27 lines
1.8 KiB
Text
27 lines
1.8 KiB
Text
|
The ETC1 compressor uses modified cluster fit:
|
||
|
|
|
||
|
|
Assume that there exists an ideal base color and set of selectors for a given table.
|
||
|
|
For a given table and set of selectors, the ideal base color can be determined by subtracting the offsets from each pixel and averaging them.
|
||
|
|
Doing that is equivalent to subtracting the average offset from the average color.
|
||
|
|
Because positive and negative selectors of the same magnitude cancel out, the search space of possible average offsets is reduced: 57 unique offsets for the first table and 81 for the others.
|
||
|
|
Most of the offsets result in the same color as another average offset due to quantization of the base color, so those can be de-duplicated.
|
||
|
|
So:
|
||
|
|
- Start with a high-precision average color.
|
||
|
|
- Apply precomputed luma offsets to it.
|
||
|
|
- Quantize and de-duplicate the base colors.
|
||
|
|
- Find the ideal selectors for each base color.
|
||
|
|
|
||
|
|
Differential mode is solved by just finding the best legal combination from those attempts.
|
||
|
|
|
||
|
|
There are several scenarios where this is not ideal:
|
||
|
|
- Clamping behavior can sometimes be leveraged for a more accurate block.
|
||
|
|
- Differentials can sometimes be moved slightly closer to become legal.
|
||
|
|
- This only works when MSE is the error metric (i.e. not normal maps)
|
||
|
|
- This only works when pixel weights are of equal importance (i.e. not using weight by alpha or edge deblocking)
|
||
|
|
|
||
|
|
T and H mode just work by generating clustering assignments by computing a chrominance line and splitting the block in half by the chrominance midpoint and using those to determine the averages.
|
||
|
|
|
||
|
|
Planar mode is just solved algebraically.
|
||
|
|
|
||
|
|
If you want to emulate etc2comp's default settings, add the flag ETC_UseFakeBT709 to use its modified Rec. 709 error coefficients.
|
||
|
|
Doing that will significantly slow down encoding because it requires much more complicated quantization math.
|