OpenGL Redbook loadShaders won't load shaders

Question

I am having trouble loading the basic fragment, and vertex shaders. I have tried to run the example code from the OpenGL Red Book for OpenGL 4.3, which works fine except the triangles aren't blue.

I have loadShaders.h and loadShaders.cpp in the same directory as my main.cpp directory.

I am linking the files like this: g++ main.cpp LoadShaders.cpp -lGL -lGLEW -lGLU -lglut -o run

my: main.cpp file, triangles.vert file and triangles.frag file all look exactly the same as the sample code in the book.

Someone help me solve this problem.

main.cpp:

#include<GL/glew.h>
#include<GL/freeglut.h>
#include<GL/gl.h>
#include<iostream>

#include"LoadShaders.h"
#include"vgl.h"
using namespace std;

enum VAO_IDs {Triangles, NumVAOs };
enum Buffer_IDs {ArrayBuffer, NumBuffers };
enum Attrib_IDs {vPosition = 0};

GLuint VAOs[NumVAOs];
GLuint Buffers[NumBuffers];

const GLuint NumVertices = 6;

void init(void)
{
    glGenVertexArrays(NumVAOs, VAOs);
    glBindVertexArray(VAOs[Triangles]);

    GLfloat vertices[NumVertices] [2] = {
        {-0.90, -0.90},
        {0.85, -0.90 },
        {-0.90, 0.85 },
        {0.90, -0.85 },
        {0.90, 0.90  },
        {-0.85, 0.90 }
    };

    glGenBuffers(NumBuffers, Buffers);
    glBindBuffer(GL_ARRAY_BUFFER, Buffers[ArrayBuffer]);
    glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);

    ShaderInfo shaders[]={
        {GL_VERTEX_SHADER, "triangles.vert"},
        {GL_FRAGMENT_SHADER, "triangles.frag"},
        {GL_NONE}
    };

    GLuint program = LoadShaders(shaders);
    glUseProgram(program);

    glVertexAttribPointer(vPosition, 2, GL_FLOAT,
                GL_FALSE, 0, 0);
    glEnableVertexAttribArray(vPosition);
}

void display(void)
{
    glClear(GL_COLOR_BUFFER_BIT);
    glBindVertexArray(VAOs[Triangles]);
    glDrawArrays(GL_TRIANGLES, 0, NumVertices);

    glFlush();
}
int main(int argc, char** argv)
{
    glutInit(&argc, argv);
    glutInitDisplayMode(GLUT_RGBA);
    glutInitWindowSize(512,512);
    glutInitContextProfile(GLUT_CORE_PROFILE);
    glutCreateWindow(argv[0]);

    if (glewInit()) {
        cerr << "Unable to initialize GLEW"<<endl;
        exit(EXIT_FAILURE);
    }

    init();

    glutDisplayFunc(display);

    glutMainLoop();
}

triangles.vert:

#version 430 core

layout(location = 0) in vec4 vPosition;

void main()
{
    gl_position = vPosition;
}

triangles.frag:

#version 430 core

out vec4 fColor;

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

Answer 1

• GLSL is case-sensitive: gl_position != gl_Position
• Request a context version (via glutInitContextVersion()) that meets or exceeds the GLSL version your shaders require
• Set glewExperimental to GL_TRUE before glewInit()

This works on my (OpenGL 4.2) system:

#include <GL/glew.h>
#include <GL/freeglut.h>
#include <iostream>

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;
        if( glIsShader(obj) ) glGetShaderiv( obj, GL_COMPILE_STATUS, &status );
        if( glIsProgram(obj) ) glGetProgramiv( obj, GL_LINK_STATUS, &status );
        if( status == GL_TRUE ) return;
        GLchar log[ 1 << 15 ] = { 0 };
        if( glIsShader(obj) ) glGetShaderInfoLog( obj, sizeof(log), NULL, log );
        if( glIsProgram(obj) ) glGetProgramInfoLog( obj, sizeof(log), NULL, log );
        std::cerr << log << 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
(
    420 core,
    layout(location = 0) in vec4 vPosition;
    void main()
    {
        gl_Position = vPosition;
    }
);

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

GLuint vao = 0;
unsigned int numVerts = 0;
void init(void)
{
    glGenVertexArrays( 1 , &vao );
    glBindVertexArray( vao );

    GLfloat vertices[] = 
    {
        -0.90, -0.90,
         0.85, -0.90,
        -0.90,  0.85,
         0.90, -0.85,
         0.90,  0.90,
        -0.85,  0.90, 
    };
    numVerts = sizeof( vertices ) / sizeof( GLfloat ) / 2;

    GLuint vbo = 0;
    glGenBuffers( 1, &vbo );
    glBindBuffer( GL_ARRAY_BUFFER, vbo );
    glBufferData( GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW );

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

    glVertexAttribPointer( 0, 2, GL_FLOAT, GL_FALSE, 0, 0 );
    glEnableVertexAttribArray( 0 );
}

void display()
{
    glClear( GL_COLOR_BUFFER_BIT );
    glBindVertexArray( vao );
    glDrawArrays( GL_TRIANGLES, 0, numVerts );
    glutSwapBuffers();
}

int main( int argc, char **argv )
{
    glutInitWindowSize( 640, 480 );
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitContextVersion( 4, 2 );
    glutInitContextProfile( GLUT_CORE_PROFILE );
    glutCreateWindow( "GLUT" );
    glewExperimental = GL_TRUE;
    glewInit();
    init();
    glutDisplayFunc( display );
    glutMainLoop();
    return 0;
}

Version tester:

#include <GL/glew.h>
#include <GL/freeglut.h>
#include <iostream>

int main( int argc, char **argv )
{
    glutInitWindowSize( 640, 480 );
    glutInit( &argc, argv );
    glutInitDisplayMode( GLUT_RGBA | GLUT_DOUBLE );
    glutInitContextProfile( GLUT_CORE_PROFILE );
    glutCreateWindow( "GLUT" );
    glewExperimental = GL_TRUE;
    glewInit();
    std::cout << glGetString( GL_VERSION ) << std::endl;
    return 0;
}

Answer 2

Wipe out main's call to glutInitContextProfile. That and glutInitContextVersion(() gave me trouble and I was able to do without them by accepting the default profiles, I suppose. Then add this to the top of your fragment shader:

#version 300 es
precision mediump float;

... and your vertex shader will need to match it's version with this at the top:

#version 300 es

Also, be careful with the shader paths. I had to tweak mine with

ShaderInfo shaders[] = {
{ GL_VERTEX_SHADER, "../../triangles.vert" },
{ GL_FRAGMENT_SHADER, "../../triangles.frag" },
{ GL_NONE, NULL }
};