BTIC1G - cr88192/bgbtech_misc GitHub Wiki

BTIC1G Goals:

• Simple Encoder
• Bounded worst case bitrate
• Incremental images
• Inline image data

Will aim to have a lower average bitrate than 1C.

Colorspace:

• Either YCbCr or a scaled RCT.
• YCbCr:
  • Y=( 0.35R+0.50G+0.15B)
  • U=(-0.17R-0.33G+0.50B)+128
  • V=( 0.50R-0.42G-0.08B)+128
• ScRCT
  • Y=(0.25R+0.50G+0.25B)
  • U=(0.50B-0.50G)+128
  • V=(0.50R-0.50G)+128

Block Color

• 23 bit pair (YUV23A):
  • X: 1 (0)
  • Y: 7
  • D: 6
  • U: 5
  • V: 5
• 22 bit pair (YUV22B):
  • X: 1 (0)
  • M: 2 (Mode)
    • 0: Flat (Simple Case: D=0)
      • Bits within D are used for special cases.
    • 1: 2x2x2
    • 2: 4x4x1
    • 3: 4x4x2
  • Y: 7
  • D: 6
  • U: 4
  • V: 4
• 15 bit flat color (YUV15):
  • X: 1
  • Y: 7
  • U: 4
  • V: 4

YUV specifies the centroid color (RCT), and d=Ymax-Yavg.

• Both Y and D will be linear.
• Y and D are normally unpacked with high bits folded to low-bits to make 8-bit values.

UV Points:

• UV points will follow curves.
• 3-bit:
  • 0 56 96 120 128 136 160 200
• 4-bit:
  • 0 30 56 78 96 110 120 126 128 130 136 146 160 178 200 226
• 5-bit:
  • 0 15 30 43 56 67 78 87 96 103 110 115 120 123 126 127
  • 128 129 130 133 136 141 146 153 160 169 178 189 200 213 226 241

Extension: HF Color 0 (YUV22B Based):

• • X: 1 (0)
  • M=0: 2 (Mode, Escape)
  • Dx: 7 (Reserved)
  • M: 6
    • 0: Invalid
    • 1: 4x4x2
    • 2: 4x4x3
    • 16: HF-Flat
    • 17: 4x4x2 (Dyuv)
    • 18: 4x4x2Y+2x2U+2x2V
  • Y: 8
  • U: 8
  • V: 8
  • If(M!=16)
    • D: 8
  • If((M&16) && (M!=16))
    • Du: 8
    • Dv: 8

Pixel Data:

• Xsz x Ysz x Bits
• 1bpp
  • 0=Low Color
  • 1=High Color
• 2bpp
  • 0=Low
  • 1=2/3 Low + 1/3 High
  • 2=1/3 Low + 2/3 High
  • 3=High

Commands:

• 00-7F: Raw Block
  • / 23-bit color, 4x4x2bpp (if d!=0)
  • YUV22B, followed by pixel data.
  • Special Case: 16-bit Flat Block:
    • 000Y-YYYY YUUU-VVV1
  • Special Case 2: 24-bit Flat Block 2:
    • 000Y-YYYY YYYU-VP10 UUUU-VVVV
      • Y=8-bit, U/V=5+1 bit (P bit is shared between U and V).
      • The U/V bits located in the D field are least-significant bits.
• 80-8F: Skip
• 90-9F: Flat Color
  • Run of single flat color (YUV15)
• A0-AF: Discrete Colors
  • Each block represented by a single color.
• B0-BF: 2x2x2bpp
• C0-CF: 4x4x1bpp
• D0-DF: 4x4x2bpp
  • YUV23 color-point, followed by pixel data.
• E0-FF: Extended Commands
  • E0: End Of Data
  • E1-E6: Reserved for markers.
  • E8: Skip+Translate
    • High 3 bits:
      • 0=Extended Skip (13 bit)
        • 000n-nnnn nnnn-nnnn
      • 1=Skip+Translate (5-bit), Y:4 (Bias=8), X:4 (Bias=8)
        • 001n-nnnn yyyy-xxxx
      • 2=Skip+Translate (9-bit), Y:6 (Bias=32), X:6 (Bias=32)
        • 010n-nnnn nnnn-yyyy yyxx-xxxx
  • E9: Set Controller
  • EA: Extended Block Commands
    • ooon-nnnn
    • 00-1F: Raw Blocks With Differential Coding
      • Each block is encoded as a raw block.
      • Color.X may be used for differential colors.

For blocks, Color.X indicates the use of differential colors:

• 0: Discrete color point
• 1: Delta from prior colors.

YUV15:

• Discrete: Implicitly sets D to 0.
• Differential: Does not effect D.

Differential values will be center-biased (Identity=128), operating on logical 8-bit values and will be 8-bit modular.

Possible bitstream coded version.

Bitstream will be encoded is MSB first order.

Superseded / Became: BTIC1H

AdRice:

• Will have a unary prefix (Q) of 0+ 1 bits, terminated by a 0 bit.
• This will be followed by a k bit suffix (j).
  • Value=(Q<<k)|j;
• ></k)|j;
• >0)k=k-1;
  • 1: k is unchanged.
  • 2+: k=k+log2(Q).

Here, AdRice contexts will start with an initial value of 2 unless specified otherwise.

Signed Rice values will have the sign folded into the LSB (0, -1, 1, -2, 2, ...).

Normal Colors are 7655, flat colors are 755.

• Color points are encoded with a leading bit:
  • 0: Discrete Color, encoded as 7655 or 755.
  • 1: Diff Color, encoded as 3 or 4 AdRice coded deltas.
    • Each AdRice point has its own context.
• 7655D: Will mean 7655 or Diff (4 value).
• 755D: Will mean 755 or Diff (3 value).

Commands will start with a 3-bit tag:

• 0: Raw Block
  • 2 Bit Mode Follows
  • M=0: Flat Color, 755D Color point follows.
  • M=1: 2x2x2bpp, Followed by 7655D and 2x2x2bpp.
  • M=2: 4x4x1bpp, Followed by 7655D and 4x4x1bpp.
  • M=3: 4x4x2bpp, Followed by 7655D and 4x4x2bpp.
• 1: Skip
  • Length Follows
  • Translate bit follows:
    • 0: No Translate
    • 1: Translate
      • AdRice coded X/Y offsets follow.
• 2: Flat Color
  • Length Follows
  • Followed by 755D point.
• 3: Discrete Colors
  • Length Follows
  • Each block given as a 755D point.
• 4: 2x2x2bpp
• 5: 4x4x1bpp
• 6: 4x4x2bpp
  • Length Follows
  • Each 7655D point follows.
  • Followed by pixel data for each block.
• 7: Escape
  • Another 5 bits follows.

Possible could be to use 6-bit MTF+Rice for commands:

• High 3: Command
• Low 3: Depends on command
  • Raw: Mode (4-7=Reserved)
  • Skip, TCC
    • T=Translate
    • CC=Count, 0-2: 1-3 blocks, 3=Length follows
  • Pixel Blocks
    • Encodes a length, where 0-6=1-7 blocks, 7=Length follows.
  • Escape, Encodes op bits.