Opencv mat access element This is the one you’ll see in most of the older tutorials and educational materials. cv::Mat stores the pointer to the data in a unsigned char* data member. That is, array elements should have one of the 4 days ago · Note Format of the file is determined by its extension. What is not immediately clear, however, whether (i,j) refers to the x,y coordinate in the matrix, or the ith row and the jth column. With 3D Mat variables, your code becomes: int dims[]={640,480,48}; Mat image2(3,dims,CV_32F); Accessing an element (let's say, channel 30 of pixel 10,10): float ch30=image2. col, and others, where m is cv::Mat. hpp> #include <opencv2/opencv. at<Vec1f>( Point(1,1 )) = 5; // I used 1 channel so I defined a new element type as a workaround I adapted the code from this post opencv multi channel element access Apr 15, 2022 · Manipulating and accessing Mat data in EmguCV: how to get array elements and apply scalar? Due to this compatibility, it is possible to make a Mat header for user-allocated data and process it in-place using OpenCV functions. cols * M. I can extract one value at a time, but if I put it into a loop, the program aborts in the console at runtime. How do I access elements of a multi channel matrix? For example: Jul 12, 2013 · Accessing elements of mat in openCV Asked 11 years, 10 months ago Modified 11 years, 10 months ago Viewed 3k times Nov 23, 2012 · OpenCV access element from Mat Asked 13 years, 5 months ago Modified 12 years, 11 months ago Viewed 6k times 1 day ago · OpenCV is a computer vision library whose main focus is to process and manipulate this information. Our test case Jan 8, 2013 · OpenCV is a computer vision library whose main focus is to process and manipulate this information. Therefore, the first thing you need to be familiar with is how OpenCV stores and handles images. For information about . The ImageData interface can represent or set the underlying pixel data of an area of a canvas element. The following is going to write (255. row (0) (0,10) [1] to access the 10th element of row 0 and channel 1. CV_8U <-> uchar CV_32S <-> int CV_32F <-> float CV_64F <-> double TYPE float takes up 4 bytes, and double takes up 8 bytes in the memory. Jan 8, 2013 · Goals Learn how to read an image and how to display it in a web. at (i,j) to access the element at position i,j. Jan 8, 2013 · Matrix elements are stored row by row. at<cv::Vec2f>[y][x][0] = 1. This article describes several methods for accessing Matrix Elements in OpenCV, compares performance on a specified platform, and analyzes the Code complexity and ease of use of each matrix element access method. tif", CV_LOAD_IMAGE_ANYDEPTH); img_array = (Mat **)malloc (hor * sizeof (temp)); for (char i1 = 0;i1<hor;i1++) { img_array [i1] = (Mat *)malloc (ver * sizeof May 19, 2016 · I met this kind of situation several times which turn out to be the problem of Wrong Type of the Mat. Whether you’re filtering an image, performing matrix operations, or extracting features, you’ll inevitably need to access Dec 23, 2010 · The next important thing to learn about the matrix class is element access. For my problem, I need to access a Mat element and its neighbours (i-1,j-1) for a calculation. What is the most efficient way to acces a Mat value and its neighbour values? c++ opencv image-processing matrix asked Apr 24, 2014 at 16:40 user3392074 3113 1 Dec 15, 2019 · I cannot figure out how to properly access elements of a 3D cv::Mat array. cv::Mat_<_Tp> is a class template and so the element type is embedded in the class type. The biggest issue is the Mar 10, 2016 · Is there any reason I should not use a macro for fast element access in C++ API (like CV_MAT_ELEM_PTR_FAST in C API)? Jun 21, 2015 · Is your intention to achieve this the most efficient way with opencv, or do you want to learn to how to efficient pixel access with opencv? Apr 18, 2023 · This is a guide to OpenCV Mat. 0) to 4 bytes not 1: draw. Mat to the web or in reverse. I’d like to have an int array because I want to convert the image into ASCII art, Please see below code? Is the console running out of memory or something? Thanks in advance May 26, 2015 · Accessing elements in a multi-channel OpenCV Mat Asked 10 years, 2 months ago Modified 10 years, 2 months ago Viewed 3k times This means that 2-dimensional matrices are stored row-by-row, 3-dimensional matrices are stored plane-by-plane, and so on. Jun 17, 2011 · I currently want to read in some values into a 