am using m.conservativeResize() to do the equivalent in Eigen as the reshape function in MatLab. So let N = 3, and then...
static MatrixXd m(N*N,1);
and then I assign this matrix some values, and it looks like this:
1
1
0
1
0
1
0
1
1
and then try to reshape it...
m.conservativeResize(N,N);
So now the same values should be there, but now in N rows and N columns rather than N*N rows and one column.
However that's not what I get. The first column has the first three values in the column vector - OK so far - but then the remaining values just look like garbage values from uninitialized memory:
1 3.08116e-309 0.420085
1 -2.68156e+154 1.2461e-47
0 -2.68156e+154 0.634626
Any idea what I am doing wrong?
conservativeResize() doesn't ``move" the elements around (in other words, doesn't work like MATLABs reshape, since it performs memory re-allocation even if the initial and final sizes are the same). From the documentation:
Resizes the matrix to rows x cols while leaving old values untouched.
...
Matrices are resized relative to the top-left element. In case values need to be appended to the matrix they will be uninitialized.
These statements seem a bit confusing. What it means is the following: think about the initial matrix as a rectangle, of size A x B. Then think about the resized matrix as another rectangle of size C x D. Then mentally overlap the two rectangles, making sure the top-left corner is common to both. The common elements of the intersection are the ones that are preserved by the conservativeResize. The rest just correspond to uninitialized memory.
In case you want a true reshaping, use resize() instead (making absolutely sure that A x B == C x D, otherwise reallocation takes place and all bets are off).
I have a question concerning access speed of OpenCV matrix.
I currently need two channels of unsigned char to contain my data.
But at one point i need to split my data to process them separately (which probably results in a matrix copy)
for( auto ptr = ROI.begin<cv::Vec2b>(); ptr!=ROI.end<cv::Vec2b>();ptr++){
//insert values
}
cv::split(ROI,channels_vector)
process(channels_vector[0]);
process(channels_vector[1]);
more_stuff(ROI);
My question is the following :
Should I use two different matrix at the beginning to avoid the split or let it like this ?
Or as it may depend on my computation what is the difference of cost between two accesses of a matrix and a matrix copy ?
in OpenCV 2 and later there is method Mat::resize that let's you add any number of rows with the default value to your matrix is there any equivalent method for the column. and if not what is the most efficient way to do this.
Thanks
Use cv::hconcat:
Mat mat;
Mat cols;
cv::hconcat(mat, cols, mat);
Worst case scenario: rotate the image by 90 degrees and use Mat::resize(), making columns become rows.
Since OpenCV, stores elements of matrix rows sequentially one after another there is no direct method to increase column size but I bring up myself two solutions for the above matter,
First using the following method (the order of copying elements is less than other methods), also you could use a similar method if you want to insert some rows or columns not specifically at the end of matrices.
void resizeCol(Mat& m, size_t sz, const Scalar& s)
{
Mat tm(m.rows, m.cols + sz, m.type());
tm.setTo(s);
m.copyTo(tm(Rect(Point(0, 0), m.size())));
m = tm;
}
And the other one if you are insisting not to include even copying data order into your algorithms then it is better to create your matrix with the big number of rows and columns and start the algorithm with the smaller submatrix then increase your matrix size by Mat::adjustROI method.
How to access individual pixels in OpenCV 2.3 using C++?
For my U8C3 image I tried this:
Scalar col = I.at<Scalar>(i, j);
and
p = I.ptr<uchar>(i);
First is throwing an exception, the second one is returning some unrelated data. Also all examples I was able to find are for old IIPimage(?) for C version of OpenCV.
All I need is to get color of pixel at given coordinates.
The type you call cv::Mat::at with needs to match the type of the individual pixels. Since cv::Scalar is basically a cv::Vec<double,4>, this won't work for a U8C3 image (it would work for a F64C4 image, of course).
In your case you need a cv::Vec3b, which is a typedef for cv::Vec<uchar,3>:
Vec3b col = I.at<Vec3b>(i, j);
You can then convert this into a cv::Scalar if you really need to, but the type of the cv::Mat::at instantiation must match the type of your image, since it just casts the image data without any conversions.
Your second code snippet returns a pointer to the ith row of the image. It is no unrelated data, but just a pointer to single uchar values. So in case of a U8C3 image, every consecutive 3 elements in the data returned to p should represent one pixel. Again, to get every pixel as a single element use
Vec3b *p = I.ptr<Vec3b>(i);
which again does nothing more than an appropriate cast of the row pointer before returning it.
EDIT: If you want to do many pixel accesses on the image, you can also use the cv::Mat_ convenience type. This is nothing more than a typed thin wrapper around the image data, so that all accesses to image pixels are appropriately typed:
Mat_<Vec3b> &U = reinterpret_cast<Mat_<Vec3b>&>(I);
You can then freely use U(i, j) and always get a 3-tuple of unsigned chars and therefore pixels, again without any copying, just type casts (and therefore at the same performance as I.at<Vec3b>(i, j)).