OpenGL Does Not Render Triangle - qt

Question

I am following this tutorial with a few modifications and have got this code:
#define GLSL(src) "#version 330 core\n" #src
void MainWindow::initializeGL() {
glClearColor(0, 0, 0, 1);
// Generate buffers
GLfloat verticies[] = {
+0.0f, +1.0f, +0.0f,
-1.0f, -1.0f, +0.0f,
+1.0f, -1.0f, +0.0f,
};
GLuint vertexBufferID;
glGenBuffers(1, &vertexBufferID);
glBindBuffer(GL_ARRAY_BUFFER, vertexBufferID);
glBufferData(GL_ARRAY_BUFFER, sizeof(verticies), verticies, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void *)0);
// Generate shaders
const char *vertexShaderSrc = GLSL(
layout(location = 0) in vec3 pos;
void main() {
gl_Position.xyz = pos;
gl_Position.w = 1.0;
}
);
GLuint vertexShaderID = createGLShader(GL_VERTEX_SHADER, vertexShaderSrc);
const GLchar *fragmentShaderSrc = GLSL(
out vec4 color;
void main() {
color = vec4(0.0, 1.0, 0.0, 1.0);
}
);
GLuint fragmentShaderID = createGLShader(GL_FRAGMENT_SHADER, fragmentShaderSrc);
GLuint programID = glCreateProgram();
glAttachShader(programID, vertexShaderID);
glAttachShader(programID, fragmentShaderID);
glLinkProgram(programID);
glUseProgram(programID);
}
void MainWindow::paintGL() {
//glViewport(0, 0, width(), height());
glClear(GL_COLOR_BUFFER_BIT);
glDrawArrays(GL_TRIANGLES, 0, 3);
//glDrawElements(GL_TRIANGLES, 6, GL_UNSIGNED_SHORT, 0);
}
GLuint MainWindow::createGLShader(GLenum type, const GLchar* src) {
GLuint shaderID = glCreateShader(type);
glShaderSource(shaderID, 1, &src, 0);
glCompileShader(shaderID);
GLint vertexCompileStatus;
glGetShaderiv(shaderID, GL_COMPILE_STATUS, &vertexCompileStatus);
if (vertexCompileStatus != GL_TRUE) {
GLint infoLogLength;
glGetShaderiv(shaderID, GL_INFO_LOG_LENGTH, &infoLogLength);
GLchar buffer[infoLogLength];
glGetShaderInfoLog(shaderID, infoLogLength, 0, buffer);
qDebug(buffer);
}
return shaderID;
}
This is all contained in a QGLWidget. However when I run this code I just get a black screen. What is going wrong? I don't get an error message so the shaders are compiling.
I set up the QGLWidget:
#include "mainwindow.h"
#include <QApplication>
#include <QGLFormat>
int main(int argc, char *argv[]) {
QApplication a(argc, argv);
QGLFormat glFormat;
glFormat.setVersion(3, 3);
glFormat.setProfile(QGLFormat::CoreProfile);
MainWindow w(glFormat);
w.show();
return a.exec();
}

Answer 1

Staying with "pure" OpenGL code, you need (at least) a Vertex Array Object. That object needs to be bound when you configure the vertex arrays, and everytime you draw from the aforementioned arrays.
So, before the calls to gl*VertexAttribArray, create and bind the VAO. Add a
GLuint m_vao;
member to your class. Then in initializeGL:
glGenVertexArrays(1, &m_vao);
glBindVertexArray(m_vao);
// now configure the arrays:
glEnableVertexAttribArray...
glVertexAttribArray...
// now release the VAO and move on
glBindVertexArray(0);
Then in paintGL we need the VAO again:
glBindVertexArray(m_vao);
glDrawArrays(...);
glBindVertexArray(0);
And now your code with Qt 5 OpenGL enablers (didn't try to compile it, but you can get the idea). You tell me which one is more readable and less error prone.
#define GLSL(src) "#version 330 core\n" #src
void MainWindow::initializeGL() {
glClearColor(0, 0, 0, 1);
// Generate buffers
GLfloat verticies[] = {
+0.0f, +1.0f, +0.0f,
-1.0f, -1.0f, +0.0f,
+1.0f, -1.0f, +0.0f,
};
m_vertexBuffer = new QOpenGLBuffer(QOpenGLBuffer::VertexBuffer);
m_vertexBuffer->create();
m_vertexBuffer->setusagePatter(QOpenGLBuffer::StaticDraw);
m_vertexBuffer->bind();
m_vertexBuffer->allocate(verticies, sizeof(verticies);
m_vertexBuffer->release();
// Generate shaders
const char *vertexShaderSrc = GLSL(
layout(location = 0) in vec3 pos;
void main() {
gl_Position.xyz = pos;
gl_Position.w = 1.0;
}
);
const GLchar *fragmentShaderSrc = GLSL(
out vec4 color;
void main() {
color = vec4(0.0, 1.0, 0.0, 1.0);
}
);
m_program = new QOpenGLShaderProgram;
m_program->addShaderFromSourceCode(QOpenGLShader::Vertex, vertexShaderSrc);
m_program->addShaderFromSourceCode(QOpenGLShader::Fragment, fragmentShaderSrc);
m_program->link();
// error checking missing from the last three calls. if they return false, check log()
m_vao = new QOpenGLVertexArrayObject;
m_vao->bind();
m_program->bind();
m_vertexBuffer->bind();
m_program->enableAttributeArray("pos");
m_program->setAttributeBuffer("pos", GL_FLOAT, 0, 3);
m_vertexBuffer->release();
m_program->release();
m_vao->release();
}
void MainWindow::paintGL() {
glClear(GL_COLOR_BUFFER_BIT);
m_vao->bind();
m_program->bind();
glDrawArrays(GL_TRIANGLES, 0, 3);
m_program->release();
m_vao->release();
}