UnrealEngine/Engine/Source/ThirdParty/astcenc/4.2.0/Docs/Terminology.md

Terminology for the ASTC Encoder

Like most software, the astcenc code base has a set of naming conventions for variables which are used to ensure both accuracy and reasonable brevity.

🚧 These conventions are being used for new patches, so new code will conform to this, but older code is still being cleaned up to follow these conventions.

Counts

For counts of things prefer <x>_count rather than <x>s. For example:

• plane_count
• weight_count
• texel_count

Where possible aim for descriptive loop variables, as these are more literate than simple i or j variables. For example:

• plane_index
• weight_index
• texel_index

Ideal, Unpacked Quantized, vs Packed Quantized

Variables that are quantized, such as endpoint colors and weights, have multiple states depending on how they are being used.

Ideal values represent arbitrary numeric values that can take any value. These are often used during compression to work out the best value before any quantization is applied. For example, integer weights in the 0-64 range can take any of the 65 values available.

Quant uvalues represent the unpacked numeric value after any quantization rounding has been applied. These are often used during compression to work out the error for the quantized value compared to the ideal value. For example, QUANT_3 weights in the 0-64 range can only take one of [0, 32, 64].

Quant pvalues represent the packed numeric value in the quantized alphabet. This is what ends up encoded in the ASTC data, although note that the encoded ordering is scrambled to simplify hardware. For example, QUANT_3 weights originally in the 0-64 range can only take one of [0, 1, 2].

For example:

• weights_ideal_value
• weights_quant_uvalue
• weights_quant_pvalue

Full vs Decimated interpolation weights

Weight grids have multiple states depending on how they are being used.

full_weights represent per texel weight grids, storing one weight per texel.

decimated_weights represent reduced weight grids, which can store fewer weights and which are bilinear interpolated to generate the full weight grid.

Full weights have no variable prefix,but decimated weights are stored with a dec_ prefix.

• dec_weights_ideal_value
• dec_weights_quant_uvalue
• dec_weights_quant_pvalue

Weight vs Significance

The original encoder used "weight" for multiple purposes - texel significance (weight the error), color channel significance (weight the error), and endpoint interpolation weights. This gets very confusing in functions using all three!

We are slowly refactoring the code to only use "weight" to mean the endpoint interpolation weights. The error weighting factors used for other purposes are being updated to use the using the term "significance".

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