BC3X420 - cr88192/bgbtech_misc GitHub Wiki

Idea: Extended form of DXT5/BC3, with added 4:2:0 subsampled-chroma mode.

• IIRC, Tested: actually worked pretty ok.
• Drawback: this would need hardware support to be usable.

AlphaSubBlock:

• AlphaA:BYTE
• AlphaB:BYTE
• AlphaPixels:WORD48
• AlphaA>AlphaB
  • 0=AlphaA, 1=AlphaB, 2=6/7*AlphaA+1/7*AlphaB, ..., 7=1/7*AlphaA+6/7*AlphaB
• AlphaA<=AlphaB
  • 0=AlphaA, 1=AlphaB, 2=4/5*AlphaA+1/5*AlphaB, ..., 5=1/5*AlphaA+4/5*AlphaB, 6=0, 7=255

ColorSubBlock:

• ColorA:WORD
• ColorB:WORD
• ColorPixels:DWORD

UVSubBlock:

• UA:BYTE
• VA:BYTE
• UB:BYTE
• VB:BYTE
• AT:BYTE
• UVPixels:WORD24

AT:

• Low 2 bits=00: Interpreted as 6-bit Alpha
• Low 2 bits=01: Interpreted as 2-bit endpoint selection and 4 alpha bits.
  • Endpoints: 00=0,255; 01=0,64; 10=192,255; 11=64,192
• Low 2 bits=10: Reserved
• Low 2 bits=11: Reserved

UVPixels:

• 2x2 pixels at 3 bits/pixel U channel (12 bits)
• 2x2 pixels at 3 bits/pixel V channel (12 bits)
• Interpolation is the same as AlphaSubBlock with AlphaA>AlphaB

NormalBlock (ColorA > ColorB):

• ChanA:AlphaSubBlock
• ChanRGB:ColorSubBlock

YuvBlock (VA < VB) && (UA < UB):

• ChanY:AlphaSubBlock
• ChanUV:UVSubBlock

In the case of YUV blocks, the YUV endpoints would be converted to RGBA when the block is decoded.

Probable Colorspace (Option A):

• Forward:
  • Y=(R+2*G+B)/4
  • U=(B-Y)/2+128
  • V=(R-Y)/2+128
• Inverse:
  • B'=Y+(U-128)*2
  • R'=Y+(V-128)*2
  • G'=(4*Y-R-G)/2

Probable Colorspace (Option B):

• Forward:
  • Y=(R+2*G+B)/4
  • U=(B-G)/2+128
  • V=(R-G)/2+128
• Inverse:
  • G'=Y-(U+V-256)/2;
  • B'=G+(U-128)*2
  • R'=G+(V-128)*2