3-channel, 480 row by 640 column matrix of 8 bit unsigned integer values. I store it in a cv::Mat matrix. For information about matrix operations and This means that 2-dimensional matrices are stored row-by-row, 3-dimensional matrices are stored plane-by-plane, and so on. OpenCV provides codes of how to create a complex cv::Mat_ here (search the page for the key 113 See the first answer to Accessing a matrix element in the "Mat" object (not the CvMat object) in OpenCV C++ Then just loop over all the elements in cout << M. The idea is that we recalculate each pixel's value in an image according to a mask matrix (also known as kernel). I always end up with corruption with incoherent values. Jul 27, 2017 · I know how to access three channels cv::Mat using Vec3b. Oct 10, 2017 · Hi, I´m gonna find the maximum and the minimum value of an image pixels: // ConsoleApplication1. Alternatively and much more convenient: instead of cv Mar 24, 2025 · OpenCV follows zero-based indexing, meaning the first row and column start at index 0. Dec 30, 2018 · A Mat element is stored in Mat data pointer described in OpenCV document, and the address is mat. h" #include <iostream> #include <stdlib. step [1] + + (id-1) * mat. OpenCV is open source, contains over 2500 algorithms, and is operated by the non-profit Open Source Vision Foundation. 1 with visual studio 2010. A Mat keeps a reference count that tells if data has to be deallocated when a particular instance of Mat is destroyed. hpp> #include <opencv2/highgui. To understand how to access, it is better to learn the data types first. You need to use the 'row ()' and 'col ()'-Functions and copy them to the B-matrix which need to be initialized beforehand. js saves images as cv. i. , which elements are dynamically allocated in the heap. // #include "stdafx. Here we discuss the introduction, working of Mat() function in OpenCV and examples respectively. Here is an example for a single channel grey scale image (type 8UC1) and Nov 9, 2025 · To access individual pixels of GpuMat in kernel, you have to pass the pointer referencing the actual matrix data (which is in device memory) to the kernel and do pointer arithmetic on that pointer. M. Simply suppose you are putting e. Output We represented the output numbers with colors matching the position of their pixels respectively. I write the following code: Mat **img_array; Mat temp = imread ("Lena. g. The data is stored in contiguous blocks of memory, which makes accessing the elements of the array fast and efficient. I want to access and re-assign an element with data pointer like this: cv::Mat test = (cv::Mat_<uchar> (3,3) << 1,2,3,4,5,6,7,8,9); * (test. The cv::Mat::data member points to the first element of the first row, cv::Mat::rows contains the number of matrix rows and cv::Mat::cols - the number of matrix columns. So, the data layout in Mat is fully compatible with CvMat, IplImage, and CvMatND types from OpenCV 1. Pixel in first row and first column will have some value (b00, g00, r00) and pixel in nth row and mth 5 days ago · OpenCV (Open Source Computer Vision Library) is a cornerstone tool for developers and researchers working in computer vision and image processing. Taking your example above and assuming ind-matrix is a nx1-matrix with your row-indices of type int: cv::Mat B(ind. rows, A. Element (i, j) (i - 0-based row index, j - 0-based column index) of a matrix can be retrieved or modified using CV_MAT_ELEM macro: uchar pixval = CV_MAT_ELEM(grayimg, uchar, i, j) CV_MAT_ELEM(cameraMatrix, float, 0, 2) = image. 0 (or: test. 3 rows and 4 columns of your images on the table. cols; j++ Dynamic and Variable Storage The next stop on our journey brings us to the large array types. b1,g1,r1,b2,g2,r2,… Images in OpenCV are always stored in the format of BGR. Jan 8, 2013 · C++ Java Python Mat is a structure that keeps matrix/image characteristics (rows and columns number, data type etc) and a pointer to data. at<double>(0,0); rather than just 0,0 Or better still with the C++ interface: Oct 25, 2018 · OpenCV provides all the necessary functions and overloaded operators that you need to perform all four basic operations of addition, subtraction, multiplication, and division between two matrices or a matrix and a scalar. Basic operations with images Accessing pixel intensity values In order to get pixel intensity value, you have to know the type of an image and the number of channels. Read an image OpenCV. Oct 26, 2025 · Mat Class