为什么 VK_SAMPLE_COUNT_1_BIT 是 Vulkan 中多重采样的无效选择？

Question

Hello people of StackOverflow,

I am currently working on a games engine using the Vulkan graphics API, in the past I was just setting anti-aliasing to the max it could be. However today I was trying to turn it off (to improve performance on weaker systems). To do this I tried to set the MSAA samples on my engine to VK_SAMPLE_COUNT_1_BIT however this produced the validation error:

Validation Error: [ VUID-VkSubpassDescription-pResolveAttachments-00848 ] Object 0: handle = 0x55aaa6e32828, type = VK_OBJECT_TYPE_DEVICE; | MessageID = 0xfad6c3cb | ValidateCreateRenderPass():  Subpass 0 requests multisample resolve from attachment 0 which has VK_SAMPLE_COUNT_1_BIT. The Vulkan spec states: If pResolveAttachments is not NULL, for each resolve attachment that is not VK_ATTACHMENT_UNUSED, the corresponding color attachment must not have a sample count of VK_SAMPLE_COUNT_1_BIT (https://www.khronos.org/registry/vulkan/specs/1.2-extensions/html/vkspec.html#VUID-VkSubpassDescription-pResolveAttachments-00848)

I can work around this problem relatively easily so it isn't really an issue for me, however I was wondering why exactly this limit is put into place. If I want to set the MSAA samples to 1 why can't I?

Thanks, sckzor

Answer 1

A sample count of 1 means "not a multisampled image". And if you're doing multisample resolve, resolving into a non-multisampled image doesn't make sense. Which is also why you can't use such images for any other things that expect a multisampled image (you can't use an MS-style sampler or texture function on them).