First example in OpenGL 4.0 Shading Language Cookbook

Question

I started to learn GLSL yesterday and followed the first example in OpenGL 4.0 Shading Language Cookbook to draw 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 create and compile the shader in createAndCompileShader and the status of the compilation is success.

And I draw a triangle in render function.

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 status of link is also success. However, there's nothing drawn in the window. I'm sure the function render is right.

Is there something wrong?

Answer 1

Is your triangle visible if you do not bind any GLSL shaders?

• try glDisable(GL_CULL_FACE);
• if it helps reorder the glVertex2f calls (different polygon winding)

What graphics card and driver you have?

• for OpenGL+GLSL+Windows
• Intel is almost unusable (especially for advanced things)
• ATI/AMD are usually almost OK these days (was much worst with old ATI drivers)
• nVidia are usually without any problems

You do not apply any projection or model view matrix In the vertex shader

• your modelview is identity so it does not matter
• but projection is not
• that means you are passing non transformed coordinates to the fragment shader
• OpenGL coordinates are usually in range <-1,+1>
• and your untransformed triangle does not cover that range
• so change the triangle coordinates to that range for example
• (-0.5,-0.5),(+0.5,-0.5),(0.0,+0.5)
• and try render (also you can temporarily rem the //gluOrtho2D(0, 100, 0, 100); just to be sure
• if it helps then that is the reason
• change your vertex shader to include: gl_Position = ftransform();
• In case of core profile is that not an option anymore
• so you should pass your transform matrices via uniform variables
• and multiply usually inside vertex shader
• see simple GLSL engine example
• it have texture,normal maping,3 lights,...
• and see Understanding homogenous 4x4 transform matrices

[edit1] you are using glVertex instead VAO so you should use compatibility profile

// vertex shader
#version 400 compatibility

out vec3 Color;

void main()
    {
    Color = gl_Color.rgb;
    gl_Position = ftransform();
    }

• that should do the trick ...