Learning OpenCV

Data Persistence

OpenCV provides a mechanism for serializing and de-serializing its various data types to and from disk in either YAML or XML format. In the chapter on HighGUI, which addresses user interface functions, we will cover specific functions that store and recall our most common object: IplImages (these functions are cvSaveImage() and cvLoadImage()).

In addition, the HighGUI chapter will discuss read and write functions specific to movies: cvGrabFrame(), which reads from file or from camera; and cvCreateVideoWriter() and cvWriteFrame(). In this section, we will focus on general object persistence: reading and writing matrices, OpenCV structures, and configuration and log files.

First we start with specific and convenient functions that save and load OpenCV matrices. These functions are cvSave() and cvLoad(). Suppose you had a 5-by-5 identity matrix (0 everywhere except for 1s on the diagonal). Example 3-15 shows how to accomplish this.

Example 3-15. Saving and loading a CvMat

CvMat A = cvMat( 5, 5, CV_32F, the_matrix_data );

cvSave( "my_matrix.xml", &A );
. . .
// to load it then in some other program use ...
CvMat* A1 = (CvMat*) cvLoad( "my_matrix.xml" );

The CxCore reference manual contains an entire section on data persistence. What you really need to know is that general data persistence in OpenCV consists of creating a CvFileStorage structure, as in Example 3-16, that stores memory objects in a tree structure. You can create and fill this structure by reading from disk ...