Servo - January 2007

FIGURE 3. Example program that reads an image from a file and writes it to a second file in a different compression format.

*.hpp. There will be lots of matches. You don’t need all of them. Headers for all modules except HighGUI are in separate “include” directories inside each module. You can skip headers in the “src” directories for these modules. For HighGUI, you’ll need highgui.h, located in otherlibs/highgui.

the library path and the names of the static libraries to use. The static libraries you need to link to are cxcore.lib, cv.lib, and highgui.lib. Later, for face recognition, you’ll also link to cvaux.lib. These are in OpenCV’s “lib” directory.

OpenCV modules. Although you don’t need to do this, I like to gather them together into a single include directory.

On both Linux and Windows, you can locate the headers by searching the install directory and subdirectories for filenames that match the pattern *.h,

Programming with OpenCV: Some Basics

More about Headers and Libraries Most OpenCV programs need to include cv.h and highgui.h. Later, for face recognition, we’ll also include cvaux.h. The remaining header files are included by these top-level headers. If you’ve left the header files in multiple directories (default installation), make sure your compiler’s include path contains these directories. If you’ve gathered the headers into one include directory, make sure that directory is on your compiler’s include path.

Your linker will need both

Reading and Writing Images Image I/O is easy with OpenCV.

Figure 3 shows a complete program listing for reading an image from file and writing it as a second file, in a different compression format.

To read an image file, simply call cvLoadImage(), passing it the filename (line 14). OpenCV supports most common image formats, including JPEG, PNG, and BMP. You don’t need to provide format information. cvLoadImage() determines file format by reading the file header.

To write an image to file, call cvSaveImage(). This function decides which file format to use from the file extension. In this example, the extension is “png,” so it will write the image data in PNG format.

Both cvLoadImage() and cvSaveImage() are in the HighGUI module.

When you’re finished using the input image received from cvLoadImage(), free it by calling cvReleaseImage(), as on line 29. This function takes an address of a pointer as its input because it does a “safe release.” It frees the image structure only if it’s non-null. After freeing it, it sets the image pointer to 0.

FIGURE 4. Example program that captures live video frames and stores them as files.

Live Video Input

Capturing image frames from a webcam, or other digital video device, is nearly as easy as loading from file. Figure 4 shows a complete program listing to initialize frame capture, capture and store several video frames, and close the capture interface.

The capture interface is initialized, on line 19, by calling cvCaptureFromCAM(). This function returns a pointer to a CvCapture structure. You won’t access this structure directly. Instead, you’ll store the pointer to pass to cvQueryFrame().

When you’re finished using video input, call cvReleaseCapture() to release video resources. As with cvReleaseImage(), you pass the address of the CvCapture pointer to cvReleaseCapture().