Basics of OpenCV API

Header files

These are some of important OpenCV header files for C++ interface. As a beginner, you will need few of these header files for your application. In my following lessons, I will include only necessary header files to my example programs. If you are not sure what to include, include them all. No any penalty incurred for including all this header file except for a fact that the length of your source code will increased by few lines than necessary.

• #include "opencv2/core/core.hpp"
• #include "opencv2/flann/miniflann.hpp"
• #include "opencv2/imgproc/imgproc.hpp"
• #include "opencv2/photo/photo.hpp"
• #include "opencv2/video/video.hpp"
• #include "opencv2/features2d/features2d.hpp"
• #include "opencv2/objdetect/objdetect.hpp"
• #include "opencv2/calib3d/calib3d.hpp"
• #include "opencv2/ml/ml.hpp"
• #include "opencv2/highgui/highgui.hpp"
• #include "opencv2/contrib/contrib.hpp"

• #include "opencv2/core/core_c.h"
• #include "opencv2/highgui/highgui_c.h"
• #include "opencv2/imgproc/imgproc_c.h"

Namespace 

All OpenCV classes and functions are in cv namespace. So, you have to do one of following

• Add the 'using namespace cv' line just after including your header files (I have used this method in all my sample programs)

e.g.
                              #include "opencv2/core/core.hpp"

                              using namespace cv; 

                              int main()

                              {

                                         Mat frame = cvQueryFrame( capture );

                                         imshow( "Video", frame );

                              }

• append the cv:: specifier at the beginning of every OpenCV classes, functions and data structures in your source code

e.g.                              
                              #include "opencv2/core/core.hpp"

                              int main()

                              {

                                         cv::Mat frame = cvQueryFrame( capture );

                                         cv::imshow( "Video", frame );

                              }

Data Types for Arrays

Data type of an array defines the number of bits allocated for each element of array (pixels in an image) and how the value is represented using those bits. Any array elements should have one of following data types.

For single channel arrays : 

• CV_8U (8 bit unsigned integer)
• CV_8S (8 bit signed integer)
• CV_16U (16 bit unsigned integer)
• CV_16S (16 bit signed integer)
• CV_32S (32 bit signed integer)
• CV_32F (32 bit floating point number)
• CV_64F (64 bit float floating point number)

e.g. : Here I have illustrated a single channel array with 8 bit unsigned integers. As the datatype of this array is 8 bit unsigned integers, each element should have a value from 0 to 255.

Single Channel Array

For multi channel arrays :

We can define all of above data types for multi channel arrays (supports up to 512 channels). Here I am going to show you how to define CV_8U data type for multi channel arrays.

• CV_8UC1 (single channel array with 8 bit unsigned integers) 
• CV_8UC2 (2 channel array with 8 bit unsigned integers)
• CV_8UC3 (3 channel array with 8 bit unsigned integers)
• CV_8UC4 (4 channel array with 8 bit unsigned integers)
• CV_8UC(n) (n channel array with 8 bit unsigned integers (n can be from 1 to 512) )

e.g. 1 : Here I have illustrated a 3 channel array with 8 bit unsigned integers. As the datatype is 8 bit unsigned integers, each element should have a value from 0 to 255. Because this is a 3 channel array, array consists of tuples with 3 elements. The first tuple is {54, 0, 34}, second tuple is {58, 78, 185} and so on.

3 Channel Arrays

e.g. 2 : Here I have illustrated a 2 channel array with 8 bit signed integers. As the datatype is 8 bit signed integers, each element should have a value from -128 to 127. Because this is a 2 channel array, array consists of tuples with 2 elements. The first tuple is {-85, -127}, second tuple is {25, 23} and so on.

2 Channel Array

Note : CV_8U = CV_8UC1 = CV_8UC(1)

Example Usage :

• Mat img1(3, 5, CV_32F ); //3 x 5 single-channel array with 32 bit floating point numbers
• Mat img2(23, 53, CV_64FC(5) ); //23 x 53 5-channel array with 64 bit floating point numbers
• Mat img3(Size(100, 200), CV_16UC2 ); //100 x 200 2-channel array with 16 bit unsigned integers

Remember :

Some OpenCV functions can handle only a subset of above data types. So, be careful, when using OpenCV functions. 

Bit Depths for IplImage (C style)

• IPL_DEPTH_<bit_depth>(S|U|F)
• Here possible values for <bit_depth> are 1,8,16,32 and 64
• S = Signed
• U = Unsigned 
• F = Float
• 1 bit depth images should be unsigned
• 8 bit depth images should be signed or unsigned
• 16 bit depth images should be signed or unsigned
• 32 bit depth images should be signed or float
• 64 bit depth images should be float
• E.g.:  
• IPL_DEPTH_1U (1 bit depth and unsigned)
• IPL_DEPTH_8U (8 bit depth and unsigned)
• IPL_DEPTH_16U
• IPL_DEPTH_32F ( 32 bit depth and float )
• IPL_DEPTH_8S
• IPL_DEPTH_16S ( 16 bit depth and signed )
• IPL_DEPTH_32S
• IPL_DEPTH_64F

Bit depth means the number of bits allocated for a pixel. For example, IplImage with IPL_DEPTH_8U uses 8 bit unsigned integer per each pixel. That means each pixel can hold 0 to 255 integer numbers. 

IPL_DEPTH_8U, IPL_DEPTH_8S, IPL_DEPTH_16S,  IPL_DEPTH_32S, IPL_DEPTH_32F and IPL_DEPTH_64F are currently supported by IplImage data structure.