Question

I am trying to render the model in two windows in glut. But it is rendered in only one of the windows. Is it possible to render in two windows with different callback functions?

#include <stdlib.h>
#include <stdio.h>
#include <string.h>
#include <GL/glew.h>
#include <GL/freeglut.h>
#define WINDOW_TITLE_PREFIX "Chapter 2"

int CurrentWidth = 800,
    CurrentHeight = 600,
    WindowHandle = 0;

unsigned FrameCount = 0;

GLuint
    VertexShaderId,
    FragmentShaderId,
    ProgramId,
    VaoId,
    VboId,
    ColorBufferId;

const GLchar* VertexShader =
{
    "#version 400\n"\

    "layout(location=0) in vec4 in_Position;\n"\
    "layout(location=1) in vec4 in_Color;\n"\
    "out vec4 ex_Color;\n"\

    "void main(void)\n"\
    "{\n"\
    "   gl_Position = in_Position;\n"\
    "   ex_Color = in_Color;\n"\
    "}\n"
};

const GLchar* FragmentShader =
{
    "#version 400\n"\

    "in vec4 ex_Color;\n"\
    "out vec4 out_Color;\n"\

    "void main(void)\n"\
    "{\n"\
    "   out_Color = ex_Color;\n"\
    "}\n"
};

void Initialize(int, char*[]);
void InitWindow(int, char*[]);
void ResizeFunction(int, int);
void ResizeFunction1(int, int);
void RenderFunction(void);
void RenderFunction1(void);
void TimerFunction(int);
void TimerFunction1(int);
void IdleFunction(void);
void IdleFunction1(void);
void Cleanup(void);
void CreateVBO(void);
void DestroyVBO(void);
void CreateShaders(void);
void DestroyShaders(void);

int Wh1;

int main(int argc, char* argv[])
{
    Initialize(argc, argv);

    glutMainLoop();

    exit(EXIT_SUCCESS);
}

void Initialize(int argc, char* argv[])
{
    GLenum GlewInitResult;

    InitWindow(argc, argv);

    glewExperimental = GL_TRUE;
    GlewInitResult = glewInit();

    if (GLEW_OK != GlewInitResult) {
        fprintf(
            stderr,
            "ERROR: %s\n",
            glewGetErrorString(GlewInitResult)
        );
        exit(EXIT_FAILURE);
    }

    fprintf(
        stdout,
        "INFO: OpenGL Version: %s\n",
        glGetString(GL_VERSION)
    );

    CreateShaders();
    CreateVBO();

    glClearColor(0.0f, 0.0f, 0.0f, 0.0f);
}

void InitWindow(int argc, char* argv[])
{
    glutInit(&argc, argv);

    glutInitContextVersion(4, 0);
    glutInitContextFlags(GLUT_FORWARD_COMPATIBLE);
    glutInitContextProfile(GLUT_CORE_PROFILE);

    glutSetOption(
        GLUT_ACTION_ON_WINDOW_CLOSE,
        GLUT_ACTION_GLUTMAINLOOP_RETURNS
    );

    glutInitWindowSize(CurrentWidth, CurrentHeight);

    glutInitDisplayMode(GLUT_DEPTH | GLUT_DOUBLE | GLUT_RGBA);

    WindowHandle = glutCreateWindow(WINDOW_TITLE_PREFIX);
    std::cout<<WindowHandle<<std::endl;
    if(WindowHandle < 1) {
        fprintf(
            stderr,
            "ERROR: Could not create a new rendering window.\n"
        );
        exit(EXIT_FAILURE);
    }
    glutSetWindow(WindowHandle);
    glutReshapeFunc(ResizeFunction);
    glutDisplayFunc(RenderFunction);
    glutTimerFunc(0, TimerFunction, 0);
    glutCloseFunc(Cleanup);
#if 1
    Wh1 = glutCreateWindow("sasd"); 
//  std::cout<<i<<"s"<<std::endl;
    glutReshapeFunc(ResizeFunction1);
    glutDisplayFunc(RenderFunction1);
    glutTimerFunc(0, TimerFunction1, 0);
    glutCloseFunc(Cleanup);
    glutSetWindow(Wh1);
#endif

}

void ResizeFunction(int Width, int Height)
{
    CurrentWidth = Width;
    CurrentHeight = Height;
    glViewport(0, 0, CurrentWidth, CurrentHeight);
}


void ResizeFunction1(int Width, int Height)
{
    CurrentWidth = Width;
    CurrentHeight = Height;
    glViewport(0, 0, CurrentWidth, CurrentHeight);
}

void RenderFunction(void)
{
    ++FrameCount;

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    glutSwapBuffers();
    glutSetWindow(WindowHandle);
    glutPostRedisplay();
    glutSetWindow(Wh1);
    glutPostRedisplay();
}

void RenderFunction1(void)
{
    ++FrameCount;

    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);

    glDrawArrays(GL_TRIANGLES, 0, 3);

    glutSwapBuffers();

    glutSetWindow(WindowHandle);
    glutPostRedisplay();
    glutSetWindow(Wh1);
    glutPostRedisplay();
}

void IdleFunction(void)
{
    glutSetWindow(WindowHandle);
    glutPostRedisplay();
    glutSetWindow(Wh1);
    glutPostRedisplay();
}

void IdleFunction1(void)
{
    glutSetWindow(WindowHandle);
    glutPostRedisplay();
    glutSetWindow(Wh1);
    glutPostRedisplay();
}
void TimerFunction(int Value)
{
    if (0 != Value) {
        char* TempString = (char*)
            malloc(512 + strlen(WINDOW_TITLE_PREFIX));

        sprintf(
            TempString,
            "%s: %d Frames Per Second @ %d x %d",
            WINDOW_TITLE_PREFIX,
            FrameCount * 4,
            CurrentWidth,
            CurrentHeight
        );

