Can't draw triangle using Opengl

Question

in this code i want to draw a simple triangle on a blue background using openGL however when i compile and run the code only a window with the blue background appears (without t

Answer 1

glClearColor(0, 0, 1,1);
...
fColor = vec4(0.0, 0.0, 1.0, 1.0);

You're trying to draw a blue triangle on top of a blue background. You'll have to dial up your contrast pretty high to see that :)

Make one of them a different color, like red.

You're also:

• Missing a VAO
• Not requesting a Core context
• Using #version 320 core instead of #version 150 core

Try this:

#include 
#include 
#include 
#include 

struct Program
{
    static GLuint Load( const char* vert, const char* geom, const char* frag )
    {
        GLuint prog = glCreateProgram();
        if( vert ) AttachShader( prog, GL_VERTEX_SHADER, vert );
        if( geom ) AttachShader( prog, GL_GEOMETRY_SHADER, geom );
        if( frag ) AttachShader( prog, GL_FRAGMENT_SHADER, frag );
        glLinkProgram( prog );
        CheckStatus( prog );
        return prog;
    }

private:
    static void CheckStatus( GLuint obj )
    {
        GLint status = GL_FALSE, len = 10;
        if( glIsShader(obj) )   glGetShaderiv( obj, GL_COMPILE_STATUS, &status );
        if( glIsProgram(obj) )  glGetProgramiv( obj, GL_LINK_STATUS, &status );
        if( status == GL_TRUE ) return;
        if( glIsShader(obj) )   glGetShaderiv( obj, GL_INFO_LOG_LENGTH, &len );
        if( glIsProgram(obj) )  glGetProgramiv( obj, GL_INFO_LOG_LENGTH, &len );
        std::vector< char > log( len, 'X' );
        if( glIsShader(obj) )   glGetShaderInfoLog( obj, len, NULL, &log[0] );
        if( glIsProgram(obj) )  glGetProgramInfoLog( obj, len, NULL, &log[0] );
        std::cerr << &log[0] << std::endl;
        exit( -1 );
    }

    static void AttachShader( GLuint program, GLenum type, const char* src )
    {
        GLuint shader = glCreateShader( type );
        glShaderSource( shader, 1, &src, NULL );
        glCompileShader( shader );
        CheckStatus( shader );
        glAttachShader( program, shader );
        glDeleteShader( shader );
    }
};

#define GLSL(version, shader) "#version " #version "\n" #shader

const char* vert = GLSL
( 
    150 core,
    layout(location = 0) in vec4 vPosition;
    void
    main()
    {
        gl_Position = vPosition;
    }
);

const char* frag = GLSL
( 
    150 core,
    out vec4 fColor;
    void
    main()
    {
        fColor = vec4(1.0, 0.0, 0.0, 1.0);
    }
);

// VAO & VBO objects
GLuint VAO;
GLuint VBO;
GLuint prog;
void init()
{
    // vertex Data (position)
    float vertex[] = { -1.0, -1.0 , 0.0,
                        1.0, -1.0 , 0.0,
                        0.0,  1.0 , 0.0 };

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

    glGenBuffers(1,&VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(vertex), vertex, GL_STATIC_DRAW);

    prog = Program::Load( vert, NULL, frag );
    glUseProgram( prog );

    glEnableVertexAttribArray(0);
    glVertexAttribPointer( 0, 3, GL_FLOAT, GL_FALSE, 0, 0 );
}

void display()
{
    glClearColor(0, 0, 1,1);
    glClear(GL_COLOR_BUFFER_BIT);

    // Drawing the triangle
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glutSwapBuffers();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize(1000, 400);
    glutInitWindowPosition(100, 100);
    glutInitContextVersion(3, 2);
    glutInitContextProfile(GLUT_CORE_PROFILE);
    glutCreateWindow("My First GLUT/OpenGL Window");

    glewExperimental = GL_TRUE;
    if( GLEW_OK != glewInit() )
        return -1;

    init();

    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}

EDIT: Ported to GL 2.1:

#include 
#include 
#include 
#include 

struct Program
{
    static GLuint Load( const char* vert, const char* geom, const char* frag )
    {
        GLuint prog = glCreateProgram();
        if( vert ) AttachShader( prog, GL_VERTEX_SHADER, vert );
        if( geom ) AttachShader( prog, GL_GEOMETRY_SHADER, geom );
        if( frag ) AttachShader( prog, GL_FRAGMENT_SHADER, frag );
        glLinkProgram( prog );
        CheckStatus( prog );
        return prog;
    }

private:
    static void CheckStatus( GLuint obj )
    {
        GLint status = GL_FALSE, len = 10;
        if( glIsShader(obj) )   glGetShaderiv( obj, GL_COMPILE_STATUS, &status );
        if( glIsProgram(obj) )  glGetProgramiv( obj, GL_LINK_STATUS, &status );
        if( status == GL_TRUE ) return;
        if( glIsShader(obj) )   glGetShaderiv( obj, GL_INFO_LOG_LENGTH, &len );
        if( glIsProgram(obj) )  glGetProgramiv( obj, GL_INFO_LOG_LENGTH, &len );
        std::vector< char > log( len, 'X' );
        if( glIsShader(obj) )   glGetShaderInfoLog( obj, len, NULL, &log[0] );
        if( glIsProgram(obj) )  glGetProgramInfoLog( obj, len, NULL, &log[0] );
        std::cerr << &log[0] << std::endl;
        exit( -1 );
    }

    static void AttachShader( GLuint program, GLenum type, const char* src )
    {
        GLuint shader = glCreateShader( type );
        glShaderSource( shader, 1, &src, NULL );
        glCompileShader( shader );
        CheckStatus( shader );
        glAttachShader( program, shader );
        glDeleteShader( shader );
    }
};

#define GLSL(version, shader) "#version " #version "\n" #shader

const char* vert = GLSL
( 
    120,
    attribute vec4 vPosition;
    void main()
    {
        gl_Position = vPosition;
    }
);

const char* frag = GLSL
( 
    120,
    void main()
    {
        gl_FragColor = vec4(1.0, 0.0, 0.0, 1.0);
    }
);

// VBO objects
GLuint VBO;
GLuint prog;
void init()
{
    // vertex Data (position)
    float vertex[] = { -1.0, -1.0 , 0.0,
                        1.0, -1.0 , 0.0,
                        0.0,  1.0 , 0.0 };

    glGenBuffers(1,&VBO);
    glBindBuffer(GL_ARRAY_BUFFER, VBO);
    glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(vertex), vertex, GL_STATIC_DRAW);

    prog = Program::Load( vert, NULL, frag );
    glUseProgram( prog );

    int posLoc = glGetAttribLocation( prog, "vPosition" );
    glEnableVertexAttribArray( posLoc );
    glVertexAttribPointer( posLoc, 3, GL_FLOAT, GL_FALSE, 0, 0 );
}

void display()
{
    glClearColor(0, 0, 1,1);
    glClear(GL_COLOR_BUFFER_BIT);

    // Drawing the triangle
    glDrawArrays(GL_TRIANGLES, 0, 3);
    glutSwapBuffers();
}

int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitWindowSize(1000, 400);
    glutInitWindowPosition(100, 100);
    glutCreateWindow("My First GLUT/OpenGL Window");

    if( GLEW_OK != glewInit() )
        return -1;

    init();

    glutDisplayFunc(display);
    glutMainLoop();
    return 0;
}