Passing float array to function - c++

I am trying to load a VAO in OpenGL, but when running it doesn't draw.
When I use the code in my function directly into my main loop it runs fine but whenever I try to load the VAO via my function it doesn't work.
I already narrowed it down to something going wrong when passing the values to the function, because when I directly use the float array it does work.
I have the values defined as a static float array in my main function, and I'm loading it like this:
GLuint RenderLoader::load(float vertices[])
{
GLuint VertexArrayID;
glGenVertexArrays(1, &VertexArrayID);
glBindVertexArray(VertexArrayID);
GLuint vertexbuffer;
glGenBuffers(1, &vertexbuffer);
glBindBuffer(GL_ARRAY_BUFFER, vertexbuffer);
glBufferData(GL_ARRAY_BUFFER, sizeof(vertices), vertices, GL_STATIC_DRAW);
glVertexAttribPointer(0, 3, GL_FLOAT, GL_FALSE, 0, (void*)0);
return VertexArrayID;
}

The problem is with sizeof(vertices). Since the array is passed into a function, it then becomes a pointer, and sizeof returns the size of the first element pointer only. It is only known as an array in the scope where it was first initialised.
One solution would be to pass the size as an additional parameter, but really the only solution is to use some sort of container, like vector which has a size() function.
When using a vector, you would then do it in the following way:
GLsizei size = vertices.size() * sizeof(vertices[0]); // Size in bytes
glBufferData(GL_ARRAY_BUFFER, size, &vertices[0], GL_STATIC_DRAW);

Related

OpenGL binding order between EBO and VAO

I`m trying to create opengl triangle and I bumped up with some problem.
EBO : ElementArrayBufferObject
VAO : VertexArrayObject
while i trying to binding EBO before binding VAO it causing error and i don`t know why.
my code :
// Generate the VAO and VBO with only 1 object each
glGenVertexArrays(1, &VAO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
// Make the VAO the current Vertex Array Object by binding it
glBindVertexArray(VAO);
It causing problem while rendering.
and if i fix the code like this order.
// Generate the VAO and VBO with only 1 object each
glGenVertexArrays(1, &VAO);
// Make the VAO the current Vertex Array Object by binding it
glBindVertexArray(VAO);
glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO);
glBufferData(GL_ELEMENT_ARRAY_BUFFER, sizeof(indices), indices, GL_STATIC_DRAW);
it`s working as i expected.
I really don`t know why not working binding EBO before VAO.
The index buffer (ELEMENT_ARRAY_BUFFER) binding is stored within the Vertex Array Object. When glBindBuffer(GL_ELEMENT_ARRAY_BUFFER, EBO) is called the element buffer object is stored in the currently bound Vertex Array Object. Therefore the VAO must be bound before the element buffer with glBindVertexArray(VAO).
Note that compared to the index buffer (ELEMENT_ARRAY_BUFFER), the vertex buffer binding (ARRAY_BUFFER) is a global state.
Each attribute which is stated in the VAOs state vector may refer to a different ARRAY_BUFFER. This reference is stored when glVertexAttribPointer is called. Then the buffer which is currently bound to the target ARRAY_BUFFER, is associated to the specified attribute index and the name (value) of the object is stored in the state vector of the currently bound VAO.
However the index buffer is a state of the VAO. If a buffer is bound to the target ELEMENT_ARRAY_BUFFER, this buffer is assigned to the currently bound Vertex Array Object.

how does a VBO get attached to a VAO

VAO being Vertex Array Object and VBO being Vertex Buffer Object. The calls for creating and binding/unbinding VAOs and VBOs have a general format as given below:
GLuint VAO, VBO;
glGenVertexArrays(1, &VAO);
glBindVertexArray(VAO);
glGenBuffers(1, &VBO);
glBindBuffer(GL_ARRAY_BUFFER, VBO);
glBufferData(GL_ARRAY_BUFFER,
sizeof(GLfloat)* 9,
vertices,
GL_STATIC_DRAW);
glVertexAttribPointer(0,
3, GL_FLOAT, GL_FALSE,
0, 0);
glEnableVertexAttribArray(0);
glBindBuffer(GL_ARRAY_BUFFER, 0);
glBindVertexArray(0);
I've followed some tutorials on the internet, and they say that the VBO is bound to the VAO, but in the above code, I don't get how the "connection" or "binding" is established between the VBO and the VAO? I mean how do we know which VBO is bound to which VAO and what if a VBO is to be bound to multiple VAOs?
So, the basic question is: what does VBO is bound to VAO mean, how do we know which VBO is bound to which VAO and how is this binding established?
how does a VBO get attached to a VAO
It is attached when glVertexAttribPointer is called.
Each attribute which is stated in the Vertex Array Objects state vector may refer to a different Vertex Buffer Object. This reference is stored when glVertexAttribPointer is called.
Then the buffer which is currently bound to the target ARRAY_BUFFER is associated to the attribute and the name (value) of the object is stored in the state vector of the VAO. The ARRAY_BUFFER binding is a global state.
This behaves different to the Index Buffer (ELEMENT_ARRAY_BUFFER). The index buffer binding is stated within the Vertex Array Object. When a buffer is bound to the target ELEMENT_ARRAY_BUFFER, then this buffer is associated to the vertex array object which is currently bound.
That means when you call glBindBuffer(GL_ARRAY_BUFFER, VBO);, then a global state is set that stores VBO. glVertexAttribPointer retrieves VBO from the global state and assoicates it to VAO.
When glBindBuffer(GL_ARRAY_BUFFER, ...) is called again, then the previous binding is lost, but of course it does not change any state of the VAO.
glBindVertexArray(VAO); # bind VAO (global state)
glBindBuffer(GL_ARRAY_BUFFER, VBO1); # bind VBO1 (global state)
glVertexAttribPointer(0, ...); # associate VBO1 to attribute 0 in VAO
glBindBuffer(GL_ARRAY_BUFFER, VBO2); # bind VBO2 (change global state, VBO1 binding is lost)
glVertexAttribPointer(1, ...); # associate VBO2 to attribute 1 in VAO

Issue passing Array as Parameter

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.

State in OpenGL

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.

VBOs with std::vector

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)).