Understanding the Coordinate System in OpenCV
In image processing libraries like OpenCV, the coordinate system and order of matrix elements can be confusing. This article aims to clarify these concepts, particularly the use of Point(x,y) in OpenCV compared to at(i,j) for accessing matrix elements.
Matrix Ordering
OpenCV's cv::Mat data structure is utilized for both images and matrices. Mathematically, matrices are ordered in row-major order, meaning the first index designates the row and the second index the column.
Point Coordinates
Points in a coordinate system are commonly ordered with the x-axis first and the y-axis second (Cartesian coordinate system). However, in image processing, the axis ordering used is known as image notation, where the first value represents the x-direction (abscissa) and the second value the y-direction (ordinate).
OpenCV Matrix Access
OCV's at(i,j) method allows accessing elements using (row, column) indices, with the top-left corner of the matrix being (0,0).
Point Access
The Point(x,y) constructor in OpenCV enables accessing elements of a cv::Mat using image notation, where x represents the column and y the row.
Example
Consider a 300x200 matrix (src):
for (int i = 0; i < src.rows; i++) { for (int j = 0; j < src.cols; j++) { line(src, Point(i, j), Point(i, j), Scalar(255, 0, 0), 1, 8); } }
This code is intended to draw a white line on the entire image. However, it only fills in the upper portion of the image. This is because the line function uses the Point(i,j) constructor, which interprets i as the column and j as the row.
Using at(i,j) instead of Point(i,j) would correctly fill in the entire image with white, as it uses (row, column) ordering.
Conclusion
Understanding the distinction between row/column ordering in matrices and the image notation used for points in OpenCV is crucial for effectively manipulating images and matrices in OpenCV. The at(i,j) method and the Point(x,y) constructor provide convenient ways to access elements based on different coordinate systems, catering to the specific needs of the application.
The above is the detailed content of How Does OpenCV's `Point(x,y)` Differ from `at(i,j)` in Matrix Access?. For more information, please follow other related articles on the PHP Chinese website!

Hot AI Tools

Undress AI Tool
Undress images for free

Undresser.AI Undress
AI-powered app for creating realistic nude photos

AI Clothes Remover
Online AI tool for removing clothes from photos.

Clothoff.io
AI clothes remover

Video Face Swap
Swap faces in any video effortlessly with our completely free AI face swap tool!

Hot Article

Hot Tools

Notepad++7.3.1
Easy-to-use and free code editor

SublimeText3 Chinese version
Chinese version, very easy to use

Zend Studio 13.0.1
Powerful PHP integrated development environment

Dreamweaver CS6
Visual web development tools

SublimeText3 Mac version
God-level code editing software (SublimeText3)

Hot Topics

Yes, function overloading is a polymorphic form in C, specifically compile-time polymorphism. 1. Function overload allows multiple functions with the same name but different parameter lists. 2. The compiler decides which function to call at compile time based on the provided parameters. 3. Unlike runtime polymorphism, function overloading has no extra overhead at runtime, and is simple to implement but less flexible.

The destructor in C is used to free the resources occupied by the object. 1) They are automatically called at the end of the object's life cycle, such as leaving scope or using delete. 2) Resource management, exception security and performance optimization should be considered during design. 3) Avoid throwing exceptions in the destructor and use RAII mode to ensure resource release. 4) Define a virtual destructor in the base class to ensure that the derived class objects are properly destroyed. 5) Performance optimization can be achieved through object pools or smart pointers. 6) Keep the destructor thread safe and concise, and focus on resource release.

C has two main polymorphic types: compile-time polymorphism and run-time polymorphism. 1. Compilation-time polymorphism is implemented through function overloading and templates, providing high efficiency but may lead to code bloating. 2. Runtime polymorphism is implemented through virtual functions and inheritance, providing flexibility but performance overhead.

Implementing polymorphism in C can be achieved through the following steps: 1) use inheritance and virtual functions, 2) define a base class containing virtual functions, 3) rewrite these virtual functions by derived classes, and 4) call these functions using base class pointers or references. Polymorphism allows different types of objects to be treated as objects of the same basis type, thereby improving code flexibility and maintainability.

Yes, polymorphisms in C are very useful. 1) It provides flexibility to allow easy addition of new types; 2) promotes code reuse and reduces duplication; 3) simplifies maintenance, making the code easier to expand and adapt to changes. Despite performance and memory management challenges, its advantages are particularly significant in complex systems.

C destructorscanleadtoseveralcommonerrors.Toavoidthem:1)Preventdoubledeletionbysettingpointerstonullptrorusingsmartpointers.2)Handleexceptionsindestructorsbycatchingandloggingthem.3)Usevirtualdestructorsinbaseclassesforproperpolymorphicdestruction.4

Polymorphisms in C are divided into runtime polymorphisms and compile-time polymorphisms. 1. Runtime polymorphism is implemented through virtual functions, allowing the correct method to be called dynamically at runtime. 2. Compilation-time polymorphism is implemented through function overloading and templates, providing higher performance and flexibility.

C polymorphismincludescompile-time,runtime,andtemplatepolymorphism.1)Compile-timepolymorphismusesfunctionandoperatoroverloadingforefficiency.2)Runtimepolymorphismemploysvirtualfunctionsforflexibility.3)Templatepolymorphismenablesgenericprogrammingfo
