I am using vertex buffers and element buffers.
The following function takes vertex and element data as arrays and creates buffers out of that. My real implementation is more complicated and stores the ids for later use of course, but that does not relate to this question.
void Create(const float Vertices[], const int Elements[])
{
GLuint VertexBuffer, ElementBuffer; // ids
glGenBuffers(1, VertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, VertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(Vertices), Vertices, GL_STATIC_DRAW);
glGenBuffers(1, ElementBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ElementBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(Elements), Elements, GL_STATIC_DRAW);
}
In another function I call Create() passing two arrays which represents a cube. But nothing happens. The window opens up and I see the cornflower blue background without any cube.
float VERTICES[] = {-1.f,-1.f,1.f,1.f,0.f,0.f,.8f,1.f,-1.f,1.f,0.f,1.f,0.f,.8f,1.f,1.f,1.f,0.f,0.f,1.f,.8f,-1.f,1.f,1.f,1.f,1.f,1.f,.8f,-1.f,-1.f,-1.f,0.f,0.f,1.f,.8f,1.f,-1.f,-1.f,1.f,1.f,1.f,.8f,1.f,1.f,-1.f,1.f,0.f,0.f,.8f,-1.f,1.f,-1.f,0.f,1.f,0.f,.8f};
int ELEMENTS[] = {0,1,2,2,3,0,1,5,6,6,2,1,7,6,5,5,4,7,4,0,3,3,7,4,4,5,1,1,0,4,3,2,6,6,7,3};
Create(VERTICES, ELEMENTS);
If I move the vertex and element data inside the Create() function, everything works fine and the cube is rendered correctly.
void Create()
{
GLuint VertexBuffer, ElementBuffer;
float VERTICES[] = {-1.f,-1.f,1.f,1.f,0.f,0.f,.8f,1.f,-1.f,1.f,0.f,1.f,0.f,.8f,1.f,1.f,1.f,0.f,0.f,1.f,.8f,-1.f,1.f,1.f,1.f,1.f,1.f,.8f,-1.f,-1.f,-1.f,0.f,0.f,1.f,.8f,1.f,-1.f,-1.f,1.f,1.f,1.f,.8f,1.f,1.f,-1.f,1.f,0.f,0.f,.8f,-1.f,1.f,-1.f,0.f,1.f,0.f,.8f};
int ELEMENTS[] = {0,1,2,2,3,0,1,5,6,6,2,1,7,6,5,5,4,7,4,0,3,3,7,4,4,5,1,1,0,4,3,2,6,6,7,3};
glGenBuffers(1, VertexBuffer);
glBindBuffer(GL_ARRAY_BUFFER, VertexBuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(VERTICES), VERTICES, GL_STATIC_DRAW);
glGenBuffers(1, ElementBuffer);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, ElementBuffer);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(ELEMENTS), ELEMENTS, GL_STATIC_DRAW);
}
Therefore I assume that the problem occurs when passing the array to the Create() function. I do not get any compiler error or warning. What is wrong here?
A parameter of type const float Vertices[] is actually the same as const float Vertices*. So sizeof is just returning the size of a pointer.
Use a reference to array using templates instead:
template<std::size_t VerticesN, std::size_t ElementsN>
void Create(const float (&Vertices)[VerticesN], const int (&Elements)[ElementsN])
{
// ...
}
// Usage is the same since template argument deduction
float VERTICES[] = {-1.f,-1.f,1.f,1.f,0.f,0.f,.8f,1.f,-1.f,1.f,0.f,1.f,0.f,.8f,1.f,1.f,1.f,0.f,0.f,1.f,.8f,-1.f,1.f,1.f,1.f,1.f,1.f,.8f,-1.f,-1.f,-1.f,0.f,0.f,1.f,.8f,1.f,-1.f,-1.f,1.f,1.f,1.f,.8f,1.f,1.f,-1.f,1.f,0.f,0.f,.8f,-1.f,1.f,-1.f,0.f,1.f,0.f,.8f};
int ELEMENTS[] = {0,1,2,2,3,0,1,5,6,6,2,1,7,6,5,5,4,7,4,0,3,3,7,4,4,5,1,1,0,4,3,2,6,6,7,3};
Create(VERTICES, ELEMENTS);
The problem here is sizeof(VERTICES) and sizeof(ELEMENTS). When used in the Create() method the sizes of the arrays are known, but when you pass the arrays as a parameter (like in the Create(const float Vertices[], const int Elements[]) the array degrades to a pointer, and the sizeof is reduced to returning the size of the pointer.
One simple solution is to pass the size along with the arrays. So the function will look like this:
void Create(const float Vertices[], size_t VertSize, const int Elements[], size_t ElemSize) {
...
