“Staging Buffer” ，可翻译成暂存缓冲区或临时缓冲区，在使用诸如Vulkan、DirectX等现代图形API时，经常用于充当主机和GPU之间的桥梁，以进行高效的数据传输。要明确知道，staging buffer是显存上开辟的
以Index Buffer的创建为例：
- 主机端创建了索引数组（CPU端内存开辟），并填充数据
- 调用API在显存上创建一块区域，用
indexBuffer来指向 - 目的：把CPU端的数据拷贝到
indexBuffer中
考虑问题： 既然都是从CPU到GPU传输数据，为什么要先从CPU端拷贝到staging buffer(显存块)中，再从staging buffer拷贝到index buffer(显存块)呢，这不很多余么？
知道答案的就不用往下看了
看注释
VulkanIndexBuffer::VulkanIndexBuffer(void* data, uint64_t size): m_Size(size)
{// 这里通过Buffer类申请了堆区空间，并把形参的数据拷贝进去m_LocalData = Buffer::Copy(data, size); // 封装的一个类，用于申请显存块VulkanAllocator allocator("IndexBuffer"); if (usingStaging){// Staging Buffer// 注意usage参数：传输源（我们要从staging buffer拷贝数据到index buffer）VkBufferCreateInfo bufferCreateInfo{};bufferCreateInfo.sType = VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO;bufferCreateInfo.size = instance->m_Size;bufferCreateInfo.usage = VK_BUFFER_USAGE_TRANSFER_SRC_BIT; /bufferCreateInfo.sharingMode = VK_SHARING_MODE_EXCLUSIVE;// 这里给分配器传的flags是CPU_TO_GPU，即CPU可见VkBuffer stagingBuffer;VmaAllocation stagingBufferAllocation = allocator.AllocateBuffer(bufferCreateInfo, VMA_MEMORY_USAGE_CPU_TO_GPU, stagingBuffer);// MapMemory和UnMapMemory：将GPU显存块（CPU可见的）的地址映射到程序端的虚拟地址空间，然后通过指针就能写入数据到这块内存了uint8_t* destData = allocator.MapMemory<uint8_t>(stagingBufferAllocation);memcpy(destData, this->m_LocalData.Data, this->m_LocalData.Size);allocator.UnmapMemory(stagingBufferAllocation);// Index Buffer// 注意usage参数：传输的目标，且指明是用作Index buffer（vulkan会对此显存块进行相应优化）VkBufferCreateInfo indexBufferCreateInfo = {}; indexBufferCreateInfo.sType = VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO;indexBufferCreateInfo.size = instance->m_Size;indexBufferCreateInfo.usage = VK_BUFFER_USAGE_TRANSFER_DST_BIT | VK_BUFFER_USAGE_INDEX_BUFFER_BIT; // 注意flags参数：仅GPU（因此它不能进行映射，也就是CPU不可见，不能传输数据，因此需要staging buffer）this->m_MemoryAllocation = allocator.AllocateBuffer(indexBufferCreateInfo, VMA_MEMORY_USAGE_GPU_ONLY, instance->m_IndexBuffer); // 创建/获取 支持传输指令的command buffer来记录传输指令（vkCmdCopyBuffer)// true为调用vkBeginCommandBuffer函数，即创建该commandbuffer的同时开始指令记录VkCommandBuffer copyCmd = device->GetCommandBuffer(true);// 记录指令VkBufferCopy copyRegion = {};copyRegion.size = this->m_LocalData.Size;vkCmdCopyBuffer(copyCmd, stagingBuffer,this->m_IndexBuffer, 1, ©Region);// Flush意味着提交给指令队列并销毁该comman buffer 因为只是一个复制指令而已，用完就销毁该commandbufferdevice->FlushCommandBuffer(copyCmd);// staging buffer已无用，须销毁allocator.DestroyBuffer(stagingBuffer, stagingBufferAllocation);}else{VkBufferCreateInfo indexbufferCreateInfo = {};indexbufferCreateInfo.sType = VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO;indexbufferCreateInfo.size = instance->m_Size;indexbufferCreateInfo.usage = VK_BUFFER_USAGE_INDEX_BUFFER_BIT;// 如果不用stagingbuffer，那indexBuffer就需要CPU可见，可读可写，会影响效率，禁用很多优化auto bufferAlloc = allocator.AllocateBuffer(indexbufferCreateInfo, VMA_MEMORY_USAGE_CPU_TO_GPU, instance->m_VulkanBuffer);void* dstBuffer = allocator.MapMemory<void>(bufferAlloc);memcpy(dstBuffer, instance->m_LocalData.Data, instance->m_Size);allocator.UnmapMemory(bufferAlloc);}
}