Relevant source files Purpose and Scope This document provides a comprehensive technical reference for the Mat and Mat<T> classes in OpenCvSharp, which are the primary data structures for representing n-dimensional dense arrays and images. Background Ideally in image processing we want to access pixel by pixel information as fast as possible. I am on Ubuntu 14. Shortly, CV_<bit-depth>{U|S|F}C(<number_of_channels>) is the basic structure of a type. The aim of the Mat object is to represent a matrix. a 2D array of images. data + 2 * test. Deprecated 5 days ago · assignTo () channels () returns element type, similar to CV_MAT_CN (cvmat->type) Feb 22, 2017 · The number of channels can be considered as a third dimension of your matrix; so you can treat it just like any other dimension (width, depth). step [1]) = *test. at<float[3]>(i) or m. Note: If you want to avoid having to specify the type for accessing the elements, you can use cv::Mat_<_Tp> class instead of cv::Mat. How I can access cv::Mat channels now? Aug 26, 2017 · I need to access the real part specific element of a cv::Mat that contains std::complex<double>'s. I am trying to benchmark the performance of different ways to access OpenCV mat element. at<float>(i,j Feb 12, 2013 · If I have understood you correctly you'd like to have the rows specified in your ind-matrix in a new matrix. Extract pixel value from Mat not working (for me) Mat element pixel access at 2k views 1 answer 5 votes 2013-07-31 18:19:48 -0600 Mathieu Barnachon Jan 8, 2013 · While Mat is sufficient in most cases, Mat_ can be more convenient if you use a lot of element access operations and if you know matrix type at the compilation time. Mar 26, 2013 · This is with a typical 8UC1 mat. These classes wrap OpenCV's cv::Mat and provide managed memory handling, type-safe element access, and operator overloading. data + 10; if the element type of Mat is Mar 12, 2012 · vect. Is this a OpenCV is the world's biggest computer vision library. step [id - 1] . Example # To access pixel values in an OpenCV cv::Mat object, you first have to know the type of your matrix. In this How To article I discuss and demonstrate construction of the OpenCV C++ Mat object from array and vector data types. In an RGB image (which I believe OpenCV typically stores as BGR), and assuming your cv::Mat variable is called frame, you could get the blue value at location (x, y) (from the top left) this way: Mar 6, 2013 · I'm confused by the OpenCV Mat element types. May 26, 2015 · Also, to access them in group you can check the same book, chapter 4, Section Accessing Array Elements by Block, or the OpenCV docs for methods of m. At the heart of OpenCV lies `cv::Mat`—a versatile data structure used to store images and multi-dimensional arrays. Or replace it with cv::Scalar(0). I need to process cv::Mat elements and there is not special OpenCV function that performs my task. Feb 18, 2023 · CV::Mat Class: The CV::Mat class can be used for arrays with higher dimensions. Usually I would create a for loop and I would take for each elements of B, the right elements of A. Here are some pairs that you may keep in mind. 20 The low-level way would be to access the matrix data directly. 3 and -O3 flag with the latest OpenCV in the trunk. When the memory is not allocated continuously, a low-level block access like m. In those days to store the image in the memory they used a C structure entitled IplImage. I'm trying to get the value type for an element of my matrix and avoid having to hardcode it in order to maximize portability and compatibility with possible future changes. How to do it? I am using openCV 2. x. at<float> (i, j) I get random numbers (sometimes garbage value). data + i0 * mat. This is from the docs: There is a limited fixed set of primitive data types the library can operate on. Mar 6, 2023 · Hello, I’m new to OpenCV, and I’m trying to extract the int values of a CV_8U type (1 channel) Mat object. By passing a mutable reference to the Mat to this function your closure will get called with the read reference and a write references to the same Mat. data gives a pointer which refers to the original data matrix. Apr 15, 2022 · Manipulating and accessing Mat data in EmguCV: how to get array elements and apply scalar? C++ csharp FupaTroopa April 15, 2022, 7:54pm Apr 25, 2017 · I was working on homography and whenever I try to check the values of H matrix (type CV_64F) using H. dims-1] is minimal and always equal to the element