First try - graphical program

Question

I used a book called Computer Graphices using OpenGL.

at page number 51 i found this code

#include <windows.h>
#include "glut.h" 

//<<<<<<<<<<<<<<<<< method(s) >>>>>>>>>>>>>>>>>>>
void My_Display(void); 
void My_Inti(void); 

//<<<<<<<<<<<<<<<<< main method >>>>>>>>>>>>>>>>>>
int main(int argc, char ** argv)
{
    glutInit(&argc, argv); 
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB); 
    glutInitWindowSize(640, 480); 
    glutInitWindowPosition(100, 150); 
    glutCreateWindow("my second try "); 
    glutDisplayFunc(My_Display);

    My_Inti(); 
    glutMainLoop(); 



    return 0;
}

//<<<<<<<<<<<<<<<<<<<<<<<< IMPLEMENTING METHOD(S) >>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>>.

//<<<<<<<<<<<<<<<< My_Inti >>>>>>>>>>>>>>>>>>>>>>>>>
void My_Inti(void)
{
    glClearColor(1, 1, 1, 0); // white color 
    glColor3f(0, 0, 0); // Black color 
    glPointSize(10); // point size is 10 pixel this is big .
    glMatrixMode(GL_PROJECTION);  
    glLoadIdentity(); 
    gluOrtho2D(0, 640, 0, 480); 
}


//<<<<<<<<<<<<<<<< My_Display >>>>>>>>>>>>>>>>>>>>>>
void My_Display(void)
{
    glClear(GL_COLOR_BUFFER_BIT); 
    glBegin(GL_POINT);
    glVertex2i(100, 50);
    glVertex2i(100, 130);
    glVertex2i(150, 130); 
    glEnd();
    glFlush();
}

all what i add to this code is the comments and i change a little at the variable ; nothing more .

When we come to the problem this code work fine but it didn't creat the three points at the display method ?

Answer 1

The problem is just one missing letter. Instead of this:

glBegin(GL_POINT);

The correct value is:

glBegin(GL_POINTS);

The first thing I would do any time you get no rendering, or not the expected rendering, is to call glGetError(), and see if it returns an error. I admit that I didn't see this problem initially, but calling glGetError() would have pointed it out quickly.

BTW: In case anybody else is surprised that there are both GL_POINT and GL_POINTS enums in OpenGL. GL_POINT is one of the possible arguments to glPolygonMode(), as opposed to GL_POINTS which denotes one of the possible primitive types for draw calls.

Answer 2

It does not work because there is an error in the source code (unlikely) or you mis-copied the argument for glBegin(GL_POINT); (likely).

Using

glBegin(GL_POINTS);

I got this image: