What is the difference between normalized, scaled and integer VkFormats?

Question

Let's take the following 6 VkFormats for example:

VK_FORMAT_R8_UNORM
VK_FORMAT_R8_SNORM
VK_FORMAT_R8_USCALED
VK_FORMAT_R8_SSCALED
VK_FORMAT_R8_UINT
VK_FORMAT_R8_SINT

All of these specify a one-component 8-bit format that has a single 8-bit R component.

The formats differ in whether they are (a) normalized, (b) scaled; or (c) integer. What does that mean? What are the differences between those three things? Where is that specified?

Are all 256 possible values of 8-bits meaningful and valid in all six formats?

(They also differ in whether they are signed or unsigned. I assume this means whether their underlying types are like the C types int8_t or uint8_t ?)

Answer 1

Refer to Identification of Formats and Conversion from Normalized Fixed-Point to Floating-Point in the specification.

• UNORM is a float in the range of [0, 1].
• SNORM is the same but in the range of [-1, 1]
• USCALED is the unsigned integer value converted to float
• SSCALED is the integer value converted to float
• UINT is an unsigned integer
• SINT is a signed integer

I.e. for the VK_FORMAT_R8_*:

• for UNORM raw 0 would give 0.0f, raw 255 would give 1.0f
• for SNORM raw -127 (resp. 129) would give -1.0f, raw 127 would give 1.0f
• USCALED raw 0 would give 0.0f, raw 255 would give 255.0f
• SSCALEDraw -128 (resp. 128) would give -128.0f, raw 127 would give 127.0f

-128 (-2^n-1) is not meaningful in SNORM, and simply clamps to -1.0f.