size M. 9. Access to the data member looks like: c I read this article, but I can't find how to access Mat element like this img [y] [x]=100; is it possible? And another question is how can I don't omit type of Mat, but check types on compilation time. row, m. What is the more effective way of elementwise access? cv::Mat iterators, cv::Mat::at () method, maybe pointer arithmetic by hand or something else? Jul 12, 2013 · Accessing elements of mat in openCV Asked 11 years, 10 months ago Modified 11 years, 10 months ago Viewed 3k times Nov 23, 2012 · OpenCV access element from Mat Asked 13 years, 5 months ago Modified 12 years, 11 months ago Viewed 6k times Dec 4, 2009 · How to access elements by row, col in OpenCV 2. The header contains information about the matrix itself (like the size of the matrix, the data type, number of channels etc…) The data contains the pointer to the real Sep 12, 2012 · What is the standard way to access and modify individual elements of a Mat in OpenCV4Android? Also, what is the format of the data for BGR (which is the default, I think) and grayscale? edit: Let' Mar 20, 2013 · The at-statement should work, e. Along with that, it is important to understand Vec Jan 11, 2021 · Okay, trying to subtract element by element two matrixes. io This means that 2-dimensional matrices are stored row-by-row, 3-dimensional matrices are stored plane-by-plane, and so on. Now, I want to store values that can range from -1500 to 1500. I want to access pixel valu Oct 13, 2017 · Hello, I want to add a dynamic array of Mat variebles, i. We can use array to initialize the constructor: int arr[3] = {4,3,2}; Mat L(3, arr, CV_8UC(1), Scalar::all(0)); We specified its dimension of 3, then pass a pointer containing the size for each dimension. at<uchar> (i,j) something like this img [i] [j] Dec 1, 2009 · I'm trying to learn how to use OpenCV's new C++ interface. I want to store image (Width x Height x 1-depth) in Mat and looping access each OpenCV 3, the Mat object In this post we will introduce the most used object of the new OpenCV3 library, using C++. While Mat is sufficient in most cases, Mat_ can be more convenient if you use a lot of element access operations and if you know matrix type at the compilation time. OpenCV has for Dec 2, 2015 · I've seen a lot of OpenCV code which accesses the data member of a cv::Mat directly. 0's new "Mat" class? The documentation is linked below, but I have not been able to make any sense of it. cn To find the max element one can use Mat src; double minVal, maxVal; minMaxLoc(src opencv What is the standard way to access and modify individual elements of a Mat in OpenCV4Android? Also, what is the format of the data for BGR (which is the default, I think) and grayscale? Mar 11, 2018 · 1 Suppose I have a Mat of indices (locations) called B, We can say that this Mat has dimensions of 1 x 100 and We suppose to have another Mat, called A, full of data of the same dimensions of B. In this data matrix, all the pixel values are stored in 1D array. at<> (). http://opencv Mar 19, 2018 · Understanding 3D Mat::at<T> Accessors with OpenCV 3. at<char>(row, column). Is it possible to write sort-of template function in this case?? which is independent of Mat's element type?? Apr 15, 2022 · Manipulating and accessing Mat data in EmguCV: how to get array elements and apply scalar? C++ csharp berak April 15, 2022, 7:42pm Apr 10, 2013 · I have a cv::Mat of type CV_8UC1 (8-bit single channel image) and I would like to access elements using the at<> operator as follows: image. Oct 8, 2013 · Hi, I need to apply a function to each element of a matrix (in a Mat object). 4 6 minute read Some things simply do not come naturally when we attempt to process data in OpenCV. width*0. rows; i++) { Mi = aCvMat. The problem with this is that it brings to the table all the minuses of the C language. Oct 24, 2024 · Using the wrong type would lead to undefined behavior. : test. 04 with gcc 4. hpp> #include <opencv2/imgcodecs. at<Vec3b>(y, x)[i]; So the size in terms of elements of elementary type is M. at<cv::Vec3f>(i) is dangerous and should be avoided as you say. From a mathematical point of view we make a weighted average, with our specified values. So I'm using a CV_32F mat. hpp> int main () { Mat img1; double minVal, maxVal; int Jul 14, 2013 · OpenCV- Cant access element of Mat (of descriptors) Asked 11 years, 7 months ago Modified 11 years, 7 months ago Viewed 688 times Dec 16, 2012 · When the matrix's