Qt FrameBuffer Object don't render even vertices - c++

Question

I'm learning to use OpenGL in Qt with the QOpenGLFramebufferObject， and tried to draw a triangle using the following code:
In render() :
glUseProgram(m_program);
GLfloat vertices[] = {
-1.0f, -1.0f, // first
0.0f, -1.0f, // second
0.0f, 1.0f // third
};
unsigned int VAO;
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), vertices);
glBindVertexArray(0);
glDisable(GL_DEPTH_TEST);
glClearColor(0.2f, 0.3f, 0.3f, 1.0f);
glClear(GL_COLOR_BUFFER_BIT);
glBindVertexArray(VAO);
glDrawArrays(GL_TRIANGLES, 0, 3);
And the shaders are in initShader() :
const GLchar* vfSource[] = {
"#version 330 core\n"
"layout (location = 0) in vec2 aPos;\n"
"void main()\n"
"{\n"
" gl_Position = vec4(aPos.x, aPos.y, 0.0, 1.0);\n"
"}\n\0"
};
const GLchar* fsSource[] = {
"#version 330 core\n"
"out vec4 FragColor;\n"
"void main()\n"
"{\n"
" FragColor = vec4(1.0f, 0.5f, 0.2f, 1.0f);\n"
"}\n\0"
};
Only the first and third vertex rendered correctly. The second vertex were located in the center of my screen.
And if I changed the vertices[] to
-1.0f, -1.0f, // 1
0.0f, -1.0f, // 2
0.0f, 1.0f // 3
1.0f, 1.0f, // 4
1.0f, 0.0f // 5
and the last line to
glDrawArrays(GL_TRIANGLES, 0, 5);
The output is a triangle produced by the data in line 1 3 and 5.
I have no idea about what's wrong with this code. Anyone can help me?
If more code is needed, just let me know.

Answer 1

The OpenGL 3.3 Core Profile (which your shaders are targeting) doesn't allow you to draw directly from client memory. The last parameter of glVertexAttribPointer is a offset into the currently bound GL_ARRAY_BUFFER. Setting it to something else than zero should trigger a GL_INVALID_OPERATION when no GL_ARRAY_BUFFER is bound.
In order to get your example working your need to generate a Vertex Buffer Object (VBO) and attach it to the VAO.
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
glGenBuffers(1, &vbo);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 2, GL_FLOAT, GL_FALSE, 2 * sizeof(float), 0);
glBindVertexArray(0);