glDrawArrays GL_LINES not drawing lines correctly

Question

I am trying to draw a single line with the code below and it works :

// Include standard headers
#include <stdio.h>
#include <stdlib.h>

// Include GLEW
#include <GL/glew.h>

// Include GLFW
#include <GL/glfw.h>

// Include GLM
#include <glm/glm.hpp>
using namespace glm;

// shaders
#include "shader.hpp"

int main( void )
{
    // Initialise GLFW
    if( !glfwInit() )
    {
        fprintf( stderr, "Failed to initialize GLFW\n" );
        return -1;
    }

    glfwOpenWindowHint(GLFW_FSAA_SAMPLES, 4);
    glfwOpenWindowHint(GLFW_OPENGL_VERSION_MAJOR, 3);
    glfwOpenWindowHint(GLFW_OPENGL_VERSION_MINOR, 3);
    glfwOpenWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);

    // Open a window and create its OpenGL context
    if( !glfwOpenWindow( 1024, 768, 0,0,0,0, 32,0, GLFW_WINDOW ) )
    {
        fprintf( stderr, "Failed to open GLFW window. If you have an Intel GPU, they are not 3.3 compatible. Try the 2.1 version of the tutorials.\n" );
        glfwTerminate();
        return -1;
    }

    glewExperimental=GL_TRUE;

    // Initialize GLEW
    if (glewInit() != GLEW_OK) {
        fprintf(stderr, "Failed to initialize GLEW\n");
        return -1;
    }

    glfwSetWindowTitle( "Tutorial 02" );

    // Ensure we can capture the escape key being pressed below
    glfwEnable( GLFW_STICKY_KEYS );

    // Dark blue background
    glClearColor(0.0f, 0.0f, 0.3f, 0.0f);

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

    // Create and compile our GLSL program from the shaders
    GLuint programID = LoadShaders( "SimpleVertexShader.vertexshader",
                                    "SimpleFragmentShader.fragmentshader" );

    static const GLfloat g_vertex_buffer_data[] = {
                0.0f,  1.0f, 0.0f,
                0.0f,  0.0f, 0.0f
    };

    GLuint vertexbuffer;
    glGenBuffers(1, &vertexbuffer);
    glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
    glBufferData(GL_ARRAY_BUFFER, sizeof(g_vertex_buffer_data), g_vertex_buffer_data, GL_STATIC_DRAW);

    do{

        // Clear the screen
        glClear( GL_COLOR_BUFFER_BIT );

        // Use our shader
        glUseProgram(programID);

        // 1rst attribute buffer : vertices
        glEnableVertexAttribArray(0);
        glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);

        glVertexAttribPointer(
            0,                  // attribute 0. No particular reason for 0, but must match the layout in the shader.
            2,                  // size
            GL_FLOAT,           // type
            GL_FALSE,           // normalized?
            0,                  // stride
            (void*)0            // array buffer offset
        );

        // Draw the line !
        glDrawArrays(GL_LINES, 0, 2); // 2 indices for the 2 end points of 1 line


        glDisableVertexAttribArray(0);

        // Swap buffers
        glfwSwapBuffers();

    } // Check if the ESC key was pressed or the window was closed
    while( glfwGetKey( GLFW_KEY_ESC ) != GLFW_PRESS &&
           glfwGetWindowParam( GLFW_OPENED ) );

    // Close OpenGL window and terminate GLFW
    glfwTerminate();

    // Cleanup VBO
    glDeleteBuffers(1, &vertexbuffer);
    glDeleteVertexArrays(1, &VertexArrayID);

    return 0;
}

I see a line from the middle of the screen to the top middle. However when I change the order of vertices in g_vertex_buffer_data[] to :

static const GLfloat g_vertex_buffer_data[] = {
                0.0f,  0.0f, 0.0f,
                0.0f,  1.0f, 0.0f
    };

I just see a blue screen with no lines! Nowhere in the opengl docs is there any mention of a particular order of vertices for GL_LINES so this should work too.

Answer 1

You define your vertices like this:

glVertexAttribPointer(
            0,                  // attribute 0. No particular reason for 0, but must match the layout in the shader.
            2,                  // size
            GL_FLOAT,           // type
            GL_FALSE,           // normalized?
            0,                  // stride
            (void*)0            // array buffer offset
        );

i.e. 2 floats per vertex and no stride. So if you render only two vertices, it only reads 4 floats. When you swap the verts over, the first 4 floats are all zero, and you get two identical positions, and thus no line.