element type differs, the function cannot work. at<float>(10,10,30); As the matrix lines are continuous, you can also use line Accessing elements of a cv::Mat with at<float> (i, j). It is composed by two parts: the header and the data. Note that Mat::at(int y,int x) and Mat_::operator()(int y,int x) do absolutely the same and run at the same speed, but the latter is certainly shorter: Sep 4, 2012 · Hi, I am a newbie opencv user, and I wanted to ask how to access the elements of a Mat. The first one is usin The main purpose of this class is to convert compilation-time type infor mat ion to an OpenCV-compatible data type identifier, for example: Sep 13, 2025 · Mat and UMat Data Structures Relevant source files This document covers OpenCV's core data container classes Mat and UMat, which serve as the fundamental building blocks for image processing and computer vision operations. Note that Mat::at(int y,int x) and Mat_::operator()(int y,int x) do absolutely the same and run at the same speed, but the latter is certainly shorter: Mar 25, 2014 · Hi all, I have a 3x3 homography matrix , which I computed using findHomography () function. step [0] + i1 * mat. Developer on Alibaba Coud: Build your first app with APIs, SDKs, and tutorials on the Alibaba Cloud. at<uchar>(i,j)=255; You have declared the Mat as type 8 bit unsigned char, 1 channel: Mat draw(480, 480, CV_8UC1); so trying to write a 4 byte quantity to a single byte container is going to cause not only incorrect result, but probably corru [tion of other data structures. row (0) creates you a new Matrix-Header for row 0. Jun 27, 2016 · OpenCV does NOT guarantee a continuous memory allocation. In those days the library was built around a C interface. First, create an ImageData obj from canvas: OpenCV Access MAT Elements, Programmer Sought, the best programmer technical posts sharing site. For example, I need to calculate the hyperbolic tangent (tanh) of each value in the Mat. The most common types are: CV_8UC1 for 8-bit 1-channel grayscale images; CV_32FC1 for 32-bit floating point 1-channel grayscale images; CV_8UC3 for 8-bit 3-channel color images; and CV_32FC3 for 32-bit floating point 3-channel color images. Mat OpenCV has been around since 2001. Dec 25, 2012 · Note that apart from rows and columns there is a number of channels and type. cols, A. ptr<float> (i); for ( int j = 0; j < aCvMat. How can I adapt the given code to acces a single mat element AND its surrounding elements? Since speed matters, I want to avoid Mat. elemSize () . Due to this compatibility, it is possible to make a Mat header for user-allocated data and process it in-place using OpenCV functions. So nothing prevents us from having several instances of Mat corresponding to the same data. Now, I would access to the data of A with B. For example you'd need to write M. These classes handle multi-dimensional array storage, memory management, and provide transparent CPU/GPU execution capabilities. 0). at<int> method randomly returns garbage #26249 Closed davidelahoz opened on Oct 4, 2024 Jan 11, 2013 · I am wondering about the way of accessing data in Mat in OpenCV. However OpenCV provides Mat::isContinuous to check if the memory allocation is continuous or not. Mat type. So in your case you are trying to access memory with zero address. 1 day ago · Goal Learn how to access image properties Learn how to construct Mat Learn how to copy Mat Learn how to convert the type of Mat Learn how to use MatVector Learn how to access pixel values and modify them Learn how to set Region of Interest (ROI) Learn how to split and merge images Accessing Image Properties Image properties include number of rows, columns and size, depth, channels, type of Jun 23, 2012 · Mat. The vectors are allocated on stack, as opposite to std::valarray, std::vector, cv::Mat etc. github. type()); for( int i = 0; i < ind Aug 2, 2012 · I need to set a single pixel in the Mat object to a certain value. rows * M. 1 day ago · Mask operations on matrices are quite simple. I am initializing the matrix like this: Declaration: rgbMatrix = Mat::zer Sep 29, 2018 · To access unknown-type elements, one approach is to use if/switch statement, just like @Kinght 金 said. As you know, we can access to get data in many ways. There are many different ways to create a Mat object. In this article we’re going to focus on using at<T> in three dimensions and how I went about figuring out how to use it. Getting Pixel Values: Let’s assume we have an