OpenGL mix fix pipeline and shader program (Qt) - c++

Question

I'm working on a old code that used fixed function pipeline, the scene is a bit complex but works fine. For the sake of simplicity, I replaced it with one blue triangle :
void RenduOpenGL::paintGL()
{
glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT | GL_STENCIL_BUFFER_BIT);
glViewport(0, 0, this->width(), this->height());
glBegin(GL_TRIANGLES);
glColor3d(0,0,1);
glVertex3d(0.7, 0.7, 0.0);
glVertex3d(-0.5, 0.7, 0.0);
glVertex3d(0.1, -0.7, 0.0);
glEnd();
}
Now I want to add shaders for new elements in the scene but keep the old elements of the scene like this blue triangle.
I've read here that I can mix the two to produce a scene containing the first then the second.
Therefore I want to add this code after the blue triangle :
float vertices[] = {
0.6, 0.6, 0.0,
-0.6, 0.6, 0.0,
0.0, -0.6, 0.0,
};
vbo.create(); // glGenBuffers(...);
vbo.bind(); // glBindBuffer(GL_ARRAY_BUFFER, vbo);
vbo.allocate(vertices, sizeof(vertices)); // glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(float), vertices, GL_STATIC_DRAW);
vbo.release(); // glBindBuffer(GL_ARRAY_BUFFER, 0);
prog.addShaderFromSourceFile(QOpenGLShader::Vertex, "shaders/base.vert");
prog.addShaderFromSourceFile(QOpenGLShader::Fragment, "shaders/base.frag");
vao.create(); // glGenVertexArrays(...)
vao.bind(); // glBindVertexArray(vao);
prog.enableAttributeArray("position"); // glEnableVertexAttribArray(VAO_position);
prog.setAttributeBuffer("position", GL_FLOAT, 0, 3); // (offset, size, stride=0); // glVertexAttribPointer(VAO_position, 4, GL_FLOAT, False, 0, reinterpret_cast<const void *>(offset)(0)); (False,
vao.release(); // glBindVertexArray(0);
// draw the triangle
prog.bind(); // glUseProgram(shader_program);
vao.bind(); // glBindVertexArray(vertex_array_object);
glDrawArrays(GL_TRIANGLES, 0, 3);
vao.release(); // glBindVertexArray(0);
prog.release(); // glUseProgram(0);
I use Qt to call the openGL functions, the corresponding opengl functions are in comments.
My shaders are very basic :
// base.vert
#version 330
// vertex shader
in vec3 position;
void main() {
gl_Position = vec4(position.xyz, 1);
}
// base.frag
#version 330
// fragment shader
out vec4 pixel;
void main() {
pixel = vec4(1, 0.5, 0, 1);
}
That is supposed to draw an orange triangle, but when I put the code after the blue triangle code, I don't see the orange triangle created from shaders.

Answer 1

Short (with code) answer:
The VBO and the prog.enableAttributeArray and prog.setAttributeBuffer should be in the VAO.
Something along the lines:
float vertices[] = {
0.6, 0.6, 0.0,
-0.6, 0.6, 0.0,
0.0, -0.6, 0.0,
};
prog.bind(); // glUseProgram(shader_program);
vao.create(); // glGenVertexArrays(...)
vao.bind(); // glBindVertexArray(vao);
vbo.create(); // glGenBuffers(...);
vbo.bind(); // glBindBuffer(GL_ARRAY_BUFFER, vbo);
vbo.allocate(vertices, sizeof(vertices)); // glBufferData(GL_ARRAY_BUFFER, 9 * sizeof(float), vertices, GL_STATIC_DRAW);
//vbo.release(); // glBindBuffer(GL_ARRAY_BUFFER, 0);
prog.addShaderFromSourceFile(QOpenGLShader::Vertex, "shaders/base.vert");
prog.addShaderFromSourceFile(QOpenGLShader::Fragment, "shaders/base.frag");
prog.enableAttributeArray("position"); // glEnableVertexAttribArray(VAO_position);
prog.setAttributeBuffer("position", GL_FLOAT, 0, 3); // (offset, size, stride=0); // glVertexAttribPointer(VAO_position, 4, GL_FLOAT, False, 0, reinterpret_cast<const void *>(offset)(0)); (False,
vao.release(); // glBindVertexArray(0);
// draw the triangle
prog.bind(); // glUseProgram(shader_program);
vao.bind(); // glBindVertexArray(vertex_array_object);
glDrawArrays(GL_TRIANGLES, 0, 3);
vao.release(); // glBindVertexArray(0);
prog.release(); // glUseProgram(0);
Not so long but textual answer: OpenGL is a state machine, you need to link together: the VBO and how to read its data, inside the VAO. However, IMHO, Qt people have sadly chosen their abstractions poorly: enableAttributeArray and setAttributeBuffer would be clearer as members of the VAO class instead of the prog class.