They look different formats, however an online LLM told me:
You're absolutely right to question this! The component order in the name of the Vulkan and DirectX formats is different, but the actual bitwise layout in memory is identical between DXGI_FORMAT_R10G10B10A2_UNORM and VK_FORMAT_A2R10G10B10_UNORM_PACK32. Why Are They Actually the Same in Memory?
Bit Range DirectX(R10G10B10A2_UNORM) Vulkan(VK_FORMAT_A2R10G10B10_UNORM_PACK32)
----------------------------------------------------------------------------------
[31:30] Alpha(A2) Alpha(A2)
[29:20] Red(R10) Red(R10)
[19:10] Green(G10) Green(G10)
[9:0] Blue(B10) Blue(B10)
Although Vulkan names the alpha first (A2R10G10B10), this is just a naming convention—the actual memory layout is identical to R10G10B10A2_UNORM in DirectX.
I was just wondering if this is true. The Vulkan version puts the A at the front in the name, but the bit patterns are the same?
Yes, they are the same.
The Vulkan spec description for VK_FORMAT_A2B10G10R10_UNORM_PACK32 is [spec][archive]:
VK_FORMAT_A2B10G10R10_UNORM_PACK32 specifies a four-component, 32-bit packed unsigned normalized format that has a 2-bit A component in bits 30..31, a 10-bit B component in bits 20..29, a 10-bit G component in bits 10..19, and a 10-bit R component in bits 0..9
Further, packed formats are defined to have first value be the most significant bit [spec][archive]:
Packed formats store multiple components within one underlying type. The bit representation is that the first component specified in the name of the format is in the most-significant bits and the last component specified is in the least-significant bits of the underlying type. The in-memory ordering of bytes comprising the underlying type is determined by the host endianness.
DXGI is a little more nebulous in how they define the bit ordering, but they conveniently provide a sample with this exact format [spec][archive]:
// DXGI_FORMAT_R10G10B10A2_UNORM
UINT32* pR10G10B10A2 = ...;
pR10G10B10A2 = (0x3ff) | (0x1 << 30); // R=0x3ff, and A=0x1
This sample confirms that bits 30..31 are the alpha channel, and that bits 0..9 are the red channel, just like in Vulkan.