Let's say a scene can have up to 1024 point lights. Usually, there'd be less, but the worse case is 1024. They will be added and deleted dynamically over some time or after a certain action.
Does it make sense to allocate a uniform buffer with size 1024 * sizeof(PointLight), map the buffer at the beginning and do a pool over it? Or the better idea would be to create a CPU side pool and then commit the CPU side data to the buffer using glNamedBufferSubData every frame?
Something like this, very simple, does not take a uniform buffer alignment into consideration, because it's not related to the question.
template<typename T, size_t MAX_SIZE>
struct GPUPool
{
GPUPool()
{
glCreateBuffers(1, &ubo);
glNamedBufferStorage(ubo, MAX_SIZE * sizeof(T), nullptr, GL_MAP_WRITE_BIT | GL_MAP_COHERENT_BIT | GL_MAP_PERSISTENT_BIT);
memory = (T*)glMapNamedBufferRange(ubo, 0, MAX_SIZE * sizeof(T), GL_MAP_WRITE_BIT | GL_MAP_COHERENT_BIT | GL_MAP_PERSISTENT_BIT);
}
~GPUPool()
{
glDeleteBuffers(1, &ubo);
}
[[nodiscard]] inline uint32_t insert(const T& element)
{
if (!freeIndexes.empty()) {
auto index = freeIndexes.back();
freeIndexes.pop_back();
memcpy(add(memory, index), &element, sizeof(element));
++size;
return index;
}
memcpy(add(memory, backIndex), &element, sizeof(element));
auto ret = backIndex;
backIndex += sizeof(element);
++size;
return ret;
}
void Bind(uint32_t bindingIndex)
{
glBindBuffer(GL_UNIFORM_BUFFER, bindingIndex);
}
void erase(size_t index)
{
freeIndexes.push_back(index);
--size;
}
GLuint ubo = 0;
T* memory = nullptr;
std::vector<size_t> freeIndexes;
uint32_t backIndex = 0;
size_t size = 0;
};
usage:
struct PointLight
{
vec3 position;
vec3 color;
float fallof;
float radius;
};
GPUPool<PointLight, 1024> pointLightPool;
std::vector<uint32_t> lightIndexes;
while(true)
{
if(Something_Happend_And_Lights_Are_Added()) {
for(uint32_t i = 0; i < 10; ++i)
lightIndexes.emplace_back() = pointLightPool.insert(someLight);
}
if(Something_Happend_And_Light_2_And_5_Are_Deleted()) {
//when lights get deleted, lightIndexes should also be updated, but let's leave it
//for simplicity
pointLightPool.erase(lightIndexes[1]);
pointLightPool.erase(lightIndexes[4]);
}
if(Something_Happend_And_More_Lights_Are_Added()) {
//light pool will place a new light into the old memory spot.
lightIndexes.emplace_back() = pointLightPool.insert(someLight);
}
pointLightPool.Bind(0);
//draw scene...
}