        //glutSetWindowTitle(TempString);
        free(TempString);
    }

    FrameCount = 0;
    glutTimerFunc(250, TimerFunction, 1);
}
void TimerFunction1(int Value)
{
    if (0 != Value) {
        char* TempString = (char*)
            malloc(512 + strlen(WINDOW_TITLE_PREFIX));

        sprintf(
            TempString,
            "%s: %d Frames Per Second @ %d x %d",
            WINDOW_TITLE_PREFIX,
            FrameCount * 4,
            CurrentWidth,
            CurrentHeight
        );

        //glutSetWindowTitle(TempString);
        free(TempString);
    }

    FrameCount = 0;
    glutTimerFunc(250, TimerFunction, 1);
}

void Cleanup(void)
{
    DestroyShaders();
    DestroyVBO();
}

void CreateVBO(void)
{
    GLfloat Vertices[] = {
        -0.8f, -0.8f, 0.0f, 1.0f,
         0.0f,  0.8f, 0.0f, 1.0f,
         0.8f, -0.8f, 0.0f, 1.0f
    };

    GLfloat Colors[] = {
        1.0f, 0.0f, 0.0f, 1.0f,
        0.0f, 1.0f, 0.0f, 1.0f,
        0.0f, 0.0f, 1.0f, 1.0f
    };

    GLenum ErrorCheckValue = glGetError();

    glGenVertexArrays(1, &VaoId);
    glBindVertexArray(VaoId);

    glGenBuffers(1, &VboId);
    glBindBuffer(GL_ARRAY_BUFFER, VboId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
    glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);
    glEnableVertexAttribArray(0);

    glGenBuffers(1, &ColorBufferId);
    glBindBuffer(GL_ARRAY_BUFFER, ColorBufferId);
    glBufferData(GL_ARRAY_BUFFER, sizeof(Colors), Colors, GL_STATIC_DRAW);
    glVertexAttribPointer(1, 4, GL_FLOAT, GL_FALSE, 0, 0);
    glEnableVertexAttribArray(1);

    ErrorCheckValue = glGetError();
    if (ErrorCheckValue != GL_NO_ERROR)
    {
        fprintf(
            stderr,
            "ERROR: Could not create a VBO: %s \n",
            gluErrorString(ErrorCheckValue)
        );

        exit(-1);
    }
}

void DestroyVBO(void)
{
    GLenum ErrorCheckValue = glGetError();

    glDisableVertexAttribArray(1);
    glDisableVertexAttribArray(0);

    glBindBuffer(GL_ARRAY_BUFFER, 0);

    glDeleteBuffers(1, &ColorBufferId);
    glDeleteBuffers(1, &VboId);

    glBindVertexArray(0);
    glDeleteVertexArrays(1, &VaoId);

    ErrorCheckValue = glGetError();
    if (ErrorCheckValue != GL_NO_ERROR)
    {
        fprintf(
            stderr,
            "ERROR: Could not destroy the VBO: %s \n",
            gluErrorString(ErrorCheckValue)
        );

        exit(-1);
    }
}

void CreateShaders(void)
{
    GLenum ErrorCheckValue = glGetError();

    VertexShaderId = glCreateShader(GL_VERTEX_SHADER);
    glShaderSource(VertexShaderId, 1, &VertexShader, NULL);
    glCompileShader(VertexShaderId);

    FragmentShaderId = glCreateShader(GL_FRAGMENT_SHADER);
    glShaderSource(FragmentShaderId, 1, &FragmentShader, NULL);
    glCompileShader(FragmentShaderId);

    ProgramId = glCreateProgram();
        glAttachShader(ProgramId, VertexShaderId);
        glAttachShader(ProgramId, FragmentShaderId);
    glLinkProgram(ProgramId);
    glUseProgram(ProgramId);

    ErrorCheckValue = glGetError();
    if (ErrorCheckValue != GL_NO_ERROR)
    {
        fprintf(
            stderr,
            "ERROR: Could not create the shaders: %s \n",
            gluErrorString(ErrorCheckValue)
        );

        exit(-1);
    }
}

void DestroyShaders(void)
{
    GLenum ErrorCheckValue = glGetError();

    glUseProgram(0);

    glDetachShader(ProgramId, VertexShaderId);
    glDetachShader(ProgramId, FragmentShaderId);

    glDeleteShader(FragmentShaderId);
    glDeleteShader(VertexShaderId);

    glDeleteProgram(ProgramId);

    ErrorCheckValue = glGetError();
    if (ErrorCheckValue != GL_NO_ERROR)
    {
        fprintf(
            stderr,
            "ERROR: Could not destroy the shaders: %s \n",
            gluErrorString(ErrorCheckValue)
        );

        exit(-1);
    }
}

Answer 1

glutDisplayFunc, and all of the other callbacks, set callbacks for whatever window is current at the time the function is called. So if you want to set the display callback for a certain window, call glutDisplayFunc after calling glutSetWindow.

I figured out the problem that gldrawArrays requires to create new vbo to redraw. i dunno why.

That’s because each window represents a separate OpenGL context. Unless you explicitly share objects (via a platform-specific call), then each context will have its own, separate objects. You can’t use buffers created in one in another, without explicitly sharing (and doing so before creating objects in the new context).