OpenCV: Transition guide

| OpenCV 3.4.2 Open Source Computer Vision

Transition guide

# Changes overview

This document is intended to software developers who want to migrate their code to OpenCV 3.0.

OpenCV 3.0 introduced many new algorithms and features comparing to version 2.4. Some modules have been rewritten, some have been reorganized. Although most of the algorithms from 2.4 are still present, the interfaces can differ.

This section describes most notable changes in general, all details and examples of transition actions are in the next part of the document.

##### Contrib repository

https://github.com/opencv/opencv_contrib

This is a place for all new, experimental and non-free algorithms. It does not receive so much attention from the support team comparing to main repository, but the community makes an effort to keep it in a good shape.

To build OpenCV with contrib repository, add the following option to your cmake command:

-DOPENCV_EXTRA_MODULES_PATH=<path-to-opencv_contrib>/modules

##### Headers layout

In 2.4 all headers are located in corresponding module subfolder (opencv2/<module>/<module>.hpp), in 3.0 there are top-level module headers containing the most of the module functionality: opencv2/<module>.hpp and all C-style API definitions have been moved to separate headers (for example opencv2/core/core_c.h).

##### Algorithm interfaces

General algorithm usage pattern has changed: now it must be created on heap wrapped in smart pointer cv::Ptr. Version 2.4 allowed both stack and heap allocations, directly or via smart pointer.

get and set methods have been removed from the cv::Algorithm class along with CV_INIT_ALGORITHM macro. In 3.0 all properties have been converted to the pairs of getProperty/setProperty pure virtual methods. As a result it is not possible to create and use cv::Algorithm instance by name (using generic Algorithm::create(String) method), one should call corresponding factory method explicitly.

##### Changed modules

- ml module has been rewritten
- highgui module has been split into parts: imgcodecs, videoio and highgui itself
- features2d module have been reorganized (some feature detectors has been moved to opencv_contrib/xfeatures2d module)
- legacy, nonfree modules have been removed. Some algorithms have been moved to different locations and some have been completely rewritten or removed
- CUDA API has been updated (gpu module -> several cuda modules, namespace gpu -> namespace cuda)
- OpenCL API has changed (ocl module has been removed, separate ocl:: implementations -> Transparent API)
- Some other methods and classes have been relocated

# Transition hints

This section describes concrete actions with examples.

## Prepare 2.4

Some changes made in the latest 2.4.11 OpenCV version allow you to prepare current codebase to migration:

- cv::makePtr function is now available
- opencv2/<module>.hpp headers have been created

## New headers layout

Note: Changes intended to ease the migration have been made in OpenCV 3.0, thus the following instructions are not necessary, but recommended.

1. Replace inclusions of old module headers:
   // old header
   #include "opencv2/<module>/<module>.hpp"
   // new header
   #include "opencv2/<module>.hpp"
2. If your code is using C API (`cv*` functions, `Cv*` structures or `CV_*` enumerations), include corresponding `*_c.h` headers. Although it is recommended to use C++ API, most of C-functions are still accessible in separate header files (opencv2/core/core_c.h, opencv2/core/types_c.h, opencv2/imgproc/imgproc_c.h, etc.).

## Modern way to use algorithm

1. Algorithm instances must be created with cv::makePtr function or corresponding static factory method if available:
   // good ways
   Ptr<SomeAlgo> algo = makePtr<SomeAlgo>(...);
   Ptr<SomeAlgo> algo = SomeAlgo::create(...);
   Other ways are deprecated:
   // bad ways
   Ptr<SomeAlgo> algo = new SomeAlgo(...);
   SomeAlgo * algo = new SomeAlgo(...);
   SomeAlgo algo(...);
   Ptr<SomeAlgo> algo = Algorithm::create<SomeAlgo>("name");
2. Algorithm properties should be accessed via corresponding virtual methods, getSomeProperty/setSomeProperty, generic get/set methods have been removed:
   // good way
   double clipLimit = clahe->getClipLimit();
   clahe->setClipLimit(clipLimit);
   // bad way
   double clipLimit = clahe->getDouble("clipLimit");
   clahe->set("clipLimit", clipLimit);
   clahe->setDouble("clipLimit", clipLimit);
3. Remove `initModule_<moduleName>()` calls

## Machine learning module

Since this module has been rewritten, it will take some effort to adapt your software to it. All algorithms are located in separate ml namespace along with their base class StatModel. Separate SomeAlgoParams classes have been replaced with a sets of corresponding getProperty/setProperty methods.

