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docs: add an allocation readback example #552

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35 changes: 34 additions & 1 deletion include/vk_mem_alloc.h
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Original file line number Diff line number Diff line change
Expand Up @@ -18054,7 +18054,40 @@ vmaCreateBuffer(allocator, &bufCreateInfo, &allocCreateInfo, &buf, &alloc, nullp
vmaCopyMemoryToAllocation(allocator, &constantBufferData, alloc, 0, sizeof(ConstantBuffer));
\endcode

Copy in the other direction - from an allocation to a host pointer can be performed the same way using function vmaCopyAllocationToMemory().
Copy in the other direction - from an allocation to a host pointer - can be performed with vmaCopyAllocationToMemory().
For an allocation that will be read by the CPU, use #VMA_ALLOCATION_CREATE_HOST_ACCESS_RANDOM_BIT.
If the GPU has written the data, wait for the operation to complete before reading it.

\code
void ReadBack(VmaAllocator allocator)
{
uint8_t readbackData[256] = {};

VkBufferCreateInfo readbackBufCreateInfo = { VK_STRUCTURE_TYPE_BUFFER_CREATE_INFO };
readbackBufCreateInfo.size = sizeof(readbackData);
readbackBufCreateInfo.usage = VK_BUFFER_USAGE_TRANSFER_DST_BIT;

VmaAllocationCreateInfo readbackAllocCreateInfo = {};
readbackAllocCreateInfo.usage = VMA_MEMORY_USAGE_AUTO;
readbackAllocCreateInfo.flags = VMA_ALLOCATION_CREATE_HOST_ACCESS_RANDOM_BIT;

VkBuffer readbackBuf = VK_NULL_HANDLE;
VmaAllocation readbackAlloc = VK_NULL_HANDLE;
VkResult res = vmaCreateBuffer(allocator, &readbackBufCreateInfo, &readbackAllocCreateInfo,
&readbackBuf, &readbackAlloc, nullptr);
if(res != VK_SUCCESS)
return;

// Record and submit vkCmdCopyBuffer into readbackBuf, then wait for its fence.
res = vmaCopyAllocationToMemory(allocator, readbackAlloc, 0, readbackData, sizeof(readbackData));
if(res == VK_SUCCESS)
{
// Use readbackData.
}

vmaDestroyBuffer(allocator, readbackBuf, readbackAlloc);
}
\endcode

\section memory_mapping_mapping_functions Mapping functions

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