gluLookAt（）在OpenGL中无效

Question

I'm trying to look at the square from the other side using the gluLookAt() function.
After using the function, nothing changes, although I expected that the corners of the square will change.
I set the camera point to the rightmost part of the world and look at its center, where the square is located.
He had to stretch out to the sides. Why hasn't anything changed?

Code:

#include "includes.h"

using namespace std;

constexpr auto FPS_RATE = 60;
int windowHeight = 600, windowWidth = 600, windowDepth = 600;

void init();
void idleFunction();
void displayFunction();
double getTime();

double getTime()
{
    using Duration = std::chrono::duration<double>;
    return std::chrono::duration_cast<Duration>(
        std::chrono::high_resolution_clock::now().time_since_epoch()
        ).count();
}

const double frame_delay = 1.0 / FPS_RATE;
double last_render = 0;

void init()
{
    glutDisplayFunc(displayFunction);
    glutIdleFunc(idleFunction);
    glViewport(0, 0, windowWidth, windowHeight);
    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    glOrtho(-windowWidth / 2, windowWidth / 2, -windowHeight / 2, windowHeight / 2, -windowDepth / 2, windowDepth / 2);
    glClearColor(0.0, 0.0, 0.0, 0.0);
    glEnable(GL_DEPTH_TEST);
    glDepthFunc(GL_LEQUAL);
}

void idleFunction()
{
    const double current_time = getTime();
    if ((current_time - last_render) > frame_delay)
    {
        last_render = current_time;
        glutPostRedisplay();
    }
}

void displayFunction()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glBegin(GL_POLYGON);
    gluLookAt(-300, 0, 0,
        0, 0, 0,
        0, 1, 0);
    glColor3f(1, 1, 1);
    glVertex3i(-150, 150, 0);
    glVertex3i(150, 150, 0);
    glVertex3i(150, -150, 0);
    glVertex3i(-150, -150, 0);
    glEnd();
    glutSwapBuffers();
}

int main(int argc, char* argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_DOUBLE | GLUT_RGB);
    glutInitWindowSize(windowWidth, windowHeight);
    glutInitWindowPosition((GetSystemMetrics(SM_CXSCREEN) - windowWidth) / 2, (GetSystemMetrics(SM_CYSCREEN) - windowHeight) / 2);
    glutCreateWindow("Window");
    init();
    glutMainLoop();
    return 0;
}

Answer 1

The issue is caused because gluLookAt() is call with in a glBegin / glEnd sequence. This is not allowed. You've to call gluLookAt before glBegin .
Once drawing of primitives was started by glBegin it is only allowed to specify vertex coordinates ( glVertex ) and change attributes (eg glColor , glTexCoord ...), till the drawn is ended ( glEnd ).
All other instruction will be ignored and cause a GL_INVALID_OPERATION error (error code 1282).

Further note, that glLookAt doesn't set a the current matrix. It defines a matrix and multiplies the current matrix by the new matrix. Set the matrix mode ( glMatrixMode ) and set Identity matrix by glLoadIdentity before gluLookAt .

With the view matrix

 gluLookAt(-300, 0, 0, 0, 0, 0, 0, 1, 0); 

you want "see" anything, because with that matrix the line of sight is set along the x-axis and you look at the 2 dimensional polygon from the side.

Note, the polygon is a 2D object. The size of the object appears different if you look at it from the front, from the side (then it is a line and not visible) or from an direction in between. The first 3 parameters of gluLookAt define the point of view the next 3 parameters define the point you look at. The vector from the point of view to the point you look at is the line of sight.

Probably yo want look along the z-axis:

void displayFunction()
{
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    gluLookAt(0, 0, -300, 0, 0, 0, 0, 1, 0);    

    glBegin(GL_POLYGON);
    glColor3f(1, 1, 1);
    glVertex3i(-150, 150, 0);
    glVertex3i(150, 150, 0);
    glVertex3i(150, -150, 0);
    glVertex3i(-150, -150, 0);
    glEnd();

    glutSwapBuffers();
}

You use Orthographic (parallel) projection . If you would use Perspective projection , then the projected size of the object would decrease, when the distance to the point of view increases. Perspective projection can be set by gluPerspective . eg:

glMatrixMode(GL_PROJECTION);
glLoadIdentity();
gluPerspective(90.0, (double)windowWidth / windowHeight, 0.1, 600.0);