Question

Trying to get a very simple opengl/glut/glew program up and running. Currently the display() function passed to glutDisplayFunc() is not being called. When executed, init() sets everything up and gives me a blank white window, but display never fills it with the generated points. Here is the init() function which is called just before entering the glutMainLoop():

void init()
{

    //generate points
    const int NumPoints = 5000;
    point3 points[NumPoints];

    point3 vertices[3] = {point3(-1.0, -1.0, 0.0),
                          point3(0.0, 1.0, 0.0),
                          point3(1.0, -1.0, 0.0)};


    points[0] = point3(0.25, 0.5, 0.0);

    for(int k = 1; k < NumPoints; k++)
    {
        int j = rand() % 3;
        points[k] = (points[k-1]+vertices[j])/2.0;
    }

    //load shaders and use the resulting shader program
    GLuint program = InitShader("shaders/vshader.glsl", "shaders/fshader.glsl");

    GLint linked;
    glGetProgramiv( program, GL_LINK_STATUS, &linked );
    if( !linked ){
        std::cerr << "Shader program failed to link" << std::endl;
        GLint logSize;
        glGetProgramiv( program, GL_INFO_LOG_LENGTH, &logSize);
        char *logMsg = new char[logSize];
        glGetProgramInfoLog( program, logSize, NULL, logMsg);
        std::cerr << logMsg << std::endl;
        delete [] logMsg;

        exit( EXIT_FAILURE );
    }

    glUseProgram( program );

    //create Vertex-Array object
    GLuint aBuffer;

    glGenVertexArrays(1, &aBuffer);
    glBindVertexArray((GLuint)&aBuffer);

    //create Buffer object
    GLuint buffer;

    //glGenBuffers(1, &buffer);
    glGenBuffers(1, &buffer);
    glBindBuffer(GL_ARRAY_BUFFER, buffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(points),
                 points, GL_STATIC_DRAW);

    //initialize the vertex position attribute from the vertex shader
    GLuint loc = glGetAttribLocation( program, "vPosition");
    glEnableVertexAttribArray( loc );
    glVertexAttribPointer( loc, 3, GL_FLOAT, GL_FALSE, 0, 0);

    glClearColor( 1.0, 1.0, 1.0, 1.0);  // white background


}

and here are the main() and display() functions:

void display()
{
    fprintf(stderr, "display called!\n");
    glClear(GL_COLOR_BUFFER_BIT);
    glDrawArrays(GL_POINTS, 0, 5000);
    glFlush();
}

int main(int argc, char **argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_SINGLE | GLUT_RGBA);
    glutInitWindowSize(500, 500);
    glutInitWindowPosition(100, 100);
    glutDisplayFunc(display);
    glutCreateWindow("Program 1");
    glewInit();

    init();

    glutMainLoop();
    return 0;
}

using eclipse CDT C/C++ with MinGW. Debugging shows that glutMainLoop is indeed being called but I can’t follow it past that. Could it be a shader issue? They are reportedly compiling and linking fine, but here they are

vshader.glsl

#version 150

in vec4 vPosition;

void main() {
    gl_Position = vPosition;
}

.

fshader.glsl

#version 150

out vec4 fColor;

void main() {
    fColor = vec4(1.0, 0.0, 0.0, 1.0);
}

edit: the shaders definitely work. By inserting glutIdleFunc(display); into the main(), the program executes properly and draws all the expected points. So like I originally thought, for some reason glutMainLoop() just doesnt want to call the function passed to glutDisplayFunc()? Or am I doing something terribly wrong?

Answer 1

GLUT can create multiple windows. glutDisplayFunc operates on the currently active window, so you must call glutCreateWindow before the glut…Func functions.