The following table illustrates correspondence between 2.4 and 3.0 machine learning classes.

2.4 | 3.0
--- | ---
CvStatModel | cv::ml::StatModel
CvNormalBayesClassifier | cv::ml::NormalBayesClassifier
CvKNearest | cv::ml::KNearest
CvSVM | cv::ml::SVM
CvDTree | cv::ml::DTrees
CvBoost | cv::ml::Boost
CvGBTrees | Not implemented
CvRTrees | cv::ml::RTrees
CvERTrees | Not implemented
EM | cv::ml::EM
CvANN_MLP | cv::ml::ANN_MLP
Not implemented | cv::ml::LogisticRegression
CvMLData | cv::ml::TrainData

Although rewritten ml algorithms in 3.0 allow you to load old trained models from xml/yml file, deviations in prediction process are possible.

The following code snippets from the `points_classifier.cpp` example illustrate differences in model training process:

using namespace cv;
// ======== version 2.4 ========
Mat trainSamples, trainClasses;
prepare_train_data( trainSamples, trainClasses );
CvBoost boost;
Mat var_types( 1, trainSamples.cols + 1, CV_8UC1, Scalar(CV_VAR_ORDERED) );
var_types.at<uchar>( trainSamples.cols ) = CV_VAR_CATEGORICAL;
CvBoostParams params( CvBoost::DISCRETE, // boost_type
100, // weak_count
0.95, // weight_trim_rate
2, // max_depth
false, //use_surrogates
0 // priors
);
boost.train( trainSamples, CV_ROW_SAMPLE, trainClasses, Mat(), Mat(), var_types, Mat(), params );
// ======== version 3.0 ========
Ptr<Boost> boost = Boost::create();
boost->setBoostType(Boost::DISCRETE);
boost->setWeakCount(100);
boost->setWeightTrimRate(0.95);
boost->setMaxDepth(2);
boost->setUseSurrogates(false);
boost->setPriors(Mat());
boost->train(prepare_train_data()); // 'prepare_train_data' returns an instance of ml::TrainData class

## Features detect

Some algorithms (FREAK, BRIEF, SIFT, SURF) has been moved to opencv_contrib repository, to xfeatures2d module, xfeatures2d namespace. Their interface has been also changed (inherit from `cv::Feature2D` base class).

List of xfeatures2d module classes:

- cv::xfeatures2d::BriefDescriptorExtractor - Class for computing BRIEF descriptors (2.4 location: features2d)
- cv::xfeatures2d::FREAK - Class implementing the FREAK (Fast Retina Keypoint) keypoint descriptor (2.4 location: features2d)
- cv::xfeatures2d::StarDetector - The class implements the CenSurE detector (2.4 location: features2d)
- cv::xfeatures2d::SIFT - Class for extracting keypoints and computing descriptors using the Scale Invariant Feature Transform (SIFT) algorithm (2.4 location: nonfree)
- cv::xfeatures2d::SURF - Class for extracting Speeded Up Robust Features from an image (2.4 location: nonfree)

Following steps are needed:

1. Add opencv_contrib to compilation process
2. Include `opencv2/xfeatures2d.h` header
3. Use namespace `xfeatures2d`
4. Replace `operator()` calls with `detect`, `compute` or `detectAndCompute` if needed

Some classes now use general methods `detect`, `compute` or `detectAndCompute` provided by `Feature2D` base class instead of custom `operator()`

