Passing non-interleaved data to glBufferStorage without using glBufferSubData

Question

I would like to use glBufferStorage to load data once into the vertex buffer. However sometimes I want to pass non-interleaved data from separate arrays like shown below.

glBufferStorage(GL_ARRAY_BUFFER, sizeVerts+sizeUVs, 0, 0);
glBufferSubData(GL_ARRAY_BUFFER, 0, sizeVerts, verts.data());
glBufferSubData(GL_ARRAY_BUFFER, sizeVerts, sizeUVs, uvs.data());

Because of this I have to specify GL_DYNAMIC_STORAGE_BIT flag, despite loading the data once, and thus use a less performant alternative.

Are there any workarounds to this problem?

Or does using GL_DYNAMIC_STORAGE_BIT not lead to a significant performance hit, and should I just use it despite loading data once?

Answer 1

You can put your data in one piece of memory on the CPU and then copy it in with glBufferStorage. It's just a bunch of memcpy calls.

With C++20's std::span (or the GSL version), you can even make a function to build byte-spans from any number of vector<T>s (even of different types) and do the building and memcpy's manually.

#include <vector>
#include <concepts>
#include <ranges>
#include <span>
#include <cstdint>
#include <cstring>

using byte_range = std::span<std::byte const, std::dynamic_extent>;

template<std::ranges::contiguous_range Rng>
byte_range get_span(Rng const &rng)
{
    auto sp = std::span(rng);
    return std::as_bytes(sp);
}

template<std::ranges::contiguous_range ...Rngs>
    requires (sizeof...(Rngs) > 0)
GLuint upload_to_buffer(Rngs const &...rngs)
{
    std::array<byte_range, sizeof...(rngs)> spans =
        {get_span(rngs)...};

    std::size_t buffer_size = 0;
    for(auto const& sp : spans)
        buffer_size += sp.size();

    std::vector<std::byte> byte_buff(buffer_size);

    std::size_t offset = 0;
    for(auto const& sp : spans)
    {
        std::memcpy(&byte_buff[offset], sp.data(), sp.size());
    }

    GLuint buff;
    glCreateBuffer(&buff, 1);
    glNamedBufferStorage(buff, byte_buff.size(), byte_buff.data(), 0);
    return buff;
}

```