First example in OpenGL 4.0.

I started learning GLSL yesterday and followed the first example in the OpenGL 4.0 Shading Language Cookbook for drawing a triangle step by step.

Here are my codes:

1.shader.vert

#version 400

in vec3 VertexPosition;
in vec3 VertexColor;

out vec3 Color;

void main()
{
    Color = VertexColor;
    gl_Position = vec4(VertexPosition, 1.0);
}

      

        
        
        
      

    

2.shader.frag

#version 400

in vec3 Color;
out vec4 FragColor;

void main(){
    FragColor = vec4(Color, 1.0);
}

      

        
        
        
      

    

3.main.cpp

int main(int argc, char *argv[])
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA | GLUT_DOUBLE);
    glutInitWindowSize(500, 500);
    glutCreateWindow("Project1");

    glutDisplayFunc(render);

    glMatrixMode(GL_PROJECTION);
    glLoadIdentity();
    gluOrtho2D(0, 100, 0, 100);
    glMatrixMode(GL_MODELVIEW);
    glLoadIdentity();
    glEnable(GL_POINT_SMOOTH);

    // init glew
    GLenum err = glewInit();
    if (GLEW_OK != err){
        printf("Error: %s\n", glewGetErrorString(err));
    }
    else{
        printf("OK: glew init.\n");
    }

    // check gl version
    const GLubyte *renderer = glGetString(GL_RENDERER);
    const GLubyte *vendor = glGetString(GL_VENDOR);
    const GLubyte *version = glGetString(GL_VERSION);
    const GLubyte *glslVersion = glGetString(GL_SHADING_LANGUAGE_VERSION);
    GLint major, minor;
    glGetIntegerv(GL_MAJOR_VERSION, &major);
    glGetIntegerv(GL_MINOR_VERSION, &minor);
    printf("GL Vendor   : %s\n", vendor);
    printf("GL Renderer : %s\n", renderer);
    printf("GL Version (string) : %s\n", version);
    printf("GL Version (integer): %d.%d\n", major, minor);
    printf("GLSL Version: %s\n", glslVersion);

    // vertex shader
    GLuint vertShader = createAndCompileShader("shader.vert", VERTEX);

    // fragment shader
    GLuint fragShader = createAndCompileShader("shader.frag", FRAGMENT);

    // program
    GLuint programHandle = glCreateProgram();
    if (programHandle == 0)
    {
        printf("Error creating program object.\n");
    }

    glAttachShader(programHandle, vertShader);
    glAttachShader(programHandle, fragShader);

    glLinkProgram(programHandle);

    GLint status;
    glGetProgramiv(programHandle, GL_LINK_STATUS, &status);
    if (GL_FALSE == status){
        printf("Failed to link shader program");
    }
    else{
        printf("OK\n");
        glUseProgram(programHandle);
    }

    glutMainLoop();


    return EXIT_SUCCESS;
}

      

        
        
        
      

    

I am building and compiling a shader in createAndCompileShader

and the compilation state is successful.

And I draw a triangle in render

.

void render()
{
    glLoadIdentity();
    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
    glPushMatrix();

    glBegin(GL_TRIANGLES);

    glColor3f(1.0, 0.0, 0.0);
    glVertex2f(20, 20);
    glColor3f(0.0, 1.0, 0.0);
    glVertex2f(80, 20);
    glColor3f(0.0, 0.0, 1.0);
    glVertex2f(50, 80);

    glEnd();

    glPopMatrix();

    glutSwapBuffers();
}

      

        
        
        
      

    

The link status is also successful. However, there was nothing in the window. I'm pretty sure the function is render

correct.

Something is wrong?