Identifying a C++ idiom

I am reading some code and can't identify what part of C++ this idiom is (that :fgl(NULL) that's in red below by the image::image definition).

If somebody can give me the magic phrase that I can look up (like "polymorphic initializer resolution") or whatever it's called, I'd appreciate that.

TIA,

BVK

Code:

class image
{
public:
	// initialised by init_fast_access() :
	BYTE    **fgl;
	// allways initialised :
	BYTE    *gl;
	WORD    Width,Height;
	DWORD   Size;
	// initialised by analyse():
	WORD    gravityH,gravityW;
	long    total;
	double  minGL,maxGL;
	double  pixBsurN;
	// initialised by Ceil()&analyse():
	DWORD   Surface;

	image(): fgl(NULL){}; 
	image(char *name);
	image(image *im,double scaleX,double scaleY);
	void initFrom(image *im);
	image(image *im, double ratio);
	image(WORD _Width,WORD _Height,BYTE c);
	image(imageD *im,BYTE s);
  .
   
  .
  
  .
 }
 
 image::image(image *im, double ratio) : fgl(NULL)
// attention !!!  before using this function, you must initialize
// the two variables gravityW and gravityH by calling analyse();
{
	WORD tl=im->gravityW+im->gravityH*im->Width,
  .
   
  .
  
  .
}

Constructor Initialization List

Very good, Thanks.