在 Vulkan 中确定最高可能的颜色和深度附件采样计数

Question

Do I need to set the value of VkAttachmentDescription::samples to a power of 2 or are arbitrary values allowed, as long as they don't exceed the maximum supported by the hardware?

I'm really confused about this. The samples field is of type VkSampleCountFlagBits , which is declared in the following way

typedef enum VkSampleCountFlagBits {
    VK_SAMPLE_COUNT_1_BIT = 0x00000001,
    VK_SAMPLE_COUNT_2_BIT = 0x00000002,
    VK_SAMPLE_COUNT_4_BIT = 0x00000004,
    VK_SAMPLE_COUNT_8_BIT = 0x00000008,
    VK_SAMPLE_COUNT_16_BIT = 0x00000010,
    VK_SAMPLE_COUNT_32_BIT = 0x00000020,
    VK_SAMPLE_COUNT_64_BIT = 0x00000040,
    VK_SAMPLE_COUNT_FLAG_BITS_MAX_ENUM = 0x7FFFFFFF
} VkSampleCountFlagBits;

On the other hand, the VkPhysicalDeviceLimits struct contains the fields framebufferColorSampleCounts and framebufferDepthSampleCounts , which are of type VkSampleCountFlags , which in turn is simply a typedef for uint32_t .

The vulkan-tutorial page on multisampling determines the highest bit in these fields to compute the maximal usable sampled count. I actually don't get this. What if, for example, VK_SAMPLE_COUNT_16_BIT and VK_SAMPLE_COUNT_1_BIT are both set in these fields? Doesn't that mean that the maximal usable sampled count is at least 17?

What I need to do at the end of the day is, given a uint32_t requested_sampled_count , determine whether requested_sampled_count is a possible value for VkAttachmentDescription::samples for both color and depth attachements and, if it's not, what is the highest possible value smaller than requested_sampled_count .

EDIT :

Say I have given a std::uint32_t sample_count and, from the physical device properties, VkSampleCountFlags framebuffer_color_sample_counts and want to compute the VkSampleCountFlagBits samples . Do I need to this in the following way?

if (sample_count > 64)
    /* error */;
if (sample_count > 32)
    samples = VK_SAMPLE_COUNT_32_BIT;
else if (sample_count > 16)
    samples = VK_SAMPLE_COUNT_16_BIT;
else if (sample_count > 8)
    samples = VK_SAMPLE_COUNT_8_BIT;
else if (sample_count > 4)
    samples = VK_SAMPLE_COUNT_4_BIT;
else if (sample_count > 2)
    samples = VK_SAMPLE_COUNT_2_BIT;
else if (sample_count == 1)
    samples = VK_SAMPLE_COUNT_1_BIT;
else
    /* error */;

Answer 1

The sample counts in the VkSampleCountFlagsBits enumeration are a bit-mask of the available settings for the number of sampled bits in an attachment, so in your example the hardware supports either one or 16 samples (not 17!)