}
but I think I would prefer a solution that uses the new std::array which has a size() function:
void Create(const std::array<float>& vertices, std::array<int>& elements) {
...
}
If you do not have the opportunity to work with c++ 11, the boost libraries will provide the boost::array which mirrors the behaviour of c++ 11.
This is some simple code that draws to the screen.
GLuint vbo;
glGenBuffers(1, &vbo);
glUseProgram(myProgram);
glBindBuffer(GL_ARRAY_BUFFER, vbo);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);
//Fill up my VBO with vertex data
glBufferData(GL_ARRAY_BUFFER, sizeof(vertexes), &vertexes, GL_STATIC_DRAW);
/*Draw to the screen*/
This works fine. However, I tried changing the order of some GL calls like so:
GLuint vbo;
glGenBuffers(1, &vbo);
glUseProgram(myProgram);
glEnableVertexAttribArray(0);
glVertexAttribPointer(0, 4, GL_FLOAT, GL_FALSE, 0, 0);
//Now comes after the setting of the vertex attributes.
glBindBuffer(GL_ARRAY_BUFFER, vbo);
//Fill up my VBO with vertex data
glBufferData(GL_ARRAY_BUFFER, sizeof(vertexes), &vertexes, GL_STATIC_DRAW);
/*Draw to the screen*/
This crashes my program. Why does there need to be a VBO bound to GL_ARRAY_BUFFER while I'm just setting up vertex attributes? To me, what glVertexAttribPointer does is just set up the format of vertexes that OpenGL will eventually use to draw things. It is not specific to any VBO. Thus, if multiple VBOs wanted to use the same vertex format, you would not need to format the vertexes in the VBO again.
Why does there need to be a VBO bound to GL_ARRAY_BUFFER while I'm just setting up vertex attributes?
No, you're not "just" setting up vertex attributes. You're actually creating a reference with the currently bound buffer object.
If there's no buffer object bound, then gl…Pointer will create a reference to your process address space pointed at the given address. Since this is a null pointer in your case, any attempt to dereference a vertex will cause a segmentation fault/access violation.
To me, what glVertexAttribPointer does is just set up the format of vertexes that OpenGL will eventually use to draw things.
No. It also creates a reference to where to get the data from.
It is not specific to any VBO.
Yes it actually is specific to the bound VBO, or the process address space if no VBO is bound. And changing the buffer binding will not update along the gl…Pointer references.
I believe that's because the last argument to glVertexAttribPointer is pointer which is an offset into a the VBO after glBindBuffer(nonzero) but when a VBO is not bound it's supposed to be a pointer to an actual array of data. So if you weren't using VBOs at all the last argument would be &vertexes and it wouldn't crash.
I've written a model loader in C++ an OpenGL. I've used std::vectors to store my vertex data, but now I want to pass it to glBufferData(), however the data types are wildly different. I want to know if there's a way to convert between std::vector to the documented const GLvoid * for glBufferData().
Vertex type
typedef struct
{
float x, y, z;
float nx, ny, nz;
float u, v;
}
Vertex;
vector<Vertex> vertices;
glBufferData() call
glBufferData(GL_ARRAY_BUFFER, vertices.size() * 3 * sizeof(float), vertices, GL_STATIC_DRAW);
I get the following (expected) error:
error: cannot convert ‘std::vector<Vertex>’ to ‘const GLvoid*’ in argument passing
How can I convert the vector to a type compatible with glBufferData()?
NB. I don't care about correct memory allocation at the moment; vertices.size() * 3 * sizeof(float) will most likely segfault, but I want to solve the type error first.
If you have a std::vector<T> v, you may obtain a T* pointing to the start of the contiguous data (which is what OpenGL is after) with the expression &v[0].
In your case, this means passing a Vertex* to glBufferData:
glBufferData(
GL_ARRAY_BUFFER,
vertices.size() * sizeof(Vertex),
&vertices[0],
GL_STATIC_DRAW
);
Or like this, which is the same:
glBufferData(
GL_ARRAY_BUFFER,
vertices.size() * sizeof(Vertex),
&vertices.front(),
GL_STATIC_DRAW
);
You can rely on implicit conversion from Vertex* to void const* here; that should not pose a problem.
This should do the trick:
&vertices[0]
Some prefer &vertices.front(), but that's more typing and I'm bone lazy.
To be even lazier, you could overload glBufferData thus:
template <class T>
inline void glBufferData(GLenum target, const vector<T>& v, GLenum usage) {
glBufferData(target, v.size() * sizeof(T), &v[0], usage);
}
Then you can write:
glBufferData(GL_ARRAY_BUFFER, vertices, GL_STATIC_DRAW);
and also avoid bugs (your struct is bigger than 3 * sizeof(float)).