CODEWITHSUNDEEP
vulkan
AlphaModder
AlphaModder

Reputation: 3386

What does PACK8/16/32 mean in VkFormat names?

I'm trying to understand the names of the items in the VkFormat enum, and so far I think I get all the structure of the names of all of the (non-block) formats, but I can't figure out what it means when they have a suffix of PACK8, PACK16, PACK32. If I add up the channel sizes, they always add up to 8, 16, or 32, nothing irregular, so I don't understand what it would mean to bit-pack these values, since they seem to be 100% efficient, using all their bits.

As usual, the documentation is not very helpful, just saying the format is packed without saying what that means.

Upvotes: 4

Views: 1322

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473447

The PACK fields mean exactly what the specification says they mean:

whole texels or attributes are stored in a single data element, rather than individual components occupying a single data element

Though if you find that too confusing, you could just look at the actual format descriptions. Vulkan goes into excruciating detail about them, to the point of needless repetition.

The difference between VK_FORMAT_B8G8R8A8_RGB and VK_FORMAT_B8G8R8A8_RGB_PACK32 is the same difference between a uint8_t[4] and a uint32_t. One is an array ("individual components"), while the other is a single value ("single data element") made up of smaller values.

If you have a uint8_t color[4] array, which stores B8G8R8A8, then color[0] stores the blue component. The order of the components in the array is defined by the order of the components in the format's name.

If you have a uint32_t color value, which stores B8G8R8A8, then (color & 0xFF000000) >> 24 will retrieve the blue component. The highest byte is the first, followed by the next highest and so forth.

The reason the packed-vs-not-packed distinction matters is because of endian issues. Arrays of bytes don't have endian issues. But values packed into 16 or 32-bits do have endian issues. The endian of the packed formats is always assumed to be the native endian of the host.

Upvotes: 7

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