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.
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.