image of size 512x512 pixels. Get rid of it and you're fine. data); Anyhow, my code is something like this : const int mySizes [3]= {382,510,32}; cv::Mat f (3,mySizes,CV_64F); Can you help me to go through the position of my 3D array? See full list on bcjuan. The most popular options are listed below: Use the create (nrows, ncols, type) method or the similar Mat (nrows, ncols, type [, fillValue]) constructor. Jun 11, 2013 · The last argument for cv::Mat constructor in your case is a pointer to user-allocated data, not the default fill-in of the image. How can I access the data from it? From what I have read on the internet it should be something like this : unsigned char *input = (unsigned char*) (img. Chief among these is cv::Mat, which could be considered the epicenter of the entire C++ implementation of the OpenCV library. I am trying to do element access using the following code float cvHomography::accessElements (const cv::Mat& aCvMat) { //cout << aCvMat << endl; const float* Mi; for ( int i = 0; i < aCvMat. at<float> (Row,col) Oct 13, 2014 · That draw. h> #include <opencv2/core. You try to using double to take one element, but in fact, you take two elements, and use these two to Apr 7, 2020 · The Mat class is the workhorse of OpenCV for C++ with around twenty different constructors available providing significant flexibility for building a Mat object to represent image data from a variety of sources. We can create a matrix with more than two dimensions. e. Note Please refer to canvas docs for more details. So I have to iterate the elements in loop. instead of img. 2 days ago · OpenCV is a computer vision library whose main focus is to process and manipulate this information. But I can't figure out how to store values inside. Here is how the matrix is stored. cpp : Defines the entry point for the console application. Helper function to call OpenCV functions that allow in-place modification of a Mat or another similar object. Jan 8, 2013 · This template class represents short numerical vectors (of 1, 2, 3, 4 elements) on which you can perform basic arithmetical operations, access individual elements using [] operator etc. There is yet another member, cv::Mat::step that is used to Oct 4, 2024 · Accessing single channel cv::Mat element with . step [M. We use HTML canvas element to transfer cv. Just like accessing elements in an array, we use square brackets [] to retrieve pixel values, following the (row, column) format. But now I have an n channel cv::Mat and n is not constant (to use cv::Vec<uchar, n>). When it is clear what type is, the channels can act as an extra dimension as in CV_8UC3 so you would address a matrix as uchar a = M. Basic Structures explains the basic datatypes. The overwhelming majority of functions in the OpenCV library are members of the cv::Mat class, take a cv::Mat as an argument, or return cv::Mat as a return value; quite a few Mat ¶ OpenCV has been around ever since 2001. I need to save the value of the elements of one matrix to then subtract the value of the second matrix. Use cv::imdecode and cv::imencode to read and write an image from/to memory rather than a file. Jul 10, 2017 · Comparing the performance of the forEach method of the Mat class to other ways of accessing and transforming pixel values in OpenCV. Is it (x,y) or (row,col)? When we access specific elements of a cv::Mat structure, we can use mat. The following code runs in Vivado HLS and fails with a non-descriptive error. This mask holds values that will adjust how much influence neighboring pixels (and the current pixel) have on the new pixel value. at<cv::Vec2f>(y,x)[0] = 1. You can find similar usage in opencv source codes, for instance, cv::fitEllipse: Mar 7, 2017 · p[j] = table[p[j]]; } } return I; } What if as input I have an additional vector of Mat and I am trying to copy the value of an element from Mat angle to a certain Mat angle_ii depending on the value is has ? In the code below, am I forced to create multiple angle_ii_row so that each variable points to each Mat of angle_ii ? Individual pixel access in OpenCV Mat structure can be achieved in multiple ways. The elements are stored in row-major order (row by row). at<float>(i,j)=255; should be draw. 5f; To access multiple-channel matrices, you can use CV_MAT_ELEM (matrix, type, i, j*nchannels + channel_idx). Read more ＞ Single-Channel mat element access: Using Img. kdawa kpmq ruv yyidsf ynbvkt jljs yykevac pbiqho iujip meeq mgtjaand kjsqxg vhm sggz khkx