grid not showing (it is all black)

问题

I didn't want to go back to the same question from yesterday, however before I am able to use the function to turn on and off the grid, I first need to know if my grid is actually working, I have been making new projects all night trying to display the grid but it isn't showing, the screen is always black and nothing is there at all.

#include "include\freeglut.h"   // OpenGL toolkit - in the local shared folder
#include <iostream>

//set up some constants
#define X_CENTRE 0.0      /* centre point of square */
#define Y_CENTRE 0.0
#define LENGTH   1.0      /* lengths of sides of square */

GLfloat red = 1.0, green = 1.0, blue = 1.0;
int w;
int h;

/* reshape callback function
executed when window is moved or resized */
void reshape(int width, int height)
{
glViewport(0, 0, width, height);
/* uses orthographic (parallel) projection
use xmin = -1, xmax = 1
ymin = -1, ymax = 1
znear = -1, zfar = 1 - not relevant here (2D) */
glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);
}


/* display callback function
called whenever contents of window need to be re-displayed */
//this is the all important drawing method - all drawing code goes in here
void display(void)
{
glClear(GL_COLOR_BUFFER_BIT);   /* clear window */
//glColor3f(red, green, blue);        /* white drawing objects */
glColor3f(0.8, 0.8, 0.8);

GLint i;

glEnable(GL_LINE_STIPPLE); //Activates the line-style feature

glLineStipple(1, 0xAAAA);  // Plots a dashed polyline

glBegin(GL_LINES);
for (i = 2; i <= 9; i++)
{
    glVertex3f(i * 0.1 * w, 0.0, 0.0);
    glVertex3f(i * 0.1 * w, 0.9 * h, 0.0);
}

for (i = 1; i <= 9; i++)
{
    glVertex3f(0.1 * w, i * 0.1 * h, 0.0);
    glVertex3f(w, i * 0.1 * h, 0.0);
}
glEnd();
glDisable(GL_LINE_STIPPLE);

glFlush();     /* execute drawing commands in buffer */
}

/* graphics initialisation */
void init(void)
{
glClearColor(0.0, 0.0, 0.0, 0.0);   /* window will be cleared to black */
}

//rename this to main(...) and change example 2 to run this main function
int main(int argc, char** argv)
{
/* window management code ... */
/* initialises GLUT and processes any command line arguments */
glutInit(&argc, argv);
/* use single-buffered window and RGBA colour model */
glutInitDisplayMode(GLUT_SINGLE | GLUT_RGB);
/* window width = 400 pixels, height = 400 pixels */
glutInitWindowSize(400, 400);
/* window upper left corner at (100, 100) */
glutInitWindowPosition(100, 100);
/* creates an OpenGL window with command argument in its title bar */
glutCreateWindow("Example 1");
init();
glutDisplayFunc(display);
glutReshapeFunc(reshape);

glutMainLoop();
return 0;
}

回答1:

The variables w and h a re not initialized. Initialize the variables by 1:

int w = 1;
int h = 1;

---

However, if you want to set the vertex coordinates in window space, you have to change the orthographic projection. The projection matrix defines the area (volume) with respect to the observer (viewer) which is projected onto the viewport. At orthographic projection, this area (volume) is defined by 6 distances (left, right, bottom, top, near and far) to the viewer's position.

glOrtho(-1.0, 1.0, -1.0, 1.0, -1.0, 1.0);

glOrtho(0.0, (float)w, (float)h, 0.0, -1.0, 1.0);

Function reshape:

void reshape(int width, int height)
{
    glViewport(0, 0, width, height);
    /* uses orthographic (parallel) projection
    use xmin = -1, xmax = 1
    ymin = -1, ymax = 1
    znear = -1, zfar = 1 - not relevant here (2D) */    #
    
    w = width;
    h = height;
    glMatrixMode(GL_PROJECTION);
    glOrtho(0.0, (float)w, (float)h, 0.0, -1.0, 1.0);
    glMatrixMode(GL_MODELVIEW);
}

来源：https://stackoverflow.com/questions/65037104/grid-not-showing-it-is-all-black