问题描述
我使用一个OpenGL缓冲区和一堆GLfloats作为顶点缓冲区，一切都很好。 GLfloats的格式为 [x1，y1，z1，x2，y2，z2，...] 。
但是，在遵循的同时，它会告诉我改用 glm :: vec3 ：
glBufferData GL_ARRAY_BUFFER，vertices.size（）* sizeof（GLfloat），& vertices [0]，GL_STATIC_DRAW）;
glBufferData（GL_ARRAY_BUFFER，vertices.size（）* sizeof（glm :: vec3），& vertices [0]，GL_STATIC_DRAW）;
现在这个代码是有效的，我不知道OpenGL怎么知道如何填充glm :: vec3而不是GLfloats。然后我不知道，当我从缓冲区读回数据，使用：
std :: vector< glm :: vec3&数据;
glGetBufferSubData（mTarget，offset，vertexCount * sizeof（glm :: vec3），& data [0]）;`
这会造成一堆glm :: vec3吗？所以问题是， OpenGL如何用 glm :: vec3 填充缓冲区，并且（如果是这样，它是如何读取的）
根据， glBufferData（）需要一个指向数据
让我们先来看看 glm :: vec3 的实现。如果您签出，你会看到，取决于你的编译标志， glm :: vec3 是一个，这是。
tvec3 在（由提供）和类（模板）方法在中定义。
特别是是：
template< typename T，precision P>
GLM_FUNC_QUALIFIER T& tvec3 :: operator []（typename tvec3 :: length_type i）
{
assert（i> = 0& static_cast< detail :: component_count_t> ;（i）< detail :: component_count（* this））;
return（& x）[i];
}
给定这段代码，可以假设 x 是包含坐标 glm :: vec3 的数组的第一个元素。但是，当我们回到时，我们find：
union {T x，r，s; };
union {T y，g，t; };
union {T z，b，p; };
因此 x ， y 和 z 是单独的属性。但是，由于，它们可以被视为单个数组，从& x 。
我们现在知道， glm :: vec3 （实际上 tvec3 ）以连续方式存储坐标。但是它还存储其他属性？
好，我们可以继续深入代码，或使用一个简单的程序给我们的答案：
#include< iostream>
#include< ios>
#include< glm / vec3.hpp>
int main（）
{
const glm :: vec3 v;
const size_t sizeof_v = sizeof（v）;
const size_t sizeof_xyz = sizeof（v.x）+ sizeof（v.y）+ sizeof（v.z）;
std :: cout<< sizeof（v）：< sizeof_v< std :: endl;
std :: cout<< sizeof（xyz）：< sizeof_xyz< std :: endl;
std :: cout<< sizeof（v）== sizeof（xyz）：< std :: boolalpha< （sizeof_v == sizeof_xyz）< std :: endl;
}
在我的机器上打印：
sizeof（v）：12
sizeof（xyz）：12
sizeof（v）== sizeof（xyz）：true
因此， glm :: vec3
现在，如果我们创建一个（x，y，z） std :: vector< glm :: vec3>顶点; ，可以安全地说，由& vertices [0] 指向的数据的布局是）是：vertices == [vertice1 vertice2 ...]
== [vertice1.x vertice1.y vertice1。 z vertice2.x vertice2.y vertice2.z ...]
回到原始问题 - - glBufferData（）的要求：当你传递& vertices [0] data ，正如 glBufferData（）所期望的那样。相同的逻辑适用于 glGetBufferSubData（）。
I was using an OpenGL buffer with a bunch of GLfloats as a vertex buffer and all was well. The format of the GLfloats being [x1, y1, z1, x2, y2, z2, ...].
But then, while following this tutorial, it tells me to use glm::vec3 instead:
glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(GLfloat), &vertices[0], GL_STATIC_DRAW); glBufferData(GL_ARRAY_BUFFER, vertices.size() * sizeof(glm::vec3), &vertices[0], GL_STATIC_DRAW);
Now this code is valid, and I wonder how would OpenGL know how to fill in the buffer with glm::vec3 instead of GLfloats. Then I wonder, when I read the data back from the buffer, using:
std::vector<glm::vec3> data; glGetBufferSubData(mTarget, offset, vertexCount * sizeof(glm::vec3), &data[0]);`
Will this make a bunch of glm::vec3? So the question is, how does OpenGL fill buffers with glm::vec3, and does (and if so, how does) it read it back?
According to OpenGL's documentation, glBufferData() needs a pointer to the data (i.e. an array, i.e. the coordinates of the vertices).
Let's first have a look at glm::vec3's implementation.
If you check out glm's Github repo, you'll see that, depending on your compilation flags, glm::vec3 is a typedef of highp_vec3 which is a typedef of tvec3<float, highp>.
tvec3 is declared in type_vec3.hpp (included by vec3.hpp) and the class (template) methods are defined in type_vec3.inl.
In particular, operator[]'s definition is:
template <typename T, precision P>
GLM_FUNC_QUALIFIER T & tvec3<T, P>::operator[](typename tvec3<T, P>::length_type i)
{
assert(i >= 0 && static_cast<detail::component_count_t>(i) < detail::component_count(*this));
return (&x)[i];
}
Given that piece of code, one would assume that x is the first element of the "array" containing the coordinates of glm::vec3. However, when we go back to type_vec3.h, we find:
union { T x, r, s; };
union { T y, g, t; };
union { T z, b, p; };
So x, y and z are separate attributes. But thanks to how class/struct members are laid out, they can be viewed as a single array starting from &x.
We know now, that glm::vec3 (actually tvec3) stores the coordinates in a contiguous manner. But does it also store other attributes ?
Well, we can continue to dive into the code, or use a simple program to give us the answer:
#include <iostream>
#include <ios>
#include <glm/vec3.hpp>
int main()
{
const glm::vec3 v;
const size_t sizeof_v = sizeof(v);
const size_t sizeof_xyz = sizeof(v.x) + sizeof(v.y) + sizeof(v.z);
std::cout << "sizeof(v) : " << sizeof_v << std::endl;
std::cout << "sizeof(xyz): " << sizeof_xyz << std::endl;
std::cout << "sizeof(v) == sizeof(xyz) : " << std::boolalpha << (sizeof_v == sizeof_xyz) << std::endl;
}
Which prints, on my machine:
sizeof(v) : 12 sizeof(xyz): 12 sizeof(v) == sizeof(xyz) : true
Therefore, glm::vec3 stores only the (x, y, z) coordinates.
Now, if we create a std::vector<glm::vec3> vertices;, it is safe to say that the layout of the data pointed by &vertices[0] (which, in C++11 is vertices.data()) is:
vertices == [vertice1 vertice2 ...]
== [vertice1.x vertice1.y vertice1.z vertice2.x vertice2.y vertice2.z ...]
Going back to the original issue -- glBufferData()'s requirements: when you pass &vertices[0] you are actually passing the address (i.e. pointer) of the data, just as expected by glBufferData(). The same logic applies to glGetBufferSubData().
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