Relating vertex buffers and vertex attributes in openGL - linux

Question

My entire code that is supposed to draw a triangle on the screen is:
#include <iostream>
#include <GL/glew.h>
#include <GLFW/glfw3.h>
#include <string.h>
const GLint WIDTH=800, HEIGHT=600;
GLuint VAO, VBO, shader;
//Vertex Shader
/*static const char**/
const GLchar* vShader = "\n"
"\n"
"#version 330 \n"
"layout (location=0) in vec3 pos;\n"
"void main(){\n"
"gl_Position = vec4(pos.x,pos.y,pos.z,1.0);\n"
"\n"
"}\n"
"";
// fragment shader
const GLchar* fShader = "\n"
"#version 330 \n"
"out vec4 colour;\n"
"void main(){\n"
"colour = vec4(1.0, 0.0, 0.0, 1.0);\n"
"}\n"
"\n"
"\n";
void CreateTriangle(){
GLfloat vertices[] = {
-1.0f, -1.0f, 0.0f,
1.0f, -1.0f, 0.0f,
0.0f, 1.0f, 0.0f
};
// vertex arrays
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
// vertex buffers
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER,VBO);
glBufferData(GL_ARRAY_BUFFER,sizeof(GLfloat)*9,vertices,GL_STATIC_DRAW);
glVertexAttribPointer(0,3, GL_FLOAT,GL_FALSE,0, 0);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
}
void AddShader(GLuint theProgram, const GLchar* shaderCode, GLenum shaderType){
GLuint theShader = glCreateShader(shaderType);
const GLchar* theCode[1];
theCode[0] = shaderCode;
GLint codeLength[1];
codeLength[0] = strlen(shaderCode);
glShaderSource(theShader, 1, theCode, codeLength);
glCompileShader(theShader);
GLint result=0;
GLchar eLog[1024]={};
glGetShaderiv(theShader, GL_COMPILE_STATUS, &result);
if(!result){
glGetShaderInfoLog(theShader,sizeof(eLog),NULL, eLog);
std::cout<< "Error compiling"<<shaderType<<" "<<eLog <<std::endl;
return;
}
glAttachShader(theProgram,theShader);
}
void CompileShader(){
shader = glCreateProgram();
if(!shader){
std::cout<<"Error Creating Shader Program";
return;
}
AddShader(shader, vShader,GL_VERTEX_SHADER);
AddShader(shader, fShader,GL_FRAGMENT_SHADER);
// getting error codes
GLint result=0;
GLchar eLog[1024]={0};
// Creates the executables in the graphic card
glLinkProgram(shader);
// get information if program is linked properly
glGetProgramiv(shader, GL_LINK_STATUS, &result);
if(!result){
glGetProgramInfoLog(shader,sizeof(eLog),NULL,eLog);
std::cout<<"Error linking program"<<eLog<<std::endl;
return;
}
glValidateProgram(shader);
glGetProgramiv(shader,GL_VALIDATE_STATUS,&result);
if(!result){
glGetProgramInfoLog(shader, sizeof(eLog),NULL, eLog);
std::cout<<"Error validating program"<<eLog<<std::endl;
return;
}
}
int main(void){
if(!glfwInit()){
std::cout << "glfw initialization failed" << std::endl;
glfwTerminate();
return 1;
}
// glfwWindowHint(GLFW_CONTEXT_VERSION_MAJOR,3);
// glfwWindowHint(GLFW_CONTEXT_VERSION_MINOR,3);
// glfwWindowHint(GLFW_OPENGL_PROFILE, GLFW_OPENGL_CORE_PROFILE);
// glfwWindowHint(GLFW_OPENGL_FORWARD_COMPAT, GL_TRUE);
GLFWwindow *mainWindow = glfwCreateWindow(WIDTH, HEIGHT, "NEW WINDOW", NULL, NULL);
if(!mainWindow){
std::cout<< "Window creation failed" <<std::endl;
glfwTerminate();
return 1;
}
int bufferWidth, bufferHeight;
glfwGetFramebufferSize(mainWindow, &bufferWidth, &bufferHeight);
glfwMakeContextCurrent(mainWindow);
if(glewInit() != GLEW_OK){
std::cout << "GLEW Initialization failed" << std::endl;
glfwDestroyWindow(mainWindow);
glfwTerminate();
return 1;
}
glViewport(0,0,bufferWidth, bufferHeight);
CreateTriangle();
CompileShader();
while(!glfwWindowShouldClose(mainWindow)){
glfwPollEvents();
glUseProgram(shader);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES,0,3);
glBindVertexArray(0);
glUseProgram(0);
glfwSwapBuffers(mainWindow);
std::cout<<"something"<<std::endl;
}
return 0;
}
It essentially draws a black screen and there's no error whatsoever but is supposed to draw a red triangle so I'm trying to debug this code and there is essentially some parts in the code that I don't understand.
1) How does the VBO (Vertex Buffer Object) relate to the VAO (Vertex Attribute object), we basically defined these using the following inside of the CreateTriangles() function:
...
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
// vertex buffers
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER,VBO);
glBufferData(GL_ARRAY_BUFFER,sizeof(GLfloat)*9,vertices,GL_STATIC_DRAW);
glVertexAttribPointer(0,3, GL_FLOAT,GL_FALSE,0, 0);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
...
Note that we already unbind both the VAO and VBO, but during the drawing call inside the while loop:
while(!glfwWindowShouldClose(mainWindow)){
glfwPollEvents();
glUseProgram(shader);
glClearColor(0.0f, 0.0f, 0.0f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES,0,3);
glBindVertexArray(0);
glUseProgram(0);
glfwSwapBuffers(mainWindow);
std::cout<<"something"<<std::endl;
}
we only rebind the VAO and not the VBO, which I think may be the result of the error, but I don't know for sure.
Also, the tutorial says that the VBO is bound inside the VAO but I don't see there being any linking or anything that related VBO to the VAO in the code, so I'm really being confused on how is it that we're binding them together and why we're only binding back the VAO and not the VBO during the drawing stage?
I'm using Linux OS and used the following to compile:
g++ -std=c++17 main.cpp -o main -lGL -lGLEW -lglfw && ./main

Answer 1

From comments above, credit to #Rabbid76:
When glVertexAttribPointer is called than the a name reference of the current VBO is stored in the current VAO. The current VAO is bound by glBindVertexArray(VAO); and the current VBO is bound by glBindBuffer(GL_ARRAY_BUFFER,VBO);. glVertexAttribPointer associates VBO to the resource index 0 in VAO. This association is stored in the state vector of VAO. So it is sufficient to bind VAO(glBindVertexArray(VAO)) before the draw call. Hence, only binding the VAO is sufficient.
As for the problem stated in the comments regarding the black screen try
Updating the glew version and your graphic card drive. If updating glew is not possible, then just set the glExperimental=GL_TRUE.