Pyrogenesis  trunk
Resource aliasing (overlap)

New explicit graphics APIs (Vulkan and Direct3D 12), thanks to manual memory management, give an opportunity to alias (overlap) multiple resources in the same region of memory - a feature not available in the old APIs (Direct3D 11, OpenGL).

It can be useful to save video memory, but it must be used with caution.

For example, if you know the flow of your whole render frame in advance, you are going to use some intermediate textures or buffers only during a small range of render passes, and you know these ranges don't overlap in time, you can bind these resources to the same place in memory, even if they have completely different parameters (width, height, format etc.).

Resource aliasing (overlap)

Such scenario is possible using VMA, but you need to create your images manually. Then you need to calculate parameters of an allocation to be made using formula:

Following example shows two different images bound to the same place in memory, allocated to fit largest of them.

// A 512x512 texture to be sampled.
VkImageCreateInfo img1CreateInfo = { VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO };
img1CreateInfo.imageType = VK_IMAGE_TYPE_2D;
img1CreateInfo.extent.width = 512;
img1CreateInfo.extent.height = 512;
img1CreateInfo.extent.depth = 1;
img1CreateInfo.mipLevels = 10;
img1CreateInfo.arrayLayers = 1;
img1CreateInfo.format = VK_FORMAT_R8G8B8A8_SRGB;
img1CreateInfo.tiling = VK_IMAGE_TILING_OPTIMAL;
img1CreateInfo.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
img1CreateInfo.samples = VK_SAMPLE_COUNT_1_BIT;
// A full screen texture to be used as color attachment.
VkImageCreateInfo img2CreateInfo = { VK_STRUCTURE_TYPE_IMAGE_CREATE_INFO };
img2CreateInfo.imageType = VK_IMAGE_TYPE_2D;
img2CreateInfo.extent.width = 1920;
img2CreateInfo.extent.height = 1080;
img2CreateInfo.extent.depth = 1;
img2CreateInfo.mipLevels = 1;
img2CreateInfo.arrayLayers = 1;
img2CreateInfo.format = VK_FORMAT_R8G8B8A8_UNORM;
img2CreateInfo.tiling = VK_IMAGE_TILING_OPTIMAL;
img2CreateInfo.initialLayout = VK_IMAGE_LAYOUT_UNDEFINED;
img2CreateInfo.samples = VK_SAMPLE_COUNT_1_BIT;
VkImage img1;
res = vkCreateImage(device, &img1CreateInfo, nullptr, &img1);
VkImage img2;
res = vkCreateImage(device, &img2CreateInfo, nullptr, &img2);
VkMemoryRequirements img1MemReq;
vkGetImageMemoryRequirements(device, img1, &img1MemReq);
VkMemoryRequirements img2MemReq;
vkGetImageMemoryRequirements(device, img2, &img2MemReq);
VkMemoryRequirements finalMemReq = {};
finalMemReq.size = std::max(img1MemReq.size, img2MemReq.size);
finalMemReq.alignment = std::max(img1MemReq.alignment, img2MemReq.alignment);
finalMemReq.memoryTypeBits = img1MemReq.memoryTypeBits & img2MemReq.memoryTypeBits;
// Validate if(finalMemReq.memoryTypeBits != 0)
VmaAllocationCreateInfo allocCreateInfo = {};
allocCreateInfo.preferredFlags = VK_MEMORY_PROPERTY_DEVICE_LOCAL_BIT;
res = vmaAllocateMemory(allocator, &finalMemReq, &allocCreateInfo, &alloc, nullptr);
res = vmaBindImageMemory(allocator, alloc, img1);
res = vmaBindImageMemory(allocator, alloc, img2);
// You can use img1, img2 here, but not at the same time!
vmaFreeMemory(allocator, alloc);
vkDestroyImage(allocator, img2, nullptr);
vkDestroyImage(allocator, img1, nullptr);

Remember that using resources that alias in memory requires proper synchronization. You need to issue a memory barrier to make sure commands that use img1 and img2 don't overlap on GPU timeline. You also need to treat a resource after aliasing as uninitialized - containing garbage data. For example, if you use img1 and then want to use img2, you need to issue an image memory barrier for img2 with oldLayout = VK_IMAGE_LAYOUT_UNDEFINED.

Additional considerations: