Opengl: How to map a buffer CORRECTLY? - c++

Question

I'm new to stack overflow. I have the following problem: I want to have a rotating triangle in my window.
Now, I have already managed to have my code running and I had a rotating triangle. However, I wanted to rewrite the code for learning purposes, adding the two following major things:
Updating the buffer object later on with memcpy
Having an array-of-stuctures (AoS) instead of a structure-of-arrays (SoA)
I'm referring hereby to the "OpenGL Superbible" book.
I'll provide you some code snippets:
glGenVertexArrays(1, &vao);
static const vertex vertices[] = {
{ 0.25, -0.25, 0.5, 1.0 ,
1.0, 0.0, 0.0, 1.0},
{ -0.25, -0.25, 0.5, 1.0,
0.0, 1.0, 0.0, 1.0 },
{ 0.25, 0.25, 0.5, 1.0,
0.0, 0.0, 1.0, 1.0 }
};
glCreateBuffers(1, &buffer);
glNamedBufferStorage(buffer, sizeof(vertices), NULL, 0);
glBindBuffer(GL_ARRAY_BUFFER, buffer);
void * ptr = glMapNamedBuffer(buffer, GL_WRITE_ONLY);
memcpy(ptr, vertices, sizeof(vertices));
glUnmapNamedBuffer(GL_ARRAY_BUFFER);
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vmath::vec4));
// Positions
glVertexArrayAttribBinding(vao, 0, 0);
glVertexArrayAttribFormat(vao, 0, 4, GL_FLOAT, GL_FALSE, offsetof(vertex, x));
glEnableVertexArrayAttrib(vao, 0);
// Color
glVertexArrayAttribBinding(vao, 1, 0);
glVertexArrayAttribFormat(vao, 1, 4, GL_FLOAT, GL_FALSE, offsetof(vertex, r));
glEnableVertexArrayAttrib(vao, 1);
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vertex));
I set up the vertex struct as follows:
struct vertex {
// Position
float x;
float y;
float z;
float w;
// Color
float r;
float g;
float b;
float a;
};
The first time, I had the color hard-coded in my vertex shader. And I had the position data in a data-array. I set the data directly by calling 'glNamedBufferStorage' instead of (as it is now the case) inserting NULL. Back then, it worked. But as I changed the two things, it stopped working. I know by shure that both of these major steps include some errors.
Here I'll provide you with the vertex shader. the mvp matrix works, by the way, so that's not the problem.
#version 420 core
layout (location = 0) in vec4 position;
layout (location = 1) in vec4 color;
out vec4 vs_color;
uniform mat4 mvp;
void main(void)
{
gl_Position = mvp * position;
vs_color = color;
}
Any hints would be greatly appreciated.

Answer 1

There are several problems in the code:
glUnmapNamedBuffer takes the buffer handle as parameter, not a GLenum. Change glUnmapNamedBuffer(GL_ARRAY_BUFFER); to glUnmapNamedBuffer(buffer);. (And you should check the return value, it returns false when there is a problem).
The stride for glVertexArrayVertexBuffer is wrong. Each vertex starts 8 floats after the previous one (or sizeof(vertex)). Change
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vmath::vec4));
to
glVertexArrayVertexBuffer(vao, 0, buffer, 0, sizeof(vertex));