Learning OpenCV

IplImage Data Structure

With all of that in hand, it is now easy to discuss the IplImage data structure. In essence this object is a CvMat but with some extra goodies buried in it to make the matrix interpretable as an image. This structure was originally defined as part of Intel's Image Processing Library (IPL).^[19] The exact definition of the IplImage structure is shown in Example 3-10.

Example 3-10. IplImage header structure

typedef struct _IplImage {
  int                  nSize;
  int                  ID;
  int                  nChannels;
  int                  alphaChannel;
  int                  depth;
  char                 colorModel[4];
  char                 channelSeq[4];
  int                  dataOrder;
  int                  origin;
  int                  align;
  int                  width;
  int                  height;
  struct _IplROI*      roi;
  struct _IplImage*    maskROI;
  void*                imageId;
  struct _IplTileInfo* tileInfo;
  int                  imageSize;
  char*                imageData;
  int                  widthStep;
  int                  BorderMode[4];
  int                  BorderConst[4];
  char*                imageDataOrigin;
} IplImage;

As crazy as it sounds, we want to discuss the function of several of these variables. Some are trivial, but many are very important to understanding how OpenCV interprets and works with images.

After the ubiquitous width and height, depth and nChannels are the next most crucial. The depth variable takes one of a set of values defined in ipl.h, which are (unfortunately) not exactly the values we encountered when looking at matrices. This is because for images we tend to deal with the depth and the number of channels separately (whereas in the matrix routines we tended to refer to them simultaneously). The possible depths are listed in Table 3-2.

Table 3-2. OpenCV ...