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glslvulkan
Gary Allen
Gary Allen

Reputation: 1360

Loading multi-component bytes into shader in Vulkan

Vulkan allows you to specify attributes as multi-component byte arrays such as with the qualifier "VK_FORMAT_R8G8B8_UINT". I am, however, unsure what input variable type I should use in my glsl shader. Using an ivec3 creates an error as I would expect.

Do I need to load them into an uint and then do bitwise operations do extract the variables? What are the speed implications of this?

If I want to do these bitwise operations, how can I be sure they will be endian-independent? To my understanding, the first byte on my CPU side could be stored in the first or last byte of the integer on the GPU side.

Upvotes: 0

Views: 360

Answers (1)

Nicol Bolas
Nicol Bolas

Reputation: 473232

There is nothing to "extract". You asked to pass 3 unsigned integer values per-vertex. That's what the format defines, and that's what the shader should receive. The fact that each unsigned integer value is 8 bits doesn't need to be reflected in your shader; only that they're unsigned integers and that there are 3 of them.

There are no endian issues; not unless you create them in your CPU code. The format specifies that each value of the attribute comes from an array of 3 8-bit values. The three components are read left-to-right, and that's the order the components are expected to be in in memory.

Bytes don't have endian problems. Endian only is an issue when reading a single value that takes up multiple bytes. You asked to read 3 bytes, so that's what it will do. And that's what the CPU should write.

BTW, you should avoid using misaligned types like this. Pad it out to 4 8-bit integers rather than 3.

Upvotes: 0

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