Question

I read on http://www.cs.iit.edu/~agam/cs512/lect-notes/opencv-intro/opencv-intro.html that a possible way to access the image data with c++ is:

template<class T> class Image
{
  private:
  IplImage* imgp;
  public:
  Image(IplImage* img=0) {imgp=img;}
  ~Image(){imgp=0;}
  void operator=(IplImage* img) {imgp=img;}
  inline T* operator[](const int rowIndx) {
    return ((T *)(imgp->imageData + rowIndx*imgp->widthStep));}
};

typedef struct{
  unsigned char b,g,r;
} RgbPixel;

typedef struct{
  float b,g,r;
} RgbPixelFloat;

typedef Image<RgbPixel>       RgbImage;
typedef Image<RgbPixelFloat>  RgbImageFloat;
typedef Image<unsigned char>  BwImage;
typedef Image<float>          BwImageFloat;

So I’m able to use something like:

IplImage* img=cvCreateImage(cvSize(640,480),IPL_DEPTH_8U,3);
RgbImage  imgA(img);
imgA[i][j].b = 111;
imgA[i][j].g = 111;
imgA[i][j].r = 111;

When I use:

imgA[i][j].b

My question is: how cpp knows the channels of the image? I mean, how c++ populates the

img[i][j].b as blue channel
img[i][j].g as green channel and 
img[i][j].r as red channel?

Does it have a default constructor to the structs?!

Answer 1

You should have a look at how templates (Template metaprogramming) are working.

You have a constructor but the rgb channels are populated in the

 inline T* operator[](const int rowIndx)
 {
    return ((T *)(imgp->imageData + rowIndx*imgp->widthStep));
 }

when you instantiate the Template with a RgbPixel structure your function will became:

 inline RgbPixel* operator[](const int rowIndx)
 {
    return ((RgbPixel *)(imgp->imageData + rowIndx*imgp->widthStep));
 }

Because the data in both (the image and the RgbPixel structure) is stored in a continuous block of memory you will get the correct informations.