C / C++ / MFC

      //
			glClear(
			GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);

			glDisable( GL_STENCIL_TEST);
			glEnable(GL_STENCIL_TEST);
			// clear all three buffers
			glClearStencil(0x0);
			glClear(
			GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_COLOR_BUFFER_BIT);
			glStencilMask(GL_TRUE); // MUST enable writing to stencil buffer (??)
			// turn off writing to color buffer
			glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
			// enable only to verify the stencil
			//glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
			// setup stencil function
			// function  - test always are ref and mask bogus , ignored ?
			// referrence
			// mask
			glStencilFunc(GL_ALWAYS, 1, 1);							

			// setup stencil operation
			//glStencilOp(GL_REPLACE, GL_REPLACE, GL_KEEP);			// ??
			// blank here

			//glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
			// get stencil
			// TODO need parameter here to verify 
			parameter_stencil = 42;
			parameter_stencil = Stencil(parameter_stencil);		// parameter);
			glFlush();
			// blank here
			// switch keep area
			if (parameter_stencil == 0) {
				printf(
						"\nStencil full circle parameter %i @function %s @line %i",
						parameter_stencil, __FUNCTION__, __LINE__);
				glStencilFunc(GL_EQUAL, 1, 1);
			}	// keep inside circle
			else {
				printf(
						"\nStencil last circle parameter %i @function %s @line %i",
						parameter_stencil, __FUNCTION__, __LINE__);
				glStencilFunc(GL_NOTEQUAL, 1, 1);
			}	// keep all outside of stencil
		


			{
				// apply stencil 
				//int OpenGL_Render_RealAxis(int parameter = 0)
				//OpenGL_Render_RealAxis(); // int parameter = 0)
				OpenGL_Render_Plus_IMG_Axis(); //int parameter = 0)
				//OpenGL_Render_Minus_IMG_Axis(5); //passed temporary last circle as parameter = 0)
			}

from here I am not sure how to display only the rendered fragment 
I get both stencil and fragment ONLY when writing to color buffer is enabled 

			glStencilMask(GL_FALSE);
			glClear(
						GL_STENCIL_BUFFER_BIT );
			//glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);
			//glStencilOp(GL_ZERO, GL_ZERO, GL_REPLACE);
			//GL_ZERO
			//Reenable color
			glColorMask( GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
			glClear(
									GL_STENCIL_BUFFER_BIT );
			glFlush();

#include <iostream>
using namespace std;


int main ()
{
    float Circumference;
    float Diameter;
    
    cout << "Please enter diameter: ";
    cin >> Diameter;
    
    Circumference = 3.14f * Diameter;   // use f suffix to force float type
    
    cout << " Circumference is " << Circumference << endl;
    
    return 0;
    }

#define Circumference c
#define Foo 10

cout <<  Circumference << "  " << Foo << end;

cout << c << " " << 10

int main( void )
{
  float Diameter;
  float Circumference;

  cout << "Please enter diameter: ";
  cin >> Diameter;

  Circumference = 3.14 * Diameter;

  cout << " Circumference is " << Circumference << endl;

  return 0;
}

void Something(int value) { /* code */ }

<pre>void OpenGL_Stencil(void (*Stencil)(int parameter)) {
	
// clear all three buffers - not neccessary
	glClear(GL_STENCIL_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
//enable estecil test
	glEnable(GL_STENCIL_TEST);
// turn off writing to color buffer
	glColorMask(GL_FALSE, GL_FALSE, GL_FALSE, GL_FALSE);
// enable only to verify the stencil
//glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
// setup stencil function
	glStencilFunc(GL_ALWAYS, 1, 1);							// ??
// setup stencil opertion
	glStencilOp(GL_REPLACE, GL_REPLACE, GL_REPLACE);					// ??
// ??
	glStencilOp(GL_KEEP, GL_KEEP, GL_REPLACE);
// get stencil
// TDOD need parameter here
//OpenGL_Unit_Circle(0);
	Stencil(0);	// works with OpenGL_Unit_Circle(0) parameter
	glColorMask(GL_TRUE, GL_TRUE, GL_TRUE, GL_TRUE);
//
this is where I am missing how to test the parameter 
	// switch keep area
	//Stencil->paramater == 0;
//	if (Stencil->paramater == 0)
		glStencilFunc(GL_EQUAL, 1, 1);		///keep inside circle
//	else
//		glStencilFunc(GL_NOTEQUAL, 1, 1);	 // keep all outside of stencil
//
	glStencilOp(GL_KEEP, GL_KEEP, GL_KEEP);

void OpenGL_Stencil(void (*Stencil)(int))

void OpenGL_Stencil(void (*Stencil)(int), int value)

Stencil(value)

OpenGL_Stencil_Circle_Build(parameter);         returns parameter OK

OpenGL_Stencil(OpenGL_Stencil_Circle_Build(6)); wrong syntax

Description	Resource	Path	Location	Type
Invalid arguments '
Candidates are:
int OpenGL_Stencil(void (*)(int))
'	A_STENCIL.cpp	/A_STENCIL/src	line 766	Semantic Error
invalid conversion from ‘int’ to ‘void (*)(int)’ [-fpermissive]	A_STENCIL.cpp	/A_STENCIL/src	line 766	C/C++ Problem

invalid conversion from ‘int’ to ‘void (*)(int)

OpenGL_Stencil(OpenGL_Stencil_Circle_Build(6))

OpenGL_Stencil(OpenGL_Stencil_Circle_Build, 6)

void OpenGL_Stencil(void (*func) (int), int arg)
{
   //...
   func(arg);  // this is the line that will call OpenGL_Stencil_Circle_Build with a value of 6
}