Following code snippets illustrate the difference (from `video_homography.cpp` example):

using namespace cv;
// ====== 2.4 =======
#include "opencv2/features2d/features2d.hpp"
BriefDescriptorExtractor brief(32);
GridAdaptedFeatureDetector detector(new FastFeatureDetector(10, true), DESIRED_FTRS, 4, 4);
// ...
detector.detect(gray, query_kpts); //Find interest points
brief.compute(gray, query_kpts, query_desc); //Compute brief descriptors at each keypoint location
// ====== 3.0 =======
#include "opencv2/features2d.hpp"
#include "opencv2/xfeatures2d.hpp"
using namespace cv::xfeatures2d;
Ptr<BriefDescriptorExtractor> brief = BriefDescriptorExtractor::create(32);
Ptr<FastFeatureDetector> detector = FastFeatureDetector::create(10, true);
// ...
detector->detect(gray, query_kpts); //Find interest points
brief->compute(gray, query_kpts, query_desc); //Compute brief descriptors at each keypoint location

## OpenCL

All specialized `ocl` implemetations has been hidden behind general C++ algorithm interface. Now the function execution path can be selected dynamically at runtime: CPU or OpenCL; this mechanism is also called "Transparent API".

New class cv::UMat is intended to hide data exchange with OpenCL device in a convenient way.

Following example illustrate API modifications (from OpenCV site):

- OpenCL-aware code OpenCV-2.x
  // initialization
  VideoCapture vcap(...);
  ocl::OclCascadeClassifier fd("haar_ff.xml");
  ocl::oclMat frame, frameGray;
  Mat frameCpu;
  vector<Rect> faces;
  for(;;){
  // processing loop
  vcap >> frameCpu;
  frame = frameCpu;
  ocl::cvtColor(frame, frameGray, BGR2GRAY);
  ocl::equalizeHist(frameGray, frameGray);
  fd.detectMultiScale(frameGray, faces, ...);
  // draw rectangles …
  // show image …
  }
- OpenCL-aware code OpenCV-3.x
  // initialization
  VideoCapture vcap(...);
  CascadeClassifier fd("haar_ff.xml");
  UMat frame, frameGray; // the only change from plain CPU version
  vector<Rect> faces;
  for(;;){
  // processing loop
  vcap >> frame;
  cvtColor(frame, frameGray, BGR2GRAY);
  equalizeHist(frameGray, frameGray);
  fd.detectMultiScale(frameGray, faces, ...);
  // draw rectangles …
  // show image …
  }

## CUDA

cuda module has been split into several smaller pieces:

- cuda - CUDA-accelerated Computer Vision
- cudaarithm - Operations on Matrices
- cudabgsegm - Background Segmentation
- cudacodec - Video Encoding/Decoding
- cudafeatures2d - Feature Detection and Description
- cudafilters - Image Filtering
- cudaimgproc - Image Processing
- cudalegacy - Legacy support
- cudaoptflow - Optical Flow
- cudastereo - Stereo Correspondence
- cudawarping - Image Warping
- cudev - Device layer

`gpu` namespace has been removed, use cv::cuda namespace instead. Many classes has also been renamed, for example:

- `gpu::FAST_GPU` -> cv::cuda::FastFeatureDetector
- `gpu::createBoxFilter_GPU` -> cv::cuda::createBoxFilter

## Documentation format

Documentation has been converted to Doxygen format. You can find updated documentation writing guide in Tutorials section of OpenCV reference documentation (Writing documentation for OpenCV).

## Support both versions

In some cases it is possible to support both versions of OpenCV.

### Source code

To check library major version in your application source code, the following method should be used:

#include "opencv2/core/version.hpp"
#if CV_MAJOR_VERSION == 2
// do opencv 2 code
#elif CV_MAJOR_VERSION == 3
// do opencv 3 code
#endif

Note
  Do not use CV_VERSION_MAJOR, it has different meaning for 2.4 and 3.x branches!

### Build system

It is possible to link different modules or enable/disable some of the features in your application by checking library version in the build system. Standard cmake or pkg-config variables can be used for this:

- `OpenCV_VERSION` for cmake will contain full version: "2.4.11" or "3.0.0" for example
- `OpenCV_VERSION_MAJOR` for cmake will contain only major version number: 2 or 3
- pkg-config file has standard field `Version`

Example:

if(OpenCV_VERSION VERSION_LESS "3.0")
# use 2.4 modules
else()
# use 3